# Past seminars

# 2017-2018

# Vendredi 13 Avril

## Title: Semimetals Unlimited: Unbounded electrical and thermal transport properties in nodal semimetals

# Brian Skinner (MIT, Boston, États-Unis d’Amérique).

**Abstract: **Modern electronics is built on semiconductors, whose utility comes from their ability to operate on either side of the conductor-insulator dichotomy. For practical applications, however, semiconductors face certain unavoidable limitations imposed by the physics of Anderson localization and by the disorder introduced through doping. In this talk I discuss whether these same limitations apply to nodal semimetals, which are a novel class of three-dimensional materials that have a vanishing density of states (like insulators) but no gap to electron-hole excitations (like conductors). I show that, surprisingly, in a certain class of nodal semimetals the electronic mobility can far exceed the bounds that constrain doped semiconductors, becoming divergingly large even with a finite concentration of charged impurities. I then discuss the thermoelectric effect in semimetals, and show that their electron-hole symmetry allows for a thermopower that grows without bound under the application of a strong magnetic field. This large thermopower apparently enables the development of devices with record-large thermoelectric figure of merit.

# Jeudi 29 Mars

## Title: "Emergence of hydrodynamics in integrable systems out of

equilibrium"

# Benjamin Doyon (King's College, Londres, Angleterre)

**Abstract: **I will introduce the recently developed theory of

"generalized hydrodynamics", which describes large-scale

behaviours in many-body quantum and classical integrable

systems out of equilibrium.

# Jeudi 1 Mars

## Title: Baxter Q-operators for rational spin chains

# Rouven Frassek (IHES, Paris)

**Abstract: **

After giving a short review of the quantum inverse scattering method I will discuss how Q-operators can be constructed in this framework. The approach employs an infinite dimensional auxiliary space and follows the ideas of Bazhanov, Lukyanov and Zamolodchikov. The R-matrices relevant belong to a set of degenerate solutions to the Yang-Baxter equation. The construction is exemplified for the closed and open Heisenberg chain but also for non-compact spin chains, which are relevant for high energy QCD and N=4 super Yang-Mills theory. Finally, I discuss the generalisation of the construction to higher rank Lie algebras.

# Jeudi 15 Février

## Title: The Klein-Gordon equation on curved spacetimes and its propagators.

# Jan Derezinski (Katedra Metod Matematycznych dla Fizyki, Université de Varsovie, Pologne)

**Abstract: **The Klein-Gordon equation (including an electromagnetic potential) has several natural Green's functions,

often called propagators.The so-called Feynman propagator, used in quantum field theory, has a clear

definition on static spacetimes. I will discuss, partly on a heuristic level, its possible generalizations to the non-static case. I will also describe a curious, partly open problem about the self-adjointness of the Klein-Gordon operator.

# Jeudi 25 Janvier

## Title: Generalized Gibbs Ensembles and Generalized Hydrodynamics in quantum integrable systems

# Jacopo De Nardis (Département de Physique, ENS Paris)

**Abstract: **I'll give a short review of the recent theoretical progress to explicitly construct non-thermal steady states in quantum systems as interacting bosons and spin chains.

Moreover, I'll present the recently introduced hydrodynamic description of such non-thermal steady states that allows to study (ballistic)

transport properties of many-body systems and to construct non-equilibrium steady states with persistent energy or spin currents and stronger

# Jeudi 18 Janvier

## Title: Mesoscopic Quantum electrodynamics: from atomic-like physics to quantum transport.

# Audrey Cottet (Laboratoire Pierre Aigrain, ENS Paris)

**Abstract: **

Cavity QED techniques have turned out to be instrumental to probe or manipulate coherently two level systems such as superconducting quantum bits. The success of this field relies on the implementation of a strong coupling between the two level systems and cavity photons. Recently, experiments on hybrid mesoscopic circuits embedded in coplanar microwave cavities have appeared [1, 2]. This architecture is appealing since new degrees of can be used in the context of cavity QED. In the first part of this talk, I will discuss how the strong coupling between a single charge [3, 4, 5] or spin [6, 7, 8, 9] degree of freedom in a double quantum dot and cavity photons can be obtained.

Mesoscopic circuits represent model systems for quantum transport and condensed matter phenomena due to the presence of fermionic reservoirs. In the second part of this talk, I will show that microwave cavities are also a powerful probe in that context. For a quantum dot coupled to a superconducting contact, a microwave cavity reveals photo-emission due to quasiparticle tunneling, although this effect is too weak to be detected in a transport measurement [10]. A microwave cavity could also provide a new way to test the peculiar properties of Majorana bound states induced inside a spin-orbit coupled quantum nanowire by a superconducting contact [11].

References:

[1] Delbecq et al, Phys. Rev. Lett. **107**, 256804 (2011).

[2] Frey et al, Phys. Rev. Lett. **108**, 046807 (2012).

[3] Bruhat, Cubaynes, Viennot, Dartiailh, Desjardins, Cottet and Kontos, arXiv:1612.05214

[4] Mi, Cady, Zajac, Stehlik, Edge and Petta, Science **355** 156 (2017)

[5] Stockklauser, Scarlino, Koski, Gasparinetti, Kraglund Andersen, Reichl, Wegscheider, Ihn, Ensslin, and Wallraff, Phys. Rev. X **7**, 011030 (2017)

[6] Viennot, Dartiailh, Cottet, and Kontos, Science **349**, 408 (2015).

[7] Mi, Benito, Putz, Zajac, Taylor, Burkard, Petta,, arXiv:1710.03265

[8] Landig, Koski, Scarlino, Mendes, Blais, Reichl, Wegscheider, Wallraff, Ensslin, Ihn, arXiv:1711.01932

[9] Samkharadze, Zheng, Kalhor, Brousse, Sammak, Mendes, Blais, Scappucci, Vandersypen, arXiv:1711.02040

[10] Bruhat, Viennot, Dartiailh, Desjardins, Kontos and Cottet, Phys. Rev. X **6**, 021014 (2016).

[11] Dartiailh, Kontos, Douçot and Cottet, Phys. Rev. Lett. **118**, 126803 (2017)

# Jeudi 30 Novembre

## Title: Localization phenomena and topological properties of atomic lattice gases with long-range interactions

# Jiri MINAR (Université de Nottingham et Université de Genève)

**Abstract: **

Recent experimental progress in cold atomic gases has allowed for creation of arbitrary lattice geometries at unit filling [1,2] which opens exciting ways for studies of the dynamics of quantum spin Hamiltonians [3].

In this talk I will discuss two examples of dynamics in spin systems with long-range interactions.

Firstly, I will discuss the dynamics of Rydberg excitations in an optical tweezer array under the so-called facilitation condition [4]. Here, the presence of positional atomic disorder results in a correlated disorder in the interatomic interaction strengths and drastically affects the facilitation dynamics. To shed light on the role of disorder in a many-body setting we show that here the dynamics is governed by an Anderson-Fock model, i.e., an Anderson model formulated on a lattice with sites corresponding to many-body Fock states. We first consider a one-dimensional atom chain in a limit that is described by a one-dimensional Anderson-Fock model with disorder on every other site, featuring both localized and delocalized states. Next, we consider a situation in which the system maps on a two-dimensional Anderson-Fock model and observe a clear suppression of excitation propagation, which we ascribe to the localization of the many-body wave functions in Hilbert space. With the help of the developed framework we then study the excitation dynamics in the ladder geometry.

Secondly, I will describe the topological properties of a two-dimensional atomic lattice gas, where the coupling of the atoms to the radiation field gives rise to dissipation and long-range interactions beyond a simple power law [5]. This has for a consequence energy spectra with one-sided divergences in the Brillouin zone or possible breaking of the standard bulk-boundary relation in topological insulators. We show that under certain conditions, the topological properties, such as the transport of an excitation along the edge of the lattice, remain robust with respect to the presence of lattice defects and dissipation.

[1] D. Barredo, S. de Léséleuc et al., Science 354, 1021 (2016)

[2] M. Endres, H. Bernien et al., Science aah3752 (2016)

[3] H. Labuhn, D. Barredo et al., Nature 534, 667 (2016)

[4] M. Marcuzzi, J. Minář et al., Phys. Rev. Lett. 118, 063606 (2017)

[5] R. Bettles, J. Minář et al., Phys. Rev. A 96, 041603(R) (2017)

# Jeudi 23 Novembre

## Title: Quantum Transport after Inhomogeneous Quenches

# Spyros Sotiriadis (Université de Ljubljana, Slovénie)

**Abstract: **

# Jeudi 9 Novembre

## Title: Topological phases of parafermions: a model with exactly-solvable ground states

# Leonardo MAZZA (ENS Paris)

**Abstract: **

In this talk I will speak about parafermions, emergent excitations that generalize Majorana fermions and can also realize topological order.

After making an introduction on the research field, I will present a non-trivial and quasi-exactly-solvable model for a chain of parafermions in a topological phase. The ground-state wave-functions, which are matrix-product states and have a particularly elegant interpretation in terms of Fock parafermions, are computed and characterized. Using these wavefunctions, several signatures of topological order are demonstrated analytically.

This study provides a starting point for the non-approximate study of topological one-dimensional parafermionic chains in the absence of strong edge modes.

Reference:

Fernando Iemini, Christophe Mora and Leonardo Mazza, Phys. Rev. Lett. 118 170402 (2017)

# Jeudi 5 Octobre

## Title: Ondes, singularités et couches de cisaillement internes dans les fluides stratifiés tournants

# Stéphane Le Dizès (IRPHE, Marseille, France)

**Abstract: **Dans ce séminaire, je m'intéresse à la réponse d'un fluide stratifié tournant à un forçage

harmonique. Je montre la grande variété de cette dernière suivant la géométrie du domaine et

la fréquence d'excitation. Je montre également comment des singularités apparaissent

au sein du fluide dans la limite non-visqueuse. En présence de viscosité, ces singularités donnent lieu

à des couches de cisaillement internes dont les caractéristiques peuvent être déterminées.

J'analyse plus en détail celles obtenues par la libration d'un disque et d'un sphéroïde dans un milieu

tournant infini.

Les résultats sont discutés dans les contextes de la géophysique interne (écoulement au sein des

planètes généré par forçage gravitationnel) et de l'océanographie (écoulement généré par les marées).

# Jeudi 28 Septembre

## Title: Controllable sub-5nm nanomaterial synthesis and manipulation

# Xing Wu (Department of Electrical Engineering, East China Normal University)

**Abstract: **Two-dimensional (2D) ultra-thin materials like graphene with rich physical properties and unique layered structures are promising for applications in electronics,

chemistry, energy, and bioscience, etc. In this talk, I will mainly introduce the controllable synthesis of 2D materials, device fabrication and

electronic transport. Also, I will talk about manipulate 2D materials at atomic scale by using multiple-fields transmission electron microscopy (TEM).

# Jeudi 21 Septembre

## Title: Bulk-edge correspondence for Floquet topological insulators

# Clement Tauber (Département de Physique, ETH Zürich, Suisse)

**Abstract: **Floquet topological insulators describe independent electrons on a lattice driven out of equilibrium by a time-periodic Hamiltonian, beyond the usual adiabatic approximation. In dimension two such systems are characterized by integer-valued topological indices associated to the unitary propagator, alternatively in the bulk or at the edge of a sample. In this talk I will give new definitions of the two indices, relying neither on translation invariance nor on averaging, and show that they are equal. In particular weak disorder and defects are intrinsically taken into account. Finally indices can be defined when two driven sample are placed next to one another either in space or in time, and then shown to be equal. The edge index is interpreted as a quantized pumping occurring at the interface with an effective vacuum.

# 2016-2017

# Jeudi 22 Juin

## Title: Freezing of entanglement in alternating transverse field XY model

# Debasis SADHUKAN (Harish-Chandra Research Institute — Allahabad, India)

**Abstract: **Inhomogeneity often leads to the generation of new phases in

many-body systems. The one-dimensional quantum XY model in a uniform

transverse field is known to have a quantum phase transition from

antiferromagnetic to paramagnetic phase. Introduction of an

alternating transverse field instead of a uniform one, develops a new

dimer phase in addition to the antiferromagnetic and paramagnetic

phase. I will show that the quantum correlation present in the system

can characterize all the quantum phase transitions present in the

system. I will also talk about the trends of quantum correlations in

such system, under closed and open dynamics. Finally, I will show that

bipartite entanglement can be frozen over time with a proper choice of

the many-body substrate, which is in contact with the environment via

a repetitive interaction.

# Jeudi 15 Juin

## Title: Changes large and small: The physics of stochastic resetting

# Shamik Gupta (Department of Physics, Ramakrishna Mission Vivekananda University, Belur Math, Calcutta, India)

**Abstract: **What happens when a continuously evolving stochastic process is interrupted with large changes at random intervals of time? Modeling the stochastic process by diffusion and the large changes as abrupt resets to the initial condition, this talk will unveil a wide spectrum of rich long-time behavior that the resulting dynamics exhibits, from an ever-spreading spatial distribution, to one that is time independent and characterizes a nonequilibrium stationary state. The implication of the results for physical situations of relevance will be discussed.

# Jeudi 8 Juin

## Title: Observable's Statistical Mechanics

# Fabio ANZA (Oxford University, Angleterre)

**Abstract: **The emergence of thermal equilibrium is the statistical foundations of thermodynamics. In a many-body quantum system, whose microscopic dynamics is unitary, there are two main approaches. The "typicality approach" ascribes the emergence of local thermalization to entanglement while the "Eigenstate Thermalization Hypothesis" (ETH) derives its intuition from the emergence of chaotic behavior of observables. After a brief introduction on these two topics I will argue that the ordinary notion of thermal equilibrium is experimentally unaccessible and propose a more realistic way of describing thermal equilibrium, focused on observables rather than on the state of the system. I will show that the theory that emerges is an observable-wise generalization of statistical mechanics and it provides a fresh perspective to look at the typicality approach and at ETH.

# Jeudi 18 Mai

## Title: Topology and the Pseudo-Gap phase of Cuprates

# Catherine Pépin (IPhT, CEA Saclay, Paris)

**Abstract:**The Pseudo-Gap state in under-doped cuprates remains the key mystery for the understanding of those compounds. Recently, a new concept has been introduced, that this state of matter could be controlled by topology. In this talk we review the two main forms of topological states, in real and momentum space that are specific to quantum matter. We show how each of them can account in a different way for the phase diagram of the cuprates, and in particular the under doped region between the Mott insulator at very low oxygen doping, and the metallic state at high doping. We then describe how skyrmions can emerge in the pseudo-spin space, related to an emerging SU(2) symmetry, and argue that proliferation of such skyrmions can account for a number of experimental properties of the pseudo-gap phase.

# Jeudi 15 Mai

## Title: 2D CFT blocks for a class of N=1 theories in 4D

# Vladimir Mitev (Institute of Physics, Universität Mainz)

**Abstract: I**n this talk I will present our program for the search for the 2D CFT description of a large class of 4D gauge theories with superconformal N=1 symmetry. I will show how to identify the 2D CFT symmetry algebra and its representations, namely the conformal blocks of the Virasoro/W-algebra, that underlie the 2D theory and reproduce the Seiberg-Witten curves of the N=1 gauge theories. One finds that the blocks corresponding to thegauge theories under investigation involve fields in certain non-unitary representations of the Virasoro/W-algebra. These conformal blocks further give a prediction for the instanton partition functions of the 4D theories.

# Jeudi 11 Mai

## Title: Complex structures and zero-curvature equations for sigma-models

# Dimitri Bykov (Max-Planck-Institut fur Gravitationsphysik,Potsdam, Allemagne)

**Abstract: **

I will construct zero-curvature representations for the equations of motion of a class of sigma-models with complex homogeneous target spaces, not necessarily symmetric. As an example, for the case when the target space is a flag manifold and the worldsheet a sphere, I will describe all solutions to the equations of motion. Various ramifications of these results will be described.

# Jeudi 13 Avril

## Title: Fermionic matrix product states and one-dimensional topological phases

# Nick Bultinck (Gent University, Belgique)

**Abstract: **

The matrix product state (MPS) formalism has been very successful both

as the variational class underlying DMRG, and as a theoretical tool to

classify all

symmetry-protected topological phases of spin systems in one

dimension. In this talk I will explain how MPS can be extended to

describe fermionic systems.

This naturally leads to two classes characterized by the presence or

absence of Majorana edge modes. Imposing additional global symmetries

allows one to

extract discrete invariants from the MPS that lead to the full

classification of interacting symmetry-protected phases of fermions in

one dimension. The invariants

can be related to physical properties of the system and their behavior

under stacking of chains is determined by the intrinsic fermionic MPS

formalism.

# Jeudi 6 Avril

## Title: Detection of Zak phases and topological invariants in a chiral quantum walk of twisted photons

# Alexandre Dauphin (ICFO, Barcelone)

**Abstract: **Topological insulators are exotic phases going beyond the standard Landau theory of phase transitions. These phases are characterized by a global topological order and present robust conducting surface states protected by the topology of the system. Recently, a great effort has been done to quantum simulate such phases. We here focus on the quantum simulation of one dimensional topological phases with quantum walks. We discuss how topology can arise in these systems and how to detect the topological phase. Finally, we discuss the recent photonic quantum walk realized in the group of Prof. L. Marrucci and propose a realistic detection scheme of the topological invariant.

## Title: Topological aspects of the generalized sine-kernel Fredholm determinants

# Oleksandr Gamayun (Lorentz Institute, Leiden, Pays-Bas)

**Abstract: **

We consider Fredholm determinants with the so-called time-dependent generalized sine kernel introduced in [KK].

These determinants are appropriate for a description of two-point functions in a wide class of integrable models.

The long-distance/long-time asymptotic behaviour of these objects has been analysed by means of Riemann-Hilbert problem (RHP) [KK].

We re-derive this asymptotic by means of the summations of microscoptic form-factors (similar to Refs. [KK2],[KK3]). This allows us to bypass

restrictions on the kernel needed for the RHP analysis. In particular, we consider the possibility for certain periodic functions in a kernel to have a topological phase-slip.

We study how these phase-slips affect the asymptotic behaviour and demonstrate how they appear in specific physical models.

[KK] K. K. Kozlowski {\em Riemann–Hilbert approach to the time-dependent generalized sine kernel}

[arXiv:1011.5897].

[KK2] N. Kitanine, K. K. Kozlowski, J. M. Maillet, N. A. Slavnov, V. Terras {\em Form factor

approach to dynamical correlation functions in critical models} [arXiv:1206.2630]

[KK3] K. K. Kozlowski, J.-M. Maillet {\em Microscopic approach to a class of 1D quantum critical

models} [arXiv:1501.07711].

# Jeudi 30 Mars

## Title: What are the impedance combination rules in quantum circuits?

# Philippe Joyez (CEA/SPEC,Saclay)

**Abstract: **

Résumé: When several quantum electronic components are assembled in a electrical circuit, they interact with each other in a non-local and non-linear way that prevents using the usual impedance combinations rules to predict the behavior of the circuit. I will first explain qualitatively how this interaction operates. Then I will show how it can be taken into account for making detailed predictions on several simple circuits, and making a link with quantum optics.

References

[1] C. Altimiras, F. Portier and P. Joyez, Interacting electrodynamics of short coherent conductors in quantum circuits, Phys. Rev. X 6, 031002.

# Jeudi 23 Mars

## Title: Lattice deformation of Virasoro algebra : Volterra, Toda-2 and q-Toda models.

# Olivier Babelon (LPTHE, Université Pierre et Marie Curie, Paris)

**Abstract: **

# Jeudi 2 Février

## Title: Non equilibrium dynamics of quantum systems : the Loschmidt echo

# Eric Vernier (SISSA et INFN, Trieste, Italie)

**Abstract: **

The non-equilibrium dynamics of quantum many-body systems has attracted a large interest over the last decade, prompted by formidable advances in cold-atomic experiments.

# Jeudi 26 Janvier

## Title: Modified Bethe Ansatz for models without U(1) symmetry.

# Samuel Belliard (IPhT, CEA-Saclay, Paris)

**Abstract: **

# Jeudi 19 Janvier

## Title: Engineering non-abelian states from simpler topological order

# Cécile Répellin (Max Planck Institute, Dresde, Allemagne)

**Abstract: **

The possibility of realizing anyons -- quasiparticles with fractional exchange statistics -- is an exciting prospect in the field of interacting topological phases. Non-abelian anyons, whose exchange is characterized by a matrix rather than a simple phase, are of the most exotic kind. They are highly sought after as they could be used as qubits for quantum computation intrinsically immune to decoherence. While non-abelian anyons are expected to appear in the fractional quantum Hall effect, engineering systems that purposefully favor their emergence might be a better strategy to probe their properties. In this talk, I will explore two such routes. The first one stems from the concept of projective construction: a multilayer abelian system can formally be transformed into a non-abelian one by application of a non-local operator. I will review this construction in the case of a well-known fractional quantum Hall state -- the Moore-Read state -- and show how to obtain all of its topological sectors by the insertion of line defects. I will then discuss a possible physical realization of the projective construction in a quantum Hall bilayer, and provide numerical arguments for this discussion. Another route to obtain non-abelian degeneracies is to trigger a phase transition at the edge of a quantum Hall bilayer where the excitations will be localized. I will show some preliminary numerical results supporting this transition in a microscopic system.

# Jeudi 12 Janvier

## Title: Large deviations in single-file motion

# Tridib Sadhu (Department of Theoretical Physics, Tata Institute of Fundamental Research)

**Abstract: **

Transport of impenetrable particles in a crowded one-dimensional channel is referred as the single-file motion. The particles can not pass each other and this leads to sub-diffusion of individual (tagged) particles. Such constrained motion has been observed in many physical systems: motion of ions through narrow pores in cell membranes, transport of large molecules in porous medium, etc. I shall present a hydrodynamic formulation to analyze the probability distribution of the position of a tagged particle in single-file. This formulation is an application of the macroscopic fluctuation theory and applies to a large class of single-file systems. The framework enables one to calculate the large deviation function of the tagged particle position which contains the full statistics at large time. As a simple example, I shall discuss a system of Brownian point particles with hard-core repulsion and show how to derive an exact expression of the large deviation function. Then I shall present an exact solution of the problem starting from microscopic dynamics and verify the hydrodynamic results. In addition, I shall discuss connection with fractional Brownian motion and emphasize an unusual dependence on the initial state, even at large times.

# Jeudi 5 Janvier

## Title: Quantum fields with tensorial locality

# Sylvain Carrozza (Perimeter Institute, Canada)

**Abstract: **

In recent years, generalizations of matrix models known as Tensor Models and Group Field Theories have been developed into a consistent formalism. The common feature of these field theories is an abstract notion of locality, know as tensorial locality, which encodes the combinatorial structure of the elementary field interactions. It has initially been introduced in the context of quantum gravity, where indeed the absence of a non-dynamical background space-time renders the standard notion of locality inoperative. I will provide an overview of this approach, focusing on general features of the phase diagrams of tensorial theories, and of their possible applications to quantum gravity and statistical physics. I will in particular discuss the tensorial version of the Sachdev-Ye-Kitaev model recently proposed by Witten.

# Jeudi 24 Novembre

## Title:The relative locality of quantum spacetime

# Laurent Freidel (Perimeter Institute, Canada).

**Abstract: **

# Jeudi 17 Novembre

## Title: Critical behavior of open quantum systems

# Riccardo ROTA (Laboratoire Matériaux et Phénomènes Quantiques, Univ. Paris Diderot VII)

**Abstract: **

I will discuss intriguing features of dissipative phase transitions in open quantum systems. In particular, I will present recent results [1] about the critical properties of two-dimensional lattices of spins interacting via an anisotropic Heisenberg Hamiltonian and subject to incoherent spin flips. Using the recently developed corner-space renormalization method [2], I will show the finite-size scaling and critical exponent of the magnetic linear susceptibility. I will also present results for the Von Neumann entropy and the quantum Fisher information across the transition, showing that a dissipative phase transition can share properties of both thermal and quantum phase transitions.

[1] R. Rota, F. Storme, N. Bartolo, R. Fazio and C. Ciuti, arXiv:1609.02848 [quant-ph]

[2] S. Finazzi, A. Le Boité, F. Storme, A. Baksic and C. Ciuti, Phys. Rev. Lett. 115, 080604 (2015).

# Jeudi 10 Novembre

## Title: Random field Ising model out of equilibrium

# Ivan BALOG (Institute of Physics, Zagreb, Croatia).

**Abstract: **Phase transitions in RFIM are characterized by a huge number of quasidegenrate metastable

states. This is why the problem has been resisting solution for so long. To fully capture the

important physics we have used the Nonperturbative Renormalization Group (NPRG) approach. I will

present how one can describe the critical relaxation to equilibrium as well as the out of

equilibrium or hysteresis criticality of this model by starting from a dynamical formalism

that we developed within the NPRG.

# Jeudi 3 Novembre

## Title: On classical de Sitter and Minkowski string backgrounds

# David Andriot (Albert-Einstein Institut Potsdam, Allemagne).

**Abstract: **Standard paths to connect string theory to cosmology or particle physics require to find backgrounds where the space-time is a product of de Sitter or Minkowski space-time and a compact manifold. We study the existence of such backgrounds at the classical level, in the framework of type II supergravities with parallel orientifolds and D-branes. For de Sitter, we obtain highly constraining no-go theorems; it allows us to exclude a stringy realisation of a particular inflation scenario. For Minkowski, we characterise a broad class of solutions, that possibly encounters for all such backgrounds.

# Jeudi 20 Octobre

## Title: Entanglement entropies in 3d gauge theories

# Aldo Riello (Perimeter Institute)

**Abstract: **

Entanglement entropy is a valuable tool for characterizing the correlation structure of quantum field theories. When applied to gauge theories, subtleties arise which prevent the factorization of the Hilbert space underlying the notion of entanglement entropy. Borrowing techniques from extended topological field theories, I introduce a new definition of entanglement entropy for both Abelian and non–Abelian gauge theories. I will then relate this construction to earlier proposals and argue that it brings these closer to each other. I will also point out that different definitions of entanglement entropies can be related to choices of (squeezed) vacuum states and excitations there upon. Time allowing, I will briefly discuss aspects more closely related to topics in quantum gravity.

# Jeudi 13 Octobre

## Title: Higher Spins & Strings

# Matthias R. Gaberdiel (Institut für Theoretische Physik, ETH Zurich, Suisse)

**Abstract: **The conjectured relation between higher spin theories on anti de-Sitter (AdS) spaces

and weakly coupled conformal field theories is reviewed. I shall then outline the

evidence in favour of a concrete duality of this kind, relating a specific higher spin

theory on AdS3 to a family of 2d minimal model CFTs, and show how this duality fits

into the framework of the familiar stringy AdS/CFT correspondence. Finally, I shall

explain how Yangian symmetries appear in this context, hinting at an underlying

integrable symmetry.

# Jeudi 6 Octobre

## Title: Hexagons and Three-Point Functions

# Benjamin Basso (LPT, ENS, Paris, France)

**Abstract: ** I will present a framework for computing correlators of three

single trace operators in planar N=4 SYM theory that uses hexagonal

patches as building blocks. This approach allows one to exploit the

integrability of the theory and derive all loop predictions for its

structure constants. After presenting the main ideas and results, I will

discuss recent perturbative tests and open problems. Based on arXiv

1505.06745

# Jeudi 22 Septembre

## Title: Cooperativity flows and Shear-Bandings: a statistical field theory approach

# Roberto Benzi (Universita Roma Tor Vergata, Rome, Italie)

**Abstract: **

Shear band formation is an example of a material instability, corresponding to an abrupt loss of homogeneity of deformation occurring in a solid sample subject to a loading path compatible with continued uniform deformation. This phenomenology is associated with `complex materials'

as it is clearly distinct from the simpler homogeneous deformation or deformation rate fields in ideal Hookean solids and Newtonian fluids.

In this talk I show that shear bandings can be interpreted as a compact solutions emerging from the variational formulation of field theory. The order parameter of the theory is the fluidity (inverse of viscosity). Compactons coexistence with regions of zero fluidity ("non-flowing vacuum") is shown to be stabilized by the presence of mechanical noise, which ultimately shapes up the equilibrium distribution of the fluidity field.

# Jeudi 7 Juillet

## Title: Atomtronics flux qubits

# Luigi Amico (Università di Catania, Italy & Center for Quantum Technologies, Singapore)

**Abstract: **Atomtronics is an emerging field seeking to realize atomic circuits exploiting ultra-cold atoms manipulated in micro-magnetic or laser-generated micro-optical circuits. Atomtronics circuits are made of bosonic/fermionic neutrally charged carriers put in motion by applying a 'potential drop' induced by methods developed by quantum technology. The typically low decoherence/dissipation rates of cold atoms systems and the high controllability and enhanced flexibility of potentials for ultracold matter make Atomtronics very attractive for enlarging the scope of existing cold-atom quantum technology. In this talk, I will concentrate on a few specific Atomtronics schemes for quantum processing. In particular, I will discuss the quantum dynamics of Bose-Einstein condensates trapped in ring shaped potentials. Interrupting the ring with weak links, atomic analogs of SQUIDs are realized. I will discuss the issue of scalability.

# Jeudi 30 Juin

## Title: Many spins in a lattice

# Bruno NAYLOR (Laboratoire de Physique des Lasers - Paris XIII)

**Abstract: **The field of ultracold atoms offers the possibility to load atoms in a periodic potential (called optical lattice) and engineer model condensed matter like Hamiltonians. In our experiment, chromium atoms are loaded in each site of a 3D optical lattice. We study spin dynamics due to long-range dipole-dipole interactions. This dynamics is inherently many-body, as each atom is coupled to its many neighbors. We specifically study in which conditions the spin dynamics can be seen as classical, and in which conditions quantum correlations arise.

# Jeudi 23 Juin

## Title: Antiferroquadrupolar and Ising-nematic orders of a frustrated bilinear-biquadratic Heisenberg model and implications for the magnetism of FeSe.

# Rong YU (Renmin University of China, Beijing)

**Abstract: **Motivated by the magnetic properties of the iron chalcogenides, we study the phase diagram of a generalized Heisenberg model with frustrated bilinear-biquadratic interactions on a square lattice. We identify zero-temperature phases with antiferroquadrupolar and Ising-nematic orders. The effects of quantum fluctuations and interlayer couplings are analyzed. We propose the Ising-nematic order as underlying the structural phase transition observed in the normal state of FeSe, and discuss the role of the Goldstone modes of the antiferroquadrupolar order for the dipolar magnetic fluctuations in this system. Our results provide a considerably broadened perspective on the overall magnetic phase diagram of the iron chalcogenides and pnictides, and are amenable to tests by new experiments.

# Jeudi 26 Mai

## Title: Factorized solutions of the Yang-Baxter equation and the lattice models.

# Dimitri Chicherin (LAPTH, Annecy)

**Abstract: **The Yang-Baxter equation (YBE) plays a major role in the theory of completely integrable quantum systems.

It has found numerous applications in mathematical physics. We are interested in the rational, trigonometric,

and elliptic solutions of the YBE for the rank one symmetry algebras. They involve such special functions as

the noncompact quantum dilogarithm and the elliptic gamma function. We construct the general integral

solution for the principal series representations and associate it with a lattice model of statistical mechanics

with continuous spin variables. All finite-dimensional solutions of the YBE are obtained from the integral one.

We explain the underlying algebraic structure and provide the factorized form of the solutions.

# Jeudi 17 Mars

## Title: Modeling human interactions and the dynamics of epidemic spreading

# Christian Lyngby Vestergaard (Centre de Physique Théorique, Marseille)

**Abstract: ** Respiratory infections, such as the flu, spread mainly through face-to-face contacts between individuals. Recent development of portable and cheap radio-frequency receptor/emitters has enabled time-resolved measurement of physical interactions. Measured data, typically represented by a temporal network, reveal the heterogeneous dynamics at play and can be used to improve models of social behavior and inform realistic simulations of epidemic spreading. I will present some of our recent advances along these directions. First, I present a generalized version of the Doob-Gillespie algorithm that can be used for simulation of stochastic contagion processes on temporal networks. This temporal Gillespie algorithm is stochastically exact, and up to several orders of magnitude faster than traditional methods based on rejection sampling. Second, I present a simple generative modeling framework for social interactions in a well-mixed population. It allows us to study how heterogeneous dynamics emerge as the result of different memory mechanisms at the level of individuals. We propose four individual mechanisms, which together result in generally heterogeneous network dynamics, notably of contact and inter-contact durations and frequencies of contacts per link, as observed in empirical contact networks. Our modeling framework thus enables us to study the individual effect of heterogeneities on the propagation of contagion processes. I finally discuss our current efforts to include social groupings and different physical locations, which constrain who can interact with whom, in the above model. This augmented model can be applied to investigate the validity of the assumptions of homogeneity underlying the popular metapopulation models of epidemic spreading, and to study dynamic sampling effects.

# Jeudi 10 Mars

## Title: Crossing probability for directed polymers in random media: exact results and relation to random matrices

# LPTMS di Orsay

**Abstract: ** I discuss the problem of many polymers that compete in the same random potential but are forced to avoid each other. By means of replica trick, this model is mapped into the usual Lieb-Liniger model of quantum particles but with a generalized statistics. I will introduce the nested Bethe-Ansatz method which allows for an exact solution of this quantum system. The result agrees with a previous derivation based on Macdonald process. In this way, we arrive to a general Fredholm determinant formula which can be used to study general clusters of avoiding polymers. We apply this formalism to the study of the non-crossing probability P for two polymers. We compute exactly the leading large time behavior of all its moments. From this, we extract the tail of the probability distribution of the non-crossing probability. The exact formula is compared to numerical simulations, with excellent agreement.

# Jeudi 3 Mars

## Title: **Scattering amplitudes and hidden symmetries in supersymmetric gauge theory**

**Jan Plefka (HU Berlin)**

**Abstract: ** We give a brief introduction to gauge field theory - the underlying theoretical framework of elementary particle physics - and its symmetries. Then we shall discuss supersymmetry and a holographic description of gauge fields in terms of a higher dimensional string theory known as the AdS/CFT correspondence. Finally, we focus on recent results for scattering amplitudes in supersymmetric gauge theory, their string dual description and surprising hidden symmetries pointing towards an integrable structure.

# Jeudi 18 Février

## Title: Almost black holes in flowing water

# Antonin Coutant, School of Mathematical Sciences, University of Nottingham, UK

**Abstract: ** When the velocity of flowing water surpasses the speed of surface waves, these propagate in exactly the same way as radiation near a black hole horizon. In particular, it is in theory possible to reproduce in such systems the (classical) analog of the Hawking effect. Unfortunately, it is in practice quite hard to obtain controllable flows that reach the wave velocity. It turns out that if the flow accelerates, even below the threshold velocity, there is still an imprint of the Hawking effect, which has been experimentally observed in Vancouver and in Poitiers. I will describe the nature of this imprint, as well as its spectrum for low frequency waves. In particular, I will show that the production of Hawking-like modes is governed by a new type of horizon, which is reached for complex values of the position. This “complex horizon” governs both the region of this mode production and its spectrum.

# Jeudi 4 Février 2016

## Title: Fully developed isotropic turbulence from non-perturbative renormalisation group

# Léonie Canet (LPMMC Grenoble)

**Abstract: I will present a field-theoretic approach, based on the Non-Perturbative (or Functional) Renormalisation Group (NPRG), to study the regime of fully developed isotropic and homogeneous turbulence of the Navier-Stokes equation with stochastic forcing, focusing on incompressible fluids. I will review the symmetries of the associated field theory, and point out one which was not yet identified and which leads to useful identities. I will then present the NPRG flow equations within the Leading Order approximation, and show that they lead to a fixed point, both in two and three space dimensions. I will then analyze the property of this fixed-point solution, in particular in the limit of large wave-number, show that it does not entail the usual scale invariance, and explain the mechanism within NPRG for the emergence of corrections to the dimensional scaling.**

# Jeudi 21 Janvier 2016

**High Energy ideas and phenomena in quantum simulation**

# Alessio Celi (ICFO, Castelldefels)

**Abstract: ** After reviewing idea and motivations for quantum simulation, I will introduce the simulation of synthetic (background) gauge fields with ultracold atoms as paradigmatic example. Such quantum simulators allow to study quantum Hall effects, as well as the simulation of relativistic physics and other topological systems. Of all the possible strategies devoloped in order to engineer synthetic gauge fields, I will detail the one based on "synthetic dimensions" that we introduced, and has been recently experimentally realized in two different experiments. In the last part of the talk, I will briefly comment about other branches of my program, namely, how it is possible to simulate (certain) lattice gauge theories, i.e. models in which the synthetic gauge field becomes dynamical and quantum, and artificial gravity backgrounds.

# Jeudi 7 Janvier 2016

## Title: Thermodynamics of trajectories for quantum open systems : from full-counting statistics and dynamical phase transitions to fluctuation theorems

# Simon PIGEON (Queen’s University, Belfast, UK)

**Abstract: ** The description of the dynamics resulting from the interaction of a quantum system with its environment is one of the key goals of modern quantum physics. The formal description of the evolution of an open system, especially in a quantum context, is typically tackled through master equation approach. Recently, a promising approach came to light, combining the quantum master equation and large-deviation theory. Unlike others, this approach applies to any dissipative quantum systems, paving the way to a standard description of dynamic of open quantum system in terms of thermodynamics of trajectories. From two different systems, I will explore the possibility given by this approach. Starting with a small interacting spin ring, we will see how thermodynamic of trajectories predict bistable dynamical behaviour. Next I will consider a paradigmatic system in quantum mechanics, a quantum harmonic oscillators connected to various baths. I will present how our approach, based on quantum optics methods yields an analytical expression for the large- deviation function encoding the full-counting statistics of exchange between the system and the environment. Furthermore, the same approach, generalised to any network of harmonic oscillator undergoing linear dynamics allows us to, efficiently derive numerically the behaviour of energy-exchange processes between the system in a steady state and the environment. From it we can access to possible fluctuation theorem, a key thermodynamic quantities for a large variety of open systems.

# 2015-2016

# Jeudi 17 Décembre 2015

## Title: Resonant Tunneling in a Dissipative Environment: Quantum Critical Behavior

# Harold U. Baranger (Department of Physics, Duke University, Durham, NC, USA)

**Abstract: ** The role of the surroundings, or environment, in quantum mechanics has long captivated physicists' attention. Recently, quantum phase transitions (QPT)-- a qualitative change in the ground state as a function of a parameter-- have been shown to occur in systems coupled to a dissipative environment. Despite the ubiquity of QPTs in contemporary theoretical physics, obtaining clear experimental signatures has been challenging. I start by presenting a recent experiment in which it was possible to thoroughly characterize a QPT caused by coupling to an environment. The system is a single-molecule transistor built from a carbon nanotube quantum dot connected to strongly dissipative contacts. The electrical conductance of this system is highly singular as T tends to 0: the conductance is 0 except at one special point (on resonance and symmetric coupling) at which electrons are fully transmitted with unit probability. I then turn to the theoretical understanding of this QPT obtained by mapping the problem onto that of a resonant Majorana fermion level in an interacting electron liquid. The unitary transmission obtained in the experiment is seen as a competition between the two leads. The deviations from unitarity at nonzero temperature are connected to residual interactions among the Majoranas; in this way, the experiment observes a signature of Majorana critical behavior.

# Jeudi 10 Décembre 2015

## Title: A kagome map of spin liquids

# Ludovic Jaubert (Okinawa Institute of Technology)

**Abstract: ** Despite its deceptive simplicity, few concepts have more fundamental implications than chirality. In magnetic materials, spin chirality gives rise to unconventional phenomena such as the anomalous Hall effect and multiferroicity, taking an enhanced flavour in the so-called spin-liquid phases where magnetic disorder prevails. The kagome lattice sits at the crossroad of these ideas. Here we shall bring together a network of kagome spin liquids with anisotropic and Dzyaloshinskii-Moriya interactions. This network revolves around the Ising antiferromagnet and ends on (ferromagnetic) chiral spin liquids with spontaneously broken time-reversal symmetry. As for the celebrated Heisenberg antiferromagnet, it now belongs to a triad of equivalently disordered phases. The present work provides a unifying theory of kagome spin liquids with time-reversal nearest-neighbour Hamiltonians, and promising applications for rare-earth based kagome materials and optical-lattice experiments. work done in collaboration with Karim Essafi & Owen Benton from OIST, Japan

# Jeudi 3 Décembre 2015

## Title: Quantum spacetime and topological quantum field theories with defects

# Marc Geiler (Perimeter Institute, Canada)

**Abstract: ** After introducing and reviewing some recent developments in discrete non-perturbative approaches to quantum gravity, I will present the construction of new vacua and representations for the quantum algebra of observables of canonical gravity. This will highlight the unsuspected richness of this class of theories, and make more transparent their interpretation as topological quantum field theories with defects. I will then explain the relevance of this construction for the study of the continuum limit via coarse-graining and renormalization.

# Jeudi 26 Novembre 2015

## Title: The eta-deformation of the AdS_5 x S^5 superstring and supergravity.

# Ben HOARE (Institut f. Theoretische Physik, ETH Zürich SWITZERLAND)

**Abstract: ** In this talk we will discuss recent progress in understanding the extent to which the eta-deformed AdS_5 x S^5 background is a solution of Type IIB supergravity. Observing that the background itself is not a solution, but its 6-fold T-dual is, we will construct the corresponding deformed supergravity equations, of which the eta-deformed background is a solution.

# Jeudi 19 Novembre

## Title: Matrix product approximations to conformal field theories

# Volkher B. Scholz

**Abstract: ** We establish rigorous error bounds for approximating correlation functions of conformal field theories (CFTs) by certain finite-dimensional tensor networks. For chiral CFTs, the approximation takes the form of a matrix product state. For full CFTs consisting of a chiral and an anti-chiral part, the approximation is given by a finitely correlated state. We show that the bond dimension scales polynomially in the inverse of the approximation error and sub-exponentially in the ultraviolett cutoff. We illustrate our findings using Wess-Zumino-Witten models, and show that there is a one-to-one correspondence between group-covariant MPS and our approximation. (joint work with Robert Koenig (TUM), see arXiv:1509.07414)

# Jeudi 12 Novembre

## Title: Wannier functions for crystalline solids, obstruction theory, and the Z_2-invariants of topological insulators

# Gianluca PANATI (Rome)

**Abstract: ** The localization of electrons in crystalline solids is often expressed in terms of the Wannier functions, which provide an orthonormal basis of L2(Rd) canonically associated to a given periodic Schrödinger operator. The existence of exponentially localized (composite) Wannier functions might be, a priori, topologically obstructed, in view of the possible competition between regularity and periodicity of the corresponding (quasi-) Bloch functions. In a previous work (2007), we proved that the obstruction to the existence of exponentially localized Wannier functions is given exactly by the Hall conductance of the system, provided d<=3. On the other hand, for time-reversal (TR) symmetric systems such obstruction vanishes. Thus one may ask a finer question, and investigate the existence of frames of Bloch functions which are simultaneously smooth, periodic and TR-symmetric. The answer to this question depends on the fact that the TR operator is even or odd. In the latter case, an intriguing relation with the Z_2-invariants of TR-symmetric topological insulators appears.

# Jeudi 5 Novembre 2015

## Title: Quantum Algebras Based on Extensions of psl(2|2)

# Niklas Beisert

**Abstract: ** TBA

# Jeudi 15 Octobre 2015

## Title: Yang-Baxter deformations in AdS/CFT and flat space

# Stijn VAN TONGEREN (Humboldt-Universitaet zu Berlin, Allemagne)

**Abstract: ** The string on AdS5xS5 can be deformed in many ways while preserving its integrability, using the framework of Yang-Baxter deformations. Time permitting, my talk will consist of two parts. Firstly, starting from the quantum deformation of the string on AdS5xS5, by a contraction limit I will discuss the quantum deformation of strings on flat space. This contraction limit is actually identical to the so-called ``mirror limit''. This give a cute perspective on the string on AdS5xS5, as a worldsheet double Wick rotation of the quantum deformation of the simplest possible string. Secondly, I will discuss homogeneous deformations as Drinfeld twisted models, and show how to interpret (many of) them in terms of AdS/CFT.

# Jeudi 8 Octobre 2015

## Title: Theory of the many-body delocalization transition

# Romain Vasseur (Berkeley)

**Abstract: ** In this talk, I will review the physics of many-body localized systems. As disorder is weakened below a critical value, these non-thermal quantum glasses melt via a continuous dynamical phase transition into a high temperature, ergodic liquid. In contrast to classical phase transitions between two different non-zero temperature phases of matter, and to quantum phase transitions between zero temperature phases, this dynamical delocalization transition represents an entirely new type of critical point in which statistical mechanics and thermalization break down sharply at a continuous phase transition. I will describe an effective model for such quantum-to-classical transitions and use it to compute their universal critical properties.

# Jeudi 1 Octobre 2015

## Title: Interfaces in correlated Random Media: what we learn from the Directed Polymer

# Vivien Leconte (Laboratoire Probabilités et Modèles Aléatoires, Université Paris VII Denis Diderot)

**Abstract: ** One-dimensional boundary interfaces between different phases are described at macroscopic scales by a rough fluctuating line, whose geometrical properties are dictated by the disorder in the underlying medium, by the temperature of the environment, and by the elastic properties of the line. A widely used and successful model is the directed polymer in a random medium, pertaining to the Kardar-Parisi-Zhang (KPZ) universality class. Much is known for this continuous model when the disorder is uncorrelated, and it has allowed to understand the static and dynamical features of experimental systems ranging from magnetic interfaces to liquid crystals. We show that short-range correlations in the disorder at a scale ξ > 0 modify the uncorrelated (i.e. zero ξ) picture in a non-obvious way. If the geometrical fluctuations are still described by the celebrated 2/3 KPZ exponent, characteristic amplitudes are however modified even at scales much larger than ξ, in a well-controlled and rather universal manner. Our results are also relevant to describe the slow (so called `creep') motion of interfaces in random media, and more formally (trough replicae) one-dimensional gases of bosons interacting with softened delta potential. We also discuss results obtained in the same spirit for the depinning force of interfaces with long-range elasticity.

# Jeudi 24 Septembre 2015

## Title: Order by disorder in the antiferromagnetic Ising pyrochlore

# Pamela C. Guruciaga (Université Pierre et Marie Curie)

**Abstract: ** In the pyrochlore lattice, Ising-like spins occupy the vertices of corner-sharing tetrahedra, pointing along the local <111> directions. Via the dumbbell model, this spin configuration can be mapped into a system of non-conserved magnetic charges ("magnetic monopoles"), with four types of charges: positive or negative, single or double. In this way, the rotation of a magnetic moment is equivalent to the creation, annihilation or translation of a monopole in a discrete lattice. This mapping is commonly used in the context of the frustrated magnetic materials known as spin ices; in this case, we will address their antiferromagnetic counterpart and consider only nearest-neighbour interactions. Due to the system's geometry, a magnetic field applied along the [110] direction of the pyrochlore lattice couples to two spins per tetrahedron, but does not affect the other two which are perfectly orthogonal. In this situation, there is a range of field intensity in which the ground state consists of single monopoles, with double monopoles playing the role of the lowest excitations. Although the system is charge-disordered at T=0, a single monopole crystal is found at finite (low) temperature --a phenomenon known as order by disorder. We use Wang-Landau algorithm to find the density of states of the system and show that it is led to order by the excitations. Also, we perform Monte Carlo simulations with Metropolis algorithm to characterise other transitions present.

# Jeudi 17 Septembre 2015

## Title: On the integrability of strings on symmetric spaces

# Linus Wulff (Blackett Laboratory, Imperial College, Londres)

**Abstract: ** I will describe the structure of string actions on symmetric spaces and the form of the corresponding (super)isometry algebras. For vanishing NSNS three-from flux a general proof of the classical integrability of the superstring can be found. For the case of non-vanishing NSNS flux new supercoset models corresponding to strings on AdS(2,3)xS(2,3)xS(2,3)xT(2,3,4) are constructed.

# Jeudi 10 Septembre

## Title: Pure connection formulation of Twistor theory

# Yannick Herfray (Nottingham)

**Abstract: ** In the last decade, a lot of improvement have been made in the understanding of transition amplitudes in General Relativity. In some sense, GR is much simpler on shell than was expected. However it is still an open question as to whether or not there exist an off-shell formulation of GR that would make this simplicity manifest. A good candidat for this could be a twistor action. However, even if such an action exists for conformal gravity, it is still missing for full GR. In this talk, I will present a possible strategy to achieve it by using ideas coming from the pure connection formulation of GR. This is however still a work in progress that I hope to continue in the next years of my PhD between Lyon and Nottingham. I will also review some of the basics of twistor theory, and what we could expect from such a twistor action, mainly a MHV formalism.

# 2014-2015

# Jeudi 11 Juin 2015

## Title: On the subtle coexistence of charge and magnetism, and the exotic animals they can give birth

**Pierre Pujol (Laboratoire de Physique Théorique, Université Paul Sabatier)**

**Abstract: ** In this talk we are going to overview some microscopic models that give rise to coexisting charge and spin degrees of freedom. In these strongly correlated models, quantum effects play an important role, in particular for the existence of some non-trivial states of matter. We are going to present a particular field theory technique to understand many subtleties of the interplay between these two kind of degrees of freedom as well as for investigating the existence of quite counterintuitive phases.

# Jeudi 4 Juin 2015

## Title: Unraveling the nature of carrier mediated ferromagnetism in diluted magnetic semiconductors

**Georges BOUZERAR (Institut Lumière Matière, Univ. Lyon 1)**

**Abstract: ** After more than a decade of intensive research in the field of diluted magnetic semiconductors (DMS), the nature and origin of ferromagnetism, especially in III-V compounds is still controversial. Many questions and open issues are under intensive debates. Why among the broad family of III-V materials, and for a given concentration of transition metal (TM) impurities, Mn doped GaAs still exhibits the highest critical temperature? How can one understand that these temperatures are almost two orders of magnitude larger than that of hole doped (Zn,Mn)Te or (Cd,Mn)Se? Is there any intrinsic limitation or is there any hope to reach in the dilute regime room temperature ferromagnetism? How can one explain the proximity of (Ga,Mn)As to the metal insulator transition and the change from RKKY couplings in II-VI compounds to double exchange type in (Ga,Mn)N? In spite of the great success of density functional theory based studies to provide accurately the critical temperatures in various compounds, till very lately a theory that provides a coherent picture and understanding of the underlying physics was still missing. Recently, within a minimal model it has been possible to show that among the physical parameters, the key one is the position of the TM acceptor level. By tuning the value of that parameter, one is able to explain both magnetic and transport properties quantitatively in a broad family of DMS. We will see that this minimal model explains in particular the RKKY nature of the exchange in (Zn,Mn)Te/(Cd,Mn)Te and the double exchange type in (Ga,Mn)N and simultaneously the reason why (Ga,Mn)As exhibits the highest critical temperature among both II-VI and III-V DMS's.

# Jeudi 28 Mai 2015

## Title: Why are there so many interpretations of quantum mechanics?

**Pierre Hohenberg (New York University)**

**Abstract: ** Quantum mechanics is unique among physical theories in that 90 years after its introduction and general acceptance as being correct and complete, its 'interpretation' remains a subject of controversy. Unlike classical mechanics, what quantum mechanics still lacks is a clear microscopic formulation, whereby the theory is defined for a closed system S of any size in terms of concepts relating only to S itself. Such a formulation, called Compatible Quantum Theory, is presented and shown to account for and clarify the standard quantum phenomena and paradoxes. The question of physical implementation, on the other hand, requires a macroscopic theory, to account for state preparation and the measurement of system properties. It is primarily in different versions of such macroscopic implementation mechanisms that most interpretations of quantum mechanics differ.

# Jeudi 30 Avril 2015

## Title: Le conditionnement comme ''meilleure'' façon pour rendre typique un évènement rare

**Raphael Chetrite (Laboratoire J.A. Dieudonne, Univ. Nice)**

**Abstract: ** In my talk, I will present works done with Hugo Touchette. In these works, we consider the problem of conditioning a Markov process on a rare event and of representing this conditioned process by a conditioning-free process. This conditioning-free process may be seen as the process that is the closest to the initial one in the class of Markov processes that make typical the initial rare event. Our approach generalises many previous results in the mathematical literature on the spectral characterisation of positive operators, but also maximum entropy principles scattered in the physics literature.

# Jeudi 23 Avril 2015

## Title: Spin fluctuations and fragmentation of a spin 1 Bose-Einstein condensate

**Fabrice GERBIER (Laboratoire Kastler Brossel, ENS Paris, et Collège de France) **

**Abstract: ** I will discuss the magnetic properties of a Sodium Bose-Einstein condensate in a tight trap. This system can be described as a mesoscopic ensemble of a few thousands spin 1 bosons with antiferromagnetic exchange interactions. Anomalous spin fluctuations are observed for small magnetic fields and vanishing magnetizations. These fluctuations are characteristic of a condensate displaying spin fragmentation, i.e. condensation occurs in several single-particle spin state instead of just one for a "regular" condensate. This is a mesoscopic effect, which reflects the restoration of the spin rotational symmetry, explicitely broken by external fields, by spin exchange interactions. In our experiment, these spin fluctuations can be characterized by a quasi-thermal ensemble at a spin temperature Ts. For small magnetic fields, we find that Ts is well below the "kinetic" temperature Tk of the uncondensed gas surrounding the condensate. When increasing the magnetic field, Ts converges towards Tk. I will discuss how this behavior can be explained with a picture where the condensate spin acts as an isolated quantum system, which is able through spin-changing collisions to reach "on its own" a pseudo-equilibrium state.

# Jeudi 09 Avril 2015

## Title: The best quantum thermoelectric at finite power output

**Robert Whitney (LPMMC, Université Grenoble) **

**Abstract: ** Carnot efficiency is only achievable at zero power output. We ask what is the maximum efficiency at some given finite power output. It appears that this question is ill-defined in classical thermodynamics, but can be answered with a quantum theory. We use the Landauer-Buttiker scattering theory to find this maximum efficiency for heat engines and refrigerators made of thermoelectric quantum systems. We initially find the exact maximum efficiency for two-terminal systems without energy relaxation [1]. We then use phenomenological models to explore whether this maximum can be exceeded by two-terminal systems with relax- ation [2], or by three-terminal systems. We have not yet found a system which can exceed the maximum efficiency given in Ref. [1], although open questions remain.

# Jeudi 02 Avril 2015

## Title: Dynamics at a Quantum Critical Point: Combining Quantum Monte Carlo and Holography

**Erik SORENSEN (McMaster University & Université de Toulouse III) **

**Abstract: ** The real time dynamics near quantum critical points have proven very challenging to obtain both from a numerical and analytical perspective. Here we focus on the superfluid-insulator transition occurring for bosons on a lattice. New large-scale QMC results have made it possible to obtain very precise results for many quantities in particular the frequency dependent conductivity at imaginary frequencies. Since the numerical results remain confined to imaginary times/frequencies additional tools are needed to extend the results to the rest of the complex plane. Here, recent insights from conformal field theory and holography have yielded a wealth of information that combined with the QMC results yield quantitative and experimentally testable results for the frequency-dependent conductivity near the quantum critical point.

# Jeudi 19 Mars 2015

## Title: L'invariance d'échelle implique l'invariance conforme pour les modèles O(N) en trois dimensions

**Bertrand Delamotte (LPTMC, Paris 6) **

**Abstract: ** Nous montrerons grâce au groupe de renormalisation à la Wilson (aussi appelé "exact") que les modèles critiques tri-dimensionnels ne sont pas seulement invariants d'échelle mais sont invariants sous tout le groupe conforme. Après avoir fait une introduction générale au groupe de renormalisation Wilsonien, nous présenterons la "preuve" sur l'exemple des modèles O(N).

# Jeudi 12 Mars 2015

## Title: Robust quantum coherence above the Fermi sea

**Patrice Roche (CEA/SPEC) **

**Abstract: ** A new type of quantum device, relying on the one dimensional edge states of the Quantum Hall regime, where electrons mimic the photon trajectory of a laser beam, has opened a route towards electron quantum optics and manipulation of single electron excitations. Pauli statistics and interactions provide new ingredients for the physics of the electrons which are not relevant for photons. For example, when electrons are injected above the Fermi sea, it is fundamental to understand how their phase coherence will be affected by the injection energy. We explore this issue by first using a quantum dot to inject the carriers at a controllable energy into an edge state. Then an electronic Mach-Zehnder interferometer is used to monitor the quantum coherence of the electronic quasiparticle. We find that above a certain threshold the coherence is energy-independent; it is even preserved at energies fifty times larger than the electronic temperature. This is remarkable, since from simple considerations based on Fermi's golden rule, one would expect that the relaxation rate increases with the injection energy, thus reducing quantum coherence. Indeed, our simulations using recent theories predict a continuous trend of increasing relaxation. While the origin of this coherence robustness remain unidentified, it has a significant bearing for the implementation of quantum information encoded in electron trajectories. (S. Tewari, P. Roulleau, C. Grenier, F. Portier, A. Cavanna, U. Gennser, D. Mailly, and P. Roche)

# Jeudi 26 Février 2015

## Title: From Aztec diamonds to pyramids: steep tilings

**Jérémie Bouttier (IPhT, CEA)**

**Abstract: **We consider random tilings made of dominos (2x1 rectangles), and describe a general family which encompasses several known models: domino tilings of the Aztec diamond (giving rise to the celebrated "arctic circle phenomenon"), pyramid partitions, plane overpartitions... These tilings are in one-to-one correspondence with sequences of Young diagrams where, at each step, one adds or removes an horizontal or a vertical strip. Using an algebraic framework related to the boson-fermion correspondence, we compute the partition function and all correlations of the model (i.e. the probabilities of finding a given number of dominos at given positions). We furthermore provide an algorithm for the efficient generation of such random tilings. Based on joint work with Guillaume Chapuy, Sylvie Corteel and later on with Dan Betea, Cédric Boutillier, Sanjay Ramassamy and Mirjana Vuletić.

# Jeudi 13 Fevrier 2015

## Title: Electronic Correlations and Multiorbital Effects in Iron-Based Superconductors

**Rong Yu (Department of Physics, Renmin University of China)**

**Abstract: ** Identifying the role of electron correlations in iron-based superconductors is crucial in understanding the superconductivity and related normal-state properties in these systems. To this end, we study the metal-to-Mott-insulator transitions in multiorbital Hubbard models for several parent compounds of iron-based superconductors using the slave-spin mean-field method. We show that a crossover from a weakly coupled metal to a strongly coupled metal generally exists in all these models when the Hund's coupling is beyond a threshold. In the strongly coupled metallic phase, the quasiparticle spectral weights are substantially reduced from the non-interacting limit and become strongly orbital dependent. Particularly for alkaline iron selenides, we find a novel orbital-selective Mott phase, in which the Fe 3d xy orbital is Mott localized while the other Fe 3d orbitals remains itinerant. This phase is still stabilized over a range of carrier dopings, and has unique experimental signatures. We further investigate the effects of electron correlations on superconductivity. We have derived the effective exchange coupling between quasi-localized moments in the bad metal regime. This allows us to study the superconducting pairing via an effective multiorbital t-J1-J2 model. We show that the orbital dependent correlation effect results in a rich pairing phase diagram. In a certain parameter regime, it naturally gives rise to orbital selective pairing, which leads to anisotropic superconducting gaps along the electron Fermi pockets and splitting of spin resonance peak in the superconducting state.

# Jeudi 12 Février 2015

## Title: Super-symmetric spin-chains, percolation, and non-rational CFTs at c=0

**Azat Gainutdinov (Department of Mathematics Hamburg University and DESY, Germany)**

**Abstract: **I will discuss algebraic properties of periodic sl(n+1|n) spin-chains with Heisenberg-like interaction. These chains are made of alternating tensor products of the fundamental and conjugate sl(n+1|n) representations. The algebra of local Hamiltonian densities in the chain is provided by a representation of the affine or periodic Temperley-Lieb algebra at the primitive 6th root of unity. The more detailed analysis was carried out for periodic sl(2|1) spin chains (with H. Saleur, N. Read and R. Vasseur), which describe statistical properties of boundaries of 2D percolation clusters on a torus. In this case, the continuum limit of the chains was identified with a bulk Logarithmic CFT at c = 0, which is a fixed point theory of a non-linear sigma model on the complex projective superspace CP^{1|1} in the strong coupling regime. We deduced the structure of the space of states as a representation over the product of left and right Virasoro algebras. Our main result is the explicit structure of the full vacuum module/sector of the LCFT, which exhibits Jordan cells of arbitrary rank for the Hamiltonian or the dilatation operator.

# Mercredi 11 Février 2015

## Title: IIB Supergravity and the E_6(6) covariant vector tensor hierarchy

**Bernard de Wit (NIKHEF, Amsterdam) **

**Abstract: ** TBA

# Jeudi 5 Février 2015

## Title: 4d Quantum Gravity with a Cosmological Constant from SL2C Chern-Simons Theory

**Aldo Riello (Perimeter Institute, Ontario, Canada) **

**Abstract: ** In this seminar, I will discuss the first steps towards a definition of a model for simplicial 4d quantum gravity with a cosmological constant, via 3d Chern-Simons theory with defects. The proposal hinges on a "reconstruction theorem" assessing the correspondence between a class of flat connections on a S3 graph complement (related to the 4-simplex skeleton) and the geometries of constant-curvature Lorentzian 4-simplices. The main result consists in showing that in the semiclassical (WKB) approximation the Regge action of simplicial general relativity correctly appears. Time allowing, I will also discuss the relation of this construction with spinfoam models for loop quantum gravity.

# Mercredi 4 Février 2015

## Title: Fluctuations of the current and optimal profiles in the open Asymmetric Simple Exclusion Process

**Alexandre Lazarescu (Instituut voos Theoretische Fysica, KU Leuven, Belgium) **

**Abstract: ** The asymmetric simple exclusion process (ASEP), where particles perform biased random walks with hard core repulsion, is one of the most studied model in non-equilibrium statistical physics. It has the mathematical property of being integrable, which makes it a good candidate for in-depth exact calculations. The quantity of particular interest there is the current of particles that flows through the system due to the bias of the jumps. In this presentation, we will see how we can obtain information about the distribution of that current, through various techniques: integrability, macroscopic fluctuation theory, and asymptotic direct diagonalisation. This allows us to build the phase diagram for the large deviations of the current, and examine the corresponding density profiles in each of its five phases. We show that two situations arise: in most phases, the system can be described hydrodynamically, but in one phase, where the current is larger than the limit set by hydrodynamics, the system becomes highly correlated. If time allows it, we will also see how these techniques and results could be generalised to some other observables or models.

# Jeudi 29 janvier 2015

## Title: Realization of strongly interacting topological phases on lattices

**Antoine Sterdyniak (Institüt für Theorestische Physik, Innsbruck Universität) **

**Abstract: ** While fractional quantum Hall effect (FQHE) was realized experimentally thirty years ago in semiconductor heterostructures, strongly interacting chiral topological phases are still at the center of an important research effort, both as they serve as building blocks of more exotic phases such as fractional topological insulators and as a realization outside of semi-conductor physics is still missing. In this talk, I will describe realizations of these phases in cold atoms gases and in frustrated spins systems. I will first introduce optical flux lattices, which are continuous models that exhibit topological flat bands with a tunable Chern number and host fractional states beyond the FQHE. Then, I will focus on chiral spin liquids whose emergence on the kagomé lattice using a local Hamiltonian has been shown very recently. Unlike itinerant particle systems where FQHE can be understood as a consequence of interactions in a partially filled topological band, I will show that such a picture does not hold for this chiral spin liquid.

# mercredi 28 janvier 2015

## Title: Towards the Turaev-Viro amplitudes from a Hamiltonian constraint.

**Maité Dupuis (University of Waterloo, Ontario, Canada) **

**Abstract: ** I will show how the usual Loop Quantum Gravity phase space can be deformed to characterize hyperbolic discrete geometries and thus be a candidate to describe the discretization of SU(2) BF theory with a (negative) cosmological constant. The quantization of this model will then give at the kinematical level, an Hilbert space spanned by spin networks built on Uq(su(2)) (with q real). I will also build an Hamiltonian constraint and show that the Turaev-Viro amplitude with q real is a solution of the quantum Hamiltonian. This model is therefore a natural candidate to describe 3D loop quantum gravity with a (negative) cosmological constant.

# Jeudi 15 janvier 2015

## Title: Integrability for AdS3/CFT2

**Alessandro Sfondrini (Institut für Mathematik und Institut für Physik, Humboldt-Universität zu Berlin) **

**Abstract: ** Gravity theories with negative cosmological constant in three dimensions (such as AdS3) play an important role in the understanding of black hole physics, and provided an early example of holography. Their dual 2-dimensional conformal field theories (CFT2) are quite special, since they enjoy (suitable super-symmetric extensions of) Virasoro symmetry. This duality naturally emerges in string theory too, for instance as the near horizon limit of a system of D1/F1-strings and D5/NS5-branes and was much studied in the early days of the Maldacena correspondence. Recently, the interest in Ad3/CFT2 was revived when Babichenko, Stefanski and Zarembo showed that the maximally super-symmetric AdS3 backgrounds yield classically integrable string non-linear sigma models. It is natural to ask whether the worldsheet S-matrix and spin-chain integrability approaches, which work beautifully for the planar limit of AdS5/CFT4, can be applied here as well. The answer did not appear to be straightforward, due to several new features and some conceptual complications of AdS3/CFT2, and indeed eluded us for four years. In my talk I will provide substantial evidence for an affirmative answer. To do this, I will discuss in detail the simplest case of superstrings on AdS3xS3xT4 and describe the exciting future directions for this integrability program.

# Mardi 6 janvier 2015

## Title: A New Type of Quantum Criticality in the Pyrochlore Iridates

**Lucile Savary (MIT)**

**Abstract:** Magnetic fluctuations and electrons couple in intriguing ways in the vicinity of zero-temperature phase transitions—quantum critical points—in conducting materials. Quantum criticality is implicated in non-Fermi liquid behavior of diverse materials and in the formation of unconventional superconductors. Here, we uncover an entirely new type of quantum critical point describing the onset of antiferromagnetism in a nodal semimetal engendered by the combination of strong spin-orbit coupling and electron correlations, and which is predicted to occur in the iridium oxide pyrochlores. We formulate and solve a field theory for this quantum critical point by renormalization group techniques and show that electrons and antiferromagnetic fluctuations are strongly coupled and that both these excitations are modified in an essential way. This quantum critical point has many novel features, including strong emergent spatial anisotropy, a vital role for Coulomb interactions, and highly unconventional critical exponents. Our theory motivates and informs experiments on pyrochlore iridates and constitutes a singular realistic example of a nontrivial quantum critical point with gapless fermions in three dimensions.

# 2013-2014

# Jeudi 27 novembre 2014

## Title: Light-cone effect and supersonic correlations in one- and two-dimensional bosonic superfluids

**Giuseppe Carleo (Laboratoire Charles Fabry - Institut d'Optique, Paris, France)**

**Abstract:** In this talk I will present some recent results on the out-of-equilibrium dynamics of interacting lattice bosons [1]. In particular, we study how (and how fast) correlations can spread in a quantum system abruptly driven out of equilibrium by a quantum quench. This protocol can be experimentally realized with ultra-cold atoms, which allow to address fundamental questions concerning the quasi-locality principle in isolated quantum systems [2, 3]. We focus on the spreading of density-density correlations in Bose-Hubbard models after a quench of the interaction strength, using time-dependent variational Monte Carlo simulations [4]. This method gives access to unprecedented long propagation times and to dimensions higher than one. In both one and two dimensions, we demonstrate the existence of a "light-cone", characterized by the ballistic spreading of correlations with a finite propagation time. We extract accurate values of the correlation-cone velocity in the superfluid regime and show that the spreading of correlations is generally supersonic. Further, we show that in two dimensions the correlation spreading is highly anisotropic and presents nontrivial interference effects. [1] G. Carleo, F. Becca, L. Sanchez-Palencia, S. Sorella, and M. Fabrizio, Phys. Rev. A 89, 031602(R) (2014). [2] M. Cheneau et al., Nature 481, 484 (2012). [3] T. Langen et al., Nat. Phys. 9, 640 (2013). [4] G. Carleo, F. Becca, M. Schiro, and M. Fabrizio, (Nature) Sci. Rep. 2, 243 (2012).

# Jeudi 20 novembre 2014

## Title: Probing the $\nu=2/3$ fractional quantum Hall edge by momentum-resolved tunneling

**Hendrik Meier (Department of Physics, Yale University, USA)**

**Abstract: ** The nature of the fractional quantum Hall state with filling factor ?=2/3 and its edge modes continues to remain an open problem in low-dimensional condensed matter physics. In this talk, I am going to present and discuss a suggested experimental setting to probe the?=2/3 edge by tunnel-coupling it to a ?=1 integer quantum Hall edge in another layer of a two-dimensional electron gas (2DEG). In this double-layer geometry, the momentum of tunneling electrons may be boosted by an auxiliary magnetic field parallel to the two planes of 2DEGs. The current is calculated as a function of bias voltage and the boosting magnetic field. Its threshold behavior yields information about the spectral function of the ?=2/3 edge, in particular about the nature of the chiral edge modes. The theory accounts also for the effects of Coulomb interaction and disorder. Hendrik Meier, Yuval Gefen, Leonid I. Glazman, Phys. Rev. B 90, 081101(R) (2014); preprint: arXiv:1406.4517

# Jeudi 13 novembre 2014

## Title: On exact spectrum of planar N=4 super-Yang-Mills theory

**Vladimir Kazakov (LPT-ENS Paris & Université Paris-VI Jussieu, France)**

**Abstract: ** N=4 Super-Yang Mills in planar limit is the only exactly solvable theory in 4 space-time dimensions. This potentially gives a possibility to compute any physically interesting quantities, such as anomalous dimensions, correlators, Wilson loops, scattering amplitudes, at any strength of the 't Hooft coupling. Solvability is possible due to AdS/CFT duality and a hidden integrability property. Due to the efforts of many researchers over the last dozen of years, the exact equations for the spectrum of anomalous dimensions have been discovered, known as the AdS/CFT Y-system/TBA, are developed We present a new, the most concise and efficient, Riemann-Hilbert system of spectral equations -- Qantum Spectral Curve (QSC). We will review the origins of this approach and present the basic structure of QSC. We will also expose the most important results of computations of anomalous dimensions: Konishi dimension at any coupling (numerically), its strong coupling expansion and weak coupling expansion (9-loops!), as well as the Balitsky-Fadin-Kuraev-Lipatov limit of QSC reproducing the BFKL pomeron spectrum.

# Jeudi 06 novembre 2014

## Title: Stückelberg interferometry with a pair of Dirac cones

**Jean-Noël Fuchs (LPTMC Jussieu & LPS Orsay, France)**

**Abstract: ** Dirac cones are linear band contacts in crystals that are also characterized by a quantized Berry phase. The prime example of a crystal featuring a pair of Dirac cones is graphene (honeycomb lattice). Recently, artificial and tunable analogs of graphene were realized experimentally (e.g. with cold atoms). When deforming the honeycomb lattice, it is possible to manipulate the Dirac cones up to their merging and annihilation. The energy spectrum across the merging transition can be detected via Bloch oscillations and Landau-Zener tunneling as recently shown by Tarruell et al. This technique is not restricted to studying the energy spectrum and can give access to band coupling effects (Berry phases). The idea is to use a pair of Dirac cones to realize a Stückelberg interferometer. We will show that this type of interferometer contains information on band coupling in the form of an open-path (but gauge-invariant) Berry phase.

# Jeudi 30 octobre 2014

## Title: A New Broken Symmetry: Hidden (Hastatic) Order in URu2Si2

**Premela Chandra (Department of Physics, Rutgers University, USA)**

**Abstract:** The development of collective long-range order by means of phase transitions occurs by the spontaneous breaking of fundamental symmetries. Magnetism is a consequence of broken time-reversal symmetry, whereas superfluidity results from broken gauge invariance. The broken symmetry that develops below 17.5 kelvin in the heavy-fermion compound URu2Si2 has long eluded such identification. Here we show that the recent observations of Ising quasiparticles in URu2Si2 results from a spinor order parameter that breaks double time-reversal invariance, mixing states of integer and half-integer spin. Such "hastatic" order hybridizes uranium-atom conduction electrons with Ising 5f2 states to produce Ising quasiparticles; it accounts for the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. Hastatic order predicts a tiny transverse moment in the conduction-electron sea, a collosal Ising anisotropy in the nonlinear susceptibility anomaly and a resonant, energy-depedent nematicity in the tunnelling density of states. We also discuss the microscopic origin of hastatic order, identifying it as a fractionalization of three-body body bound-states into integer spin fermions and half-integer spin bosons. Work done with Piers Coleman and Rebecca Flint. References: PC, P. Coleman and R. Flint Nature 493, 421 (2013)arXiV: 1404.5920

# Jeudi 9 octobre 2014

## Title: 3D random tensor models

**Adrian Tanasa (Institut Galilée, Université Paris 13, France)**

**Abstract:** Random tensor models, seen as field theoretical models, are related on one side to group field theory, a recent candidate for quantum gravity, and on the other side they represent a natural generalization of the celebrated matrix models. These matrix models are also known to be connected to 2D statistical physics or to quantum gravity; one of the main results of their study is that their perturbative series can be reorganized in powers of 1/N (N being the matrix size). The leading order in this expansion is given by planar graphs (which are dual to triangulations of the 2-dimensional sphere S^2). In this talk I will present such a 1/N asymptotic expansion for some particular class of 3-dimensional random tensor models (called multi-orientable models). The leading order (and hence the dominant graphs, dual to particular triangulations of the three-dimensional sphere S^3), the next-to-leading order and finally some results on the combinatorics of the general term of this asymptotic expansion will be given.

# Jeudi 25 septembre 2014

## Title: Tensor Models and Renormalization

**Joseph BenGeloun (Max-Planck Institute, Potsdam, Allemagne)**

**Abstract:** A review will be provided on the renormalization program for the so-called Tensor Models for Quantum Gravity. These are non local field theories extending both the matrix models, a successful framework in statistical mechanics applied to 2D physics, and the Grosse-Wulkenhaar model in the matrix basis arising in Noncommuting Neometry. We will emphasize the Multi-scale renormalization but also report recent results on the Functional Renormalization Group Approach for these class of models.

# Jeudi 18 septembre 2014 (Salle 115)

## Title: Integrable Deformations of Strings

**Timothy J. Hollowood (Department of Physics, Swansea University, UK)**

**Abstract:** TBA

# Jeudi 17 juillet 2014

## Title: Random Quantum Spin Chains with Long-Range Interactions

**Stephan HAAS (Department of Physics & Astronomy, University of Southern California, Los Angeles)**

**Abstract: ** A real-space renormalization group technique is used to study the anisotropic Heisenberg model with long-range interactions, decaying with a power $\alpha$, which are generated by placing spin sites at random positions along the chain. This method permits a large-scale finite-size analysis of systems with up to 256 spins, along with a disorder average over up to 300,000 realizations. Analyzing the distribution of the first excitation energy from the ground state, we find a critical power-law decay where the distribution function is critical, separating an insulator phase at larger $\alpha$ where the distribution is Poissonian, and a metallic state at smaller $\alpha$, where it follows the Wigner surmise.

# Vendredi 11 juillet 2014

## Title: Exploring problems of nonlinear dynamics with Bose-Einstein condensates

**Tristram ALEXANDER (School of Physical, Environmental and Mathematical Sciences, UNSW Canberra)**

**Abstract: ** New frontiers in research concerning Bose-Einstein condensates continue to emerge, however in many ways some of the most intriguing problems are old. What are the effects of the interplay of nonlinearity and constraint? What nonlinear modes exist in a given system? What sort of excitations appear in the non-equilibrium dynamics? In this talk I will seek to answer these questions in the context of Bose-Einstein condensates in the presence of external potentials such as an optical lattice. I will discuss some of the localised states which may emerge and their possible excitations and I will look at parallels between confined BECs and familiar mechanical systems such as chains of oscillators. I will also try to highlight some of the outstanding problems in this area.

# Jeudi 3 juillet 2014

## Title: A Shortcut to Scattering Amplitudes in N=4 Super Yang-Mills via Integrability.

**Matthias STAUDACHER (Institut für Mathematik und Institut für Physik, Humboldt-Universität zu Berlin)**

**Abstract: ** We combine recent applications of the two-dimensional quantum inverse scattering method to the scattering amplitude problem in four-dimensional N = 4 Super Yang-Mills theory. Integrability allows us to obtain a general, explicit method for the derivation of the Yangian invariants relevant for scattering amplitudes in the N = 4 model. There is a beautiful connection to contour integrals defined on Grassmannian manifolds.

# Jeudi 5 juin 2014

## Title: The Physical Review: Editorial and Review Process

**Robert WIMMER (Assistant Editor, Physical Review D)**

**Abstract: ** I discuss some aspects of the physics journals of the American Physical Society and their review process. I will speak in particular about PRD, where I am an editor, and about some aspects of PRL. But also other journals will be discussed (PRA, PRB, PRC, PRE, PRX). The idea is that the review process becomes more transparent and to provide important information for authors and referees. Questions are very welcome.

# Jeudi 22 mai 2014

## Title: Time-dependent theory of nonlinear response and current fluctuations.

**Inès SAFI (LPS, U. Paris 11, Orsay)**

**Abstract: ** TBA.

# Jeudi 17 avril 2014

## Title: Information thermodynamics in a hybrid opto-mechanical system

**Alexia AUFFEVES (Institut Néel, Grenoble)**

**Abstract: ** Quantum optical hybrid systems are devices where a single quantum emitter is coupled to a mechanical degree of freedom. In these systems, many impressive results have been obtained already in the direction of achieving quantum control of the mechanics. In this letter we show that they have also unexpected capabilities in a very different domain: information thermodynamics. We evidence that the optical measurement-induced back action of the emitter on the mechanical oscillator can be interpreted as reversible conversions of elementary work into bits of information. In the proper - and realistic - experimental conditions, we find that a 2π mechanical oscillation describes in fact a closed cycle of such information-to-work conversions, with strong analogy to Landauer's erasure and Szilard engine. As an interesting consequence of these findings, we finally show that such devices can be turned into high power heat engines operating at Carnot efficiency.

# Jeudi 10 avril 2014

## Title: Orbital magnetic susceptibility and interband effects of 2D tight-binding models

**Frédéric PIECHON (LPS, Orsay) **

**Abstract: ** We review orbital magnetic properties of 2D spinless electrons in multiband systems. For systems (metal or insulator) that break time reversal invariance it is well established that the existence of a spontaneous orbital magnetization crucially depends on interband effects through the so called self-rotating orbital magnetic moment and Berry curvature. For systems that do not break time reversal invariance, in absence of spontaneaous magnetization one is forced to study orbital magnetic susceptibility. Using numerics and a recently developped a gauge invariant perturbation theory we present results we obtained for the orbital susceptibility of various coupled systems (semi-metal or insulators) in two and three band models. In particular we show that systems similar energy spectrum have orbital susceptibility that can range from dia to paramagnetism depending on their self-rotating orbital magnetic moment and Berry curvature.

# Jeudi 27 mars 2014

## Title: Fluctuations en homogénéisation

**Jean-Christophe MOURRAT (UMPA, ENS Lyon)**

**Abstract: ** Sous un changement d'échelle diffusif, la solution de l'équation de la chaleur à coefficients aléatoires converge vers la solution d'une équation de la chaleur à coefficients constants, "homogénéisés". On peut interpréter ce résultat comme une sorte de loi des grands nombres pour cette EDP. Dans l'exposé, je présenterai un résultat qui peut être vu comme un premier pas vers la description des fluctuations de la solution de l'équation à coefficients aléatoires (une sorte de théorème limite central). Plus précisément, je décrirai le comportement de grande échelle du "correcteur", qui est l'objet clé derrière l'homogénéisation de ces problèmes.

# Jeudi 20 mars 2014

## Title: Simulating condensed matter systems with tensor network states and discovery of algebraic decoherence

**Thomas BARTHEL (LPTMS, Orsay)**

**Abstract: ** The non-locality of quantum many-body systems can be quantified by entanglement measures. Studying the scaling behavior of such measures, one finds that the entanglement in most states of interest (occurring in nature) is far below the theoretical maximum. Hence, it is possible to describe such systems with a reduced set of effective degrees of freedom. This is exploited in simulation techniques based on so-called tensor network states (MPS, PEPS, or MERA). I will describe how this approach can be employed to simulate systems of all particle statistics in order to study ground states, thermal states, and non-equilibrium phenomena. Besides explaining the main ideas, I will highlight some applications. The second part of the talk focuses on an application to the decoherence in systems that are coupled to an environment. Until our recent study, it was assumed that, as long as the environment is memory-less (i.e. Markovian), the temporal coherence decay is always exponential -- to a degree that this behavior was synonymously associated with decoherence. However, the situation can change if the system itself is a many-body system. For the open spin-1/2 XXZ model, we have discovered that the interplay between dissipation and internal interactions can lead to a divergence of the decoherence time! The quantum coherence then decays according to a power law. To complement the quasi-exact numerical simulation, I will explain the result on the basis of a perturbative treatment.

# Jeudi 13 mars 2014

## Title: Improved diffusion Monte Carlo for quantum Monte Carlo, rare event simulation, data assimilation, and more

**Jonathan WEARE (Department of Statistics, The University of Chicago)**

**Abstract: ** Diffusion Monte Carlo (DMC) is a workhorse of stochastic computing. It was invented forty years ago as the central component in a Monte Carlo technique for estimating various characteristics of quantum mechanical systems. Since then it has been used in applied in a huge number of fields, often as a central component in sequential Monte Carlo techniques (e.g. the particle filter). DMC computes averages of some underlying stochastic dynamics weighted by a functional of the path of the process. The weight functional could represent the potential term in a Feynman-Kac representation of a partial differential equation (as in quantum Monte Carlo) or it could represent the likelihood of a sequence of noisy observations of the underlying system (as in particle filtering). DMC alternates between an evolution step in which a collection of samples of the underlying system are evolved for some short time interval, and a branching step in which, according to the weight functional, some samples are copied and some samples are eliminated. Unfortunately for certain choices of the weight functional DMC fails to have a meaningful limit as one decreases the evolution time interval between branching steps. We propose a modification of the standard DMC algorithm. The new algorithm has a lower variance per workload, regardless of the regime considered. In particular, it makes it feasible to use DMC in situations where the ``naive'' generalization of the standard algorithm would be impractical, due to an exponential explosion of its variance. We numerically demonstrate the effectiveness of the new algorithm on a standard rare event simulation problem (probability of an unlikely transition in a Lennard-Jones cluster), as well as a high-frequency data assimilation problem.

# Jeudi 20 février 2014

## Title: Ground state energy of noninteracting particles and of interacting bosons in a random Bernoulli potential

**Jan WEHR (The University of Arizona, Tucson)**

**Abstract: ** I will study the fluctuations of the ground state energy of the one-dimensional Anderson model, in which the random potential has the Bernoulli (i.e. two-valued) distribution. By a direct method, the statistics of the random landscape will be shown to imply a limit theorem for this quantity. I will also discuss a mean-field model of interacting bosons in such potential and derive an asymptotic formula for its ground state energy density in the limit of weak interaction. The results were obtained jointly with M. Bishop.

# Jeudi 13 février 2014

## Title: Grandes Déviations et Hors d'Equilibre

**Raphaël CHETRITE (Laboratoire J.A. Dieudonné, Nice)**

**Abstract: ** Ce séminaire contiendra deux parties. La première sera une ''scratch'' présentation de la théorie des Grandes Déviations. La deuxième partie portera sur des résultats récents, obtenus avec Hugo Touchette (PRL 111, 120601 (2013)) portant sur des processus de Markov conditionnés à des événements rares. Je démontrerai, à l'aide de la théorie des grandes déviations, qu'un tel processus conditionné peut être représenté par un processus Markovien sans conditionnement, appelé processus équivalent, ayant les mêmes propriétés typiques que le processus conditionné. La motivation Physique pour l'étude d'un tel processus conditionné découle de la question de l'équivalence et de la simulation d'ensembles Microcanonique et Canonique hors d'équilibre.

# Jeudi 30 janvier 2014

## Title: Decay of excitations in interacting one-dimensional Bose gases

**Zoran RISTIVOJEVIC (Centre de Physique Théorique, Ecole Polytechnique, Palaiseau)**

**Abstract: ** Excitation spectrum in weakly-interacting systems of bosons have the Bogoliubov form. In three dimensions, those excitations are unstable due to residual weak interactions. The resulting process is known as Beliaev decay [1,2] and has been experimentally observed [3]. The related problem of decay of excitations in one-dimensional Bose gases is a fundamental long-standing problem. In this talk I will present its solution [4]. As a result of the conservation laws in one dimension, at zero temperature the leading mechanism of decay of a quasiparticle excitation is its disintegration into three others. We find that a phonon excitation has a decay rate proportional to the seventh power of momentum. In the integrable case of contact interaction between the bosons, the decay rate vanishes. Our theory is based on studying the anharmonic effects produced by the leading integrability breaking perturbations to the Lieb-Liniger model. It is not limited to the decay of lowest momentum phonon excitations and can describe full crossover as momentum increases and the excitation spectrum approaches its quadratic form. [1] S. T. Beliaev, Sov. Phys. JETP 7, 299 (1958). [2] L. D. Landau and E. M. Lifshitz, Statistical Physics, Part 2 (Pergamon Press, Oxford, 1980). [3] N. Katz, J. Steinhauer, R. Ozeri, and N. Davidson, Phys. Rev. Lett. 89, 220401 (2002). [4] Z. Ristivojevic and K. A. Matveev, arxiv:1312.5322 (2013).

# Lundi 27 janvier 2014 à 15h30 en salle 115

## Title: N=4 super Yang Mills amplitudes and integrability

**Georgios PAPATHANASIOU (LAPTH)**

**Abstract: ** Maximally supersymmetric Yang-Mills theory stands out as an interacting 4-dimensional gauge theory which may be exactly solvable in the planar limit. In this talk, we explore the consequences of a recent, integrability-based proposal of Basso, Sever and Vieira, for a nonperturbative description of its scattering amplitudes. We prove that the integrals this proposal predicts for part of the 6-point amplitude, evaluate to transcendental functions known as Harmonic Polylogarithms at any order in the weak coupling expansion, and obtain explicit expressions up to 6 loops.

# Jeudi 23 janvier 2014

## Title: Shape Dynamics: a new tool for General Relativity, Cosmology and Quantum Gravity

**Flavio MERCATI, Perimeter Institute**

**Abstract: ** I will give an overview of the possibilities offered by a recent reformulation of General Relativity called Shape Dynamics. This theory replaces relativity of simultaneity for spatial conformal invariance, maintaining the same degree of symmetry of GR while doing without some of its shortcomings. In SD several kinds of singularities of GR (like black hole and big-bang type singularities) become unphysical gauge artifacts. Moreover quantum SD is expected to be inequivalent to a standard quantization of GR and appears to be more manageable. Finally, SD motivates an original interpretation of the evolution of the Universe in terms of growth of complexity, which could explain the arrow of time without referring to any notion of gravitational entropy and without statistically unlikely initial conditions for the Universe.

# Jeudi 16 janvier 2014

## Title: Relaxation dynamics of a coherently split one-dimensional gas

**Laura FOINI, Département de Physique de la Matière Condensée, Université de Genève**

**Abstract:** Non-equilibrium dynamics and relaxation processes in isolated quantum systems represent, at present, a vivid research direction both theoretically and experimentally. Such interest is sustained by the overwhelming progress in the field of cold atoms that allows to investigate the unitary dynamics of the system. In this talk I will review an experiment that has considered the splitting of a one-dimensional Bose gas into two coherent gases, where, ultimately, the properties of the system are probed by matter-wave interference. While previous works have focused on the independent dynamics of the two systems after the splitting, in our study we take into account the effect of a finite tunneling coupling between the two. Comparisons between the results obtained for such non-equilibrium problem and the thermal ones will be drawn.

# Jeudi 9 janvier 2014

## Title: Entropy and Mutual information in low-dimensional classical and quantum critical systems

**Jean-Marie STEPHAN (University of Virginia)**

**Abstract:** In studies of new and exotic phases of quantum matter, the Renyi entanglement entropy has established itself as an important resource.For example it is universal at one-dimensional quantum critical points: the leading term can be used to extract the central charge $c$ of the underlying conformal field theory, and thus identify the universality class. In this talk I will show how an analogous quantity defined for classical systems, the Renyi Mutual Information (RMI), can be used to access universality classes in 2d. In particular for a rectangle cut into two rectangles, the shape dependence of the RMI can be computed exactly and is proportional to $c$. This makes it possible to extract $c$ from (transfer-matrix) Monte Carlo simulations. I will also discuss how this Mutual information is related to the entanglement entropy of certain Resonating valence bond states in 2d, as well as other basis-dependent entropies in 1d quantum systems.

# Mardi 7 janvier 2014 à 14 heures en salle 116

## Title: Universal thermodynamics and fate of the amplitude mode in the quantum O(N) model

**Adam RANÇON (James Franck Institute, University of Chicago)**

**Abstract: ** The quantum O(N) model is ubiquitous in condensed matter and cold atoms and describes the behavior of a number of systems close to a quantum phase transition. In the ordered (broken-symmetry) phase far from the critical point, there are N-1 Goldstone modes and a gapped amplitude mode. In low dimensions, the system is strongly coupled close to the critical point, and the fate of the existence of the amplitude mode is guaranted. We discuss the thermodynamics of the two-dimensional quantum O(N) model for $N\geq 2$ in the vicinity of its zero-temperature quantum critical point, and in particular the universal scaling function ${\cal F}_N$ which determines the pressure $P(T)$. We show that the large-$N$ approach is unable to predict the (non-monotonuous) shape of ${\cal F}_N$ for $N\lesssim 10$, but ${\cal F}_N$ can be computed from a non-pertubative renormalization-group approach. Finally, we discuss the spectral function of the amplitude mode close to the quantum critical point and how show a well-defined mode at small N disappears as N increases.

# Jeudi 12 décembre 2013

## Title: Real Time Imaging of Quantum and Thermal Fluctuations: a Detour into Quantum Noise

**Denis Bernard (LPT-ENS)**

**Abstract: **In the last decade progresses have been achieved in realising and manipulating stable and controllable quantum systems, and these made possible to experimentally study fundamental questions posed in the early days of quantum mechanics. We shall theoretically discuss recent cavity QED experiments on non-demolition quantum measurements. While they nicely illustrate the possibility to implement efficient quantum state manipulations, these experiments pose a few questions such as: What does it mean to observe a progressive wave function collapse in real time? How to describe it? What do we learn from them? Their analysis will allow us one hand to link these experiments to basics notions of probability or information theory, and on the other hand to touch upon notions of quantum noise. As an illustration, we shall also look at quantum systems in contact with a heat bath and we shall describe the main physical features of thermally activated quantum jumps.

# Jeudi 5 décembre 2013 - Colloquium - Grande Salle du CBP

## Title: Quantum gravity: The view from particle physics

**Hermann Nicolai (Albert Einstein Institute Potsdam)**

# Mardi 3 décembre 2013

## Title: Quantum integrability of Benjamin-Ono model

**E. Sklyanin (Université de York, UK)**

**Abstract:** The classical Benjamin-Ono equation is a Hamiltonian integrable system originating from hydrodynamics. Its quantised version is discussed recently in various contexts of mathematical physics and pure algebra. The quantum Hamiltonian is diagonal on Jack symmetric functions of infinitely many variables. We present two different constructions of higher commuting Hamiltonians for the quantum B-O equation. One is based on the determinantal formula for the Hamiltonians of N-particle Calogero-Sutherland model. Another one uses the quantum Lax matrix. A generalisation for Macdonald polynomials is also discussed. The talk is based on joint work with Maxim Nazarov (York): arXiv:1212.2781,1212.2960,1309.6464.

# Jeudi 28 novembre 2013

## Title: Living on the edge: towards a general theory for pyrochlores

**Ludovic Jaubert (Okinawa Institute of Technology (Japon))**

**Abstract: **Rare earth pyrochlore oxydes is a very rich family in frustrated magnetism where each member brings its own complexity. If some materials and models exhibit topological phase transitions beyond the standard Ginzburg-Landau approach, other are able to avoid ordering such as spin ice (Dy2Ti2O7) or potential spin liquids (Tb2Ti2O7). Yb2Ti2O7 for example has generated considerable excitement as a potential example of a “quantum spin-ice”. Here we show how it is possible to construct a unified picture of ground states and excitations for any kind of nearest neighbour coupling. Our starting point is an exact mapping onto a lattice field theory which explains both the dimensional reduction seen in Yb2Ti2O7, and the ground-state selection in Er2Ti2O7. It also provides a general recipe to find emergent spin liquids. These results are combined with spin wave calculations and extensive Monte Carlo simulations to provide a comprehensive picture of the interplay between order and finite temperature and quantum excitations in pyrochlore oxides with highly-anisotropic exchange interactions.

# Jeudi 21 novembre 2013

## Title: Cosmology from quantum gravity: the universe as a Bose-Einstein condensate

**Daniele Oriti (AEI Potsdam)**

**Abstract:** We introduce the group field theory (GFT) formalism as a second quantized dynamics of the spin network states of loop quantum gravity. We explain how it provides, at the same time, a generalization of matrix models for 2d gravity and of lattice gravity path integrals. We summarize briefly some recent results obtained in this context, in particular concerning renormalizability. We then discuss the issue of the emergence of continuum spacetime from this "pre-geometric" quantum gravity formalism (and related ones) and the hypothesis that it arises from some sort of cosmological phase transition. We show that effective cosmological equations for continuum homogeneous geometries can be derived directly from fundamental group field theory models, in full generality. The relevant quantum states are GFT condensates, and a form of nonlinear quantum cosmology arises as the hydrodynamics of the system, in the same way in which Gross-Pitaevskii equations arise from the quantum microscopic dynamics of real Bose-Einstein condensates. A continuum spacetime emerges then from GFT as a quantum fluid.

# Jeudi 14 novembre 2013 en Amphi H

## Title: A numerical study of the Bose-Hubbard model with a trapping potential

**Christian Torrero (CPT, Marseille)**

**Abstract:** The seminar shows an application of the Trap Size Scaling (TSS) technique to the Bose-Hubbard model with a trapping potential in 1, 2 and 3 dimensions. Particular emphasis is put on the scaling behaviour of some observables close to phase transitions and to the determination of critical points in the phase diagram. A detailed description of the versatile algorithm employed in the study, i.e. the Directed Loop Update, will be also provided.

# Jeudi 7 novembre 2013

## Title: Quantum Dynamics and Topological Phases of Light in Circuit Quantum Electrodynamics

**Karyn Le Hur (Center for Theoretical Physics, Ecole Polytechnique & CNRS)**

**Abstract:** Systems in cavity or circuit Quantum Electrodynamics (QED) are extensively studied in the context of quantum information and quantum computing. In addition, studying the non-equilibrium dynamics of photons in these open quantum systems is a challenging theoretical question. Studying “versatile lattice versions of these systems” open new doors to quantum simulation, similar to ultra-cold atoms in optical lattices. In this Talk, we review our recent theoretical progress regarding (i) the dynamics in driven and dissipative light-matter systems beyond the weak-coupling limit [1] and (ii) the realization of topological phases of light in superconducting QED-circuit arrays through synthetic gauge fields [2]. Effects of disorder and Mott physics [3,4] will be studied. We also comment on the current experimental status in superconducting circuit systems.

[1] P. P. Orth, A. Imambekov and K. Le Hur, Phys. Rev. B 87, 014305 (2013)

[2] J. Koch et al. Phys. Rev. A 82, 043811 (2010); A. Petrescu, A. A. Houck and K. Le Hur, Phys. Rev. A 86, 053804 (2012)

[3] J. Koch and K. Le Hur, Phys. Rev. A 80, 023811 (2009)

[4] A. Petrescu and K. Le Hur, Phys. Rev. Lett. 111, 150601 (2013)

# Jeudi 17 octobre 2013

## Title: String theory reBorn

**Laurent Freidel (Perimeter Institute)**

**Abstract:** In this talk I will show how string theory is fundamentally satisfying the principle of Born reciprocity: A perfect duality between space and momentum space. I will show that String theory as a consequence, not only allow space-time to carry curvature but also allow momentum space and provides phase space with a novel geometrical structure that we call a Born geometry. This opens the way to radically new string theory backgrounds in which the usual concept of locality do not hold, locality becoming relative. I will show how to implement this formulation based on a generalization of T-duality and what are the challenges in trying to understand its formulation and its fundamental consequences on our picture of space-time locality.

# Jeudi 26 septembre 2013

## Exposé groupe de travail MaCon par D. Ferraro

# Jeudi 19 septembre 2013

## Title: Electric/magnetic duality in AdS4/CFT3

**Oscar Varela (ITF d'Utrecht)**

**Abstract:** The field theory defined on a stack of N M2-branes is thought to correspond to that first introduced by BLG/ABJM. At large N, an important sector of this theory can be described, holographically, by the SO(8)-gauged maximal supergravity in four dimensions of de Wit and Nicolai. Since its inception, the latter has been tacitly assumed to be unique. Recently, however, a one-parameter family of SO(8) gaugings of maximal supergravity has been discovered, the de Wit-Nicolai theory being just a member in this class. I will explain how this overlooked family of SO(8)-gauged supergravities is deeply related to electric/magnetic duality in four dimensions. I will then discuss some predictions that can be made about the possible family of holographic dual field theories, focusing on the structure of conformal phases and the RG flows between them..

# 2012-2013

# Jeudi 27 juin 2013

## Title: Quelques pistes cosmologiques pour tester la gravitation quantique à boucles

**Aurélien Barreau (Université Joseph Fourier (Grenoble))**

**Abstract:** Je présenterai brièvement les idées de la cosmologie quantique à boucles et la manière dont elles peuvent conduire à certaines prédictions potentiellement testables. Je montrerai comment le le Big Bang est remplacé par un grand rebond, pourquoi l'inflation est ici très naturelle et la façon dont le spectre de puissance cosmologique peut être modifié par cette théorie. Je conclurai sur quelques autres pistes concernant les trous noirs.

# Mardi 25 juin 2013 à 14h en salle des thèses

## Title: Séparation des variables et facteurs de forme des modèles intégrables quantiques

**Soutenance de thèse de Nicolas Grosjean (ENS Lyon))**

# Lundi 24 juin 2013 à 13h30 en salle de réunion du labo

## Title:N=2 SUSY gauge theories and quantized moduli spaces of flat connections

**Jorge Teshner (DESY, Hamburg)**

**Abstract:** Supersymmetry allows us to reduce the path integrals representing expectation values of supersymmetric observables in certain classes of N=2 SUSY gauge theories to expectation values in an effective zero mode quantum mechanics. It turns out that this quantum mechanics is related to the quantization of the moduli spaces of flat PSL(2,R)-connections. The correspondences between N=2 SUSY gauge theories and conformal field theory discovered by Alday, Gaiotto and Tachikaw (AGT) may then be understood using existing relations between the quantized moduli spaces of flat connections and conformal field theory.

# Mardi 18 juin 2013 à 14h en salle 115

## Title: Bose gas in the vicinity of the Mott transition

**Nicolas Dupuis (LPTMC, Université Pierre et Marie Curie (Jussieu))**

**Abstract:** A Bose gas in an optical lattice undergoes a quantum phase transition between a superfluid phase and a Mott-insulating state as the interaction strength or the density is varied. Using a non-perturbative renormalization-group approach to the Bose-Hubbard model, we obtain a phase diagram in very good quantitative agreement with quantum Monte Carlo simulations and recover the two universality classes of the Mott transition. We then compute the pressure in the vicinity of the transition. Due to the proximity of a quantum critical point, the equation of state takes a universal form with only a few nonuniversal parameters. We discuss both the density-driven and the interaction-driven transitions, and compute the associated universal scaling functions. Finally, we discuss the experimental consequences of our results for ultracold bosons in an optical lattice.

# Jeudi 13 juin 2013

## Title: Roton-phonon interaction: from superfluid helium to quantum magnets

**Mike Zhitomirsky (CEA, Grenoble)**

**Abstract:** High-energy gapped quasiparticles (rotons) interacting with low-energy acoustic excitations (phonons) are ubiquitous in condensed matter physics. I discuss two experimentally relevant examples. The high-precision neutron spin-echo measurements of rotons in the superfluid helium reveal a non-monotonous temperature dependence of the roton gap [1]. The new phenomenon is explained by competition between the standard roton-roton scattering, which is effective above ~1K, and the roton-phonon three-particle processes, which appear due to the presence of the Bose-Einstein condensate in the superfluid helium and dominate in the sub-Kelvin region.

In the second example, an optical magnon in an easy-plane collinear antiferromagnet interacts with acoustic spin waves. I consider the effect of disorder in such systems and demonstrate that it has a profound effect on the temperature dependence of the relaxation rate of optical magnons. The usual random-potential scattering yields a T-independent contribution whereas the impurity-assisted magnon scattering gives the leading temperature correction, which exceeds greatly the effect of magnon-magnon interaction in the bulk. Our theoretical prediction finds confirmation in the high-resolution neutron measurements on BaNi2(PO4)2 [2].

[1] B. Fak, T. Keller, M. E. Zhitomirsky, and A. L. Chernyshev, "*Roton-Phonon Interactions in Superfluid 4He*", Phys. Rev. Lett. **109**, 155305 (2012)

[2] A. L. Chernyshev, M. E. Zhitomirsky, N. Martin, and L.-P. Regnault "*Lifetime of Gapped Excitations in a Collinear Quantum Antiferromagnet*", Phys. Rev. Lett. **109**, 097201 (2012).

# Jeudi 18 avril 2013

## Title: The Physical Church-Turing Thesis and the Principles of Quantum Theory

**Pablo Arrighi (LIP ENS Lyon et Université Joseph Fourier (Grenoble))**

**Abstract:** Quantum Computation shatters the question of "How fast can we compute the solution to a given problem?" (namely, Complexity theory). But notoriously, it remains innocuous to the question of "Whether the solution a given problem can be computed at all, even with unbounded time and space resources" (namely, Computability theory). Any answer to the latter question must depend on your definition of what a computer is. This is why Computability theory crucially relies upon the celebrated Church-Turing thesis, which states that: "*Anything that can be computed can be computer by a Turing machine*".

Yet, several works have shown how quantum theory as it stands could breach the physical Church-Turing thesis. We draw a clear line as to when this is the case, in a way that is inspired by Gandy. Gandy formulates postulates about physics, such as homogeneity of space and time, bounded density and velocity of information --- and proves that the physical Church-Turing thesis is a consequence of these postulates. We provide a quantum version of this theorem. The approach exhibits a formal, non-trivial interplay between theoretical physics symmetries and computability.

# Jeudi 4 avril 2013

## Title: Beyond time-dependent charge transport: noise and thermoelectric effects.

**Janine Splettstoesser (Institut für Theorie der Statistischen Physik , AAchen)**

**Abstract:** Nanoscale systems driven by time-dependent signals, such as quantum pumps, have recently attracted a lot of attention since they can serve as controlled sources of single particles [1]. Furthermore it can be shown, that time-dependent transport provides an intriguing spectroscopy tool, revealing quantum effects that are not accessible from a stationary state measurement [2].

In this talk I will present different examples for these particular characteristics of time-dependently driven quantum dot devices. Due to the smallness of these setups many-body effects like the Coulomb interaction, as well as quantum fluctuations play an important role for the transport properties and their signatures are observable in charge transport.

In addition to the transported charge I will discuss the transport noise induced by the time-dependent modulation[3]. Interestingly, there can be pumping noise even in the absence of charge pumping, which gives additional insight into the underlying transport mechanism.

Finally, I will talk about the thermoelectric performance of driven quantum dots [4]. Under certain conditions not only quantized charge pumping can be realized, but also the heat current exhibits plateaus, related to the spin degeneracy of the system. This renders possible the operation of time-dependently driven quantum dot devices as nanoscale engines, in particular as battery chargers, cooling devices or heat engines.

[1] G. Feve et al., Science **316**, 1169 (2007); M. D. Blumenthal, et al., Nature Physics **3**, 343 (2007); V. F. Maisi, et al., New J. Phys. **11**, 113057 (2009).

[2] F. Reckermann, J. Splettstoesser, and M. R. Wegewijs, Phys. Rev. Lett. **104**, 226803 (2010).

[3] R.-P. Riwar, J. Splettstoesser, and J. Konig, arxiv:1212.3545.

[4] S. Juergens, F. Haupt, M. Moskalets, and J. Splettstoesser, in preparation.

# Jeudi 14 mars 2013

## Title: Tomonaga-Luttinger physics in electronic quantum circuits

**Mathias Albert (Laboratoire de Physique des Solides, Orsay)**

**Abstract:** In one-dimensional conductors, Coulomb interactions result in correlated electronic systems called Tomonaga-Luttinger liquids (TLL). The TLL physics also applies to other many-body phenomena, providing complementary viewpoints while benefiting from the powerful TLL framework.

One such phenomenon is the low-energy conductance suppression of a quantum coherent conductor embedded in a dissipative circuit, an effect called dynamical Coulomb blockade. Here we investigate the basic class of mesoscopic circuits constituted by a short single-channel quantum conductor in series with a resistance R. Remarkably, such circuits can be mapped on a TLL of interaction parameter 1/(1 + Re2 /h), with an impurity. From this mapping, generalized to realistic dissipative circuits, a scaling law for the suppressed conductance is derived at R = h/e2 , and small deviations computed for R ? h/e2 using the thermodynamic Bethe-Ansatz exact solution [1]. We find that the scaling law is obeyed by our data [2] for arbitrary quantum channels, emulated by a Ga(Al)As quantum point contact, and by the recent data [3] obtained using a carbon nanotube. This demonstrates a highly tunable test-bed for TLL physics, and consolidates a recently proposed phenomenological expression for the conductance of a quantum channel in a linear circuit.

[1] P. Fendley, A.W.W. Ludwig and H. Saleur Phys. Rev. **B** 52, 8934?8950 (1995).

[2] Jezouin et al, arXiv:1301.4159 (2013).

[3] Mebrahtu et al., Nature **488**, 61 (2012).

# Jeudi 7 mars 2013

## Title: Cavity QED of two-dimensional electron systems subjected to a perpendicular magnetic field

**David Hagenmüller (MPQ, Paris 7)**

**Abstract:** Cavity quantum electrodynamics (cavity QED) is the study of the interaction between light confined in a reflective cavity and atoms or other particles, under conditions where the quantum nature of light photons is significant. If the coupling between light and matter is sufficiently strong, the resulting quantum eigenstates are entangled states. The whole system exhibits new resonances, with energies that are different from the ones of the bare excitations.

In this talk, I will describe the coupling between a two-dimensional electron gas subjected to a perpendicular magnetic field and the cavity confined optical modes. In particular, I will show that such a system can reach an unprecedented ultrastrong coupling regime, in which the vacuum Rabi frequency (quantifying the strength of the light-matter interaction) becomes comparable to the cyclotron transition frequency between two consecutive Landau levels: the vacuum Rabi frequency scales with the square root of the Landau levels filling factor[1]. This physical prediction has been quantitatively demonstrated by recent spectroscopy experimental results in the THz domain[2].

Moreover, a clearly intriguing problem is to explore how graphene behaves when embedded in a cavity resonator. I will show that if it is possible to achieve ultrastrong coupling between the graphene cyclotron transition and cavity photons, it leads to strong qualitative differences with respect to the case of massive fermions in semiconductors. In particular, the former can undergo a quantum phase transition analogous to the one occuring in the Dicke model for superradiance[3].

[1] D. Hagenmüller, S. De Liberato and C. Ciuti, Phys. Rev. B **81**, 235303 (2010).

[2] G. Scalari *et al.*, Science **335**, 1323-1326 (2012).

[3] D. Hagenmüller and C. Ciuti, Phys. Rev. Lett., in press (2012).

# Jeudi 21 février 2013

## Title: Transport properties of thin superconducting films

**Aleksandra Petkovic (LPTHE, Paris)**

**Abstract:** Transport properties of 2D superconducting systems can be very different from those of bulk superconductors because thermal and quantum fluctuations of superconducting order parameter are more pronounced and play a crucial role. First we focus on influence of superconducting fluctuations on dynamics, while the system is in the normal state but close to the superconducting transition. In the fluctuational regime, we derive Ginzburg-Landau-type action under far-from equilibrium conditions. Then, utilizing it, we calculate fluctuation-induced density of states and Maki-Thomson- and Aslamazov-Larkin-type contributions to the in-plane electrical conductivity. We propose an experimental setup where our results can be tested.

Then, we concentrate on transport at lower temperatures in close-to-equilibrium conditions investigating influence of quantum fluctuations on unbinding of vortex-antivortex pairs. We determine the temperature below which quantum fluctuations dominate over thermal fluctuations and describe the transport in this quantum regime. The crossover from quantum to classical regime is discussed and the quantum correction to the classical current-voltage relation is determined.

# Jeudi 14 février 2013

## Title: Classical and quantum integrability : from spin chains to the AdS/CFT duality.

**Sébastien Laurent (Imperial College, Londres)**

**Abstract:** Various quantum systems are integrable and have a large number of conserved quantities, related to each other by an equation called the Hirota equation. This equation is also known in the context of classical integrability, and its generic solution can be expressed in terms of operators called Q-operators. We will start from some simple spin chains, and generalize to the finite size spectrum of some field theories and the AdS/CFT duality ; we will see that these systems are strongly constrained by the analytic properties of the eigenvalues of the Q-operators, and that these constraints allow to solve the system in the sense that they give relatively simple equations encoding the spectrum. However, the physical origin of these analytical conditions is not always very well understood.

# Jeudi 7 février 2013

## Title: Avalanches

**Kay Wiese (LPTENS, Paris)**

**Abstract:** Magnetic domain walls, charge density waves, contact lines, and cracks are all elastic systems, pinned by disorder. Changing an external parameter, they remain stuck before advancing in sudden rapid motion, termed avalanche. After an introduction into the phenomenology, I present work based on the functional renormalization group, which allows to go beyond the usual toy-model description: avalanche-size distributions in any dimension, the distribution of velocities in an avalanche, and their shape. These techniques also lead to an exact solution for the decay of 2-dimensional Burgers turbulence.

# Jeudi 28 février 2013

## Congés d'Hivers: pas de séminaire

# Jeudi 31 janvier 2013 (14h30-16h30, amphi Schrödinger)

## Title: Everything you wanted to know for Mathematica (but were afraid to ask)

**Marc Magro (ENS Lyon)**

**Abstract:** In two hours, Master Magro will turn you from Mathematica Novices into Mathematica Padawans... Pour participer à cette introduction, munissez vous d'un ordinateur portable sur lequel le logiciel Mathematica est installé!

# Jeudi 24 janvier 2013

## Journée de l'équipe de théorie

# Jeudi 17 janvier 2013

## Title: Dark energy: an effective field theory approach

**Federico Piazza (APC, Paris)**

**Abstract:** The discovery of the accelerating expansion of the Universe is triggering an impressive amount of theoretical and observational activity. After briefly reviewing the problems and challenges of ``Dark Energy", I will focus on recent and ongoing works in which my collaborators and I propose a unifying description of dark energy and modified gravity models that makes use of effective field theory (EFT) techniques. EFT allows to isolate the relevant low energy degrees of freedom and to efficiently study their dynamics. By extending the ``Effective field theory (EFT) of inflation" formalism to late time cosmology, we write the most general action for cosmological perturbations in the presence of an additional scalar degree of freedom. As we argue, in fact, cosmological perturbations are the relevant low energy degrees of freedom in a cosmological set-up. I will focus on a few operators that are quadratic in the perturbations and which appear in non-minimally coupled scalar-tensor gravity and ``Galileon" theories and I will describe the mixing between gravity and the scalar degree of freedom that such operators produce.

# Jeudi 22 novembre 2012

## Title: Avalanches in systems with quenched disorder: Mean-field models and beyond

**Alexander Dobrinevski (LPT ENS)**

**Abstract:** Disordered systems typically respond non-smoothly to external driving. This leads to phenomena like Barkhausen noise in the motion of magnetic domain walls, earthquakes in the motion of tectonic plates, and avalanches in granular media under shear stress. A simple mean-field model for such avalanche phenomena is a particle driven by a spring on a Brownian random-force landscape. It was developed originally in the context of magnetic domain walls and is also known as the Alessandro-Beatrice-Bertotti-Montorsi model. I will present some analytical results on statistics of avalanches in several variants of this model (e.g. including memory effects). I will then discuss, using RG methods, the relationship of this mean-field model to a spatially extended elastic interface in a disordered environment.

# Jeudi 15 novembre 2012

## Title: Quartet correlations between Cooper pairs in a mesoscopic double Josephson junction

**Régis Mélin (Institut Néel, Grenoble)**

**Abstract:** Two Josephson junctions separated by less than the superconducting coherence length involve higher order processes that can be quite sizeable for more transparent junctions. Those processes originate from crossed Andreev reflections (CAR), where Cooper pairs are split into two spin-entangled electrons. In a Sa-S-Sb bijunction, double CAR simultaneously produces two Cooper pairs, one in each junction. This amounts to producing nonlocal quartets in the superconductors Sa and Sb [1]. Cooper pairs being pseudo-bosons, this mechanism bears some similarity with the emission of pairs of time-correlated photons in Quantum Optics, thus the name "superconducting beam splitter" given to the basic bijunction set-up. Energy conservation implies that a coherent dc quartet current can flow even if Sa and Sb are biased at respective voltages V and -V, thus offering the possibility of dc Josephson effect in biased junction arrays. The appearance of the quartet currents at equilibrium rely on a superconducting circuit having 3 (and not 2) current terminals. Dc quartet current can be detected by the synchronization of the ac Josephson oscillations in each contact a and b, either with Va=V and Vb=-V, or adding a small dc and ac components such as to achieve Shapiro steps for quartet motion. Special attention will be paid to the microscopic theory for those quartet and higher order resonances in connection with preliminay experimental results obtained in Grenoble [group of F. Lefloch (CEA-Grenoble, INAC)) and H. Courtois (NEEL)]. A phenomenological circuit model for an overdamped superconducting triode will also be discussed in connection with those experiments.

[1] A. Freyn, B. Douçot, D. Feinberg and R. Mélin, Phys. Rev. Lett. **106**, 257005 (2011).

# Jeudi 8 novembre 2012

## Title: Stochastic Thermodynamics of Biased Diffusions

**Mattéo Smerlak (Albert Einstein Institute, Postdam (Allemagne))**

**Abstract:** A unifying framework for the thermodynamics of fluctuating systems with Fokker-Planck dynamics has been developed by Seifert using the notion of "stochastic entropy". I will consider the extension of this formalism to the case of geometric/entropic ratchets: inhomogeneous systems in which freely diffusing particles do not reach a Boltzmann-Gibbs equilibrium, even at constant temperature, and thus violate the naive law that "the entropy of a system at local equilibrium cannot decrease". I will introduce to this effect the notion of "relative stochastic entropy", and use it to generalize (i) Jaynes? maximum-entropy principle for the canonical ensemble, (ii) the second law of thermodynamics and (iii) Seifert?s integral fluctuation theorem. These results apply e.g. to colloidal particles dragged in viscous fluids with space-dependent viscosity or in asymmetric confined geometries.

# Jeudi 1er novembre 2012

## Congés d'Automne: pas de séminaire

# 2011-2012

# Jeudi 14 juin 2012

## Title: TBA

**Sasha Chernyshev (University of California at Irvine)**

# Du 5 au 8 juin 2012

## Workshop theory of quantum gases and quantum coherences

**Local organizers:** Edmond Orignac et Tommaso Roscilde

This workshop is dedicated to the theoretical challenges in the field of quantum gases, with a strong connection to condensed matter physics - including strongly correlated systems, low-dimensional systems, disorder effects etc.

One of the main goals of this workshop is to bring together young researchers coming from Europe and overseas. The program includes 5 overview lectures by leading senior scientists in the field of cold atoms and condensed matter, about 20 lectures by junior scientists selected by the advisory committee, and a poster session open to all the participants. A special invited session is organized this year on the subject of Bose-Einstein condensation in condensed matter systems.

# Jeudi 31 mai 2012

## Title: Casimir force induced by imperfect Bose gas

**Jaroslaw Piaseki (Institut de Physique Théorique de l'Université de Varsovie)**

**Abstract:** We present a study of the Casimir effect in an imperfect (mean-field) Bose gas filling the volume contained between two infinite parallel plane walls. In the one-phase region the Casimir force decays exponentially fast with increasing distance between the walls. However, when Bose-Einstein phase transition is approached the decay length in the exponential law diverges. In the two phase region the Casimir force is long range, and decays following a power law. We clarify the relation between the range of the Casimir forces and the bulk correlation length.

# Jeudi 24 mai 2012

## Title: Majorana Edge States in One-Dimensional Systems

**Pascal Simon (LPS Orsay)**

**Abstract:** We study a one-dimensional wire with strong spin-orbit coupling, which supports Majorana fermions when subject to a Zeeman magnetic field and in proximity of a superconductor. We evaluate the local density of states, as well as the spin polarization in this system using an exact numerical diagonalization approach. Moreover, we define and calculate a local "Majorana polarization" and "Majorana density". We find that the spatial dependence of the Majorana polarization is proportional to that of the spin polarization parallel to the chain and we propose to test the presence of Majorana fermions in a 1D system by a spin-polarized density of states measurement [1].

We then discuss the effect of electron-electron interactions on one-dimensional electron systems that support Majorana edge states. Strong interactions generically destroy the induced superconducting gap that stabilizes the Majorana edge states. For weak interactions, the renormalization of the gap is nonuniversal and allows for a regime in which the Majorana edge states persist. We present strategies of how this regime can be reached [2].

[1] C. Bena, D. Sticlet, P. Simon, arXiv:1109.5697.

[2] S. Gangadharaiah, B. Braunecker, P. Simon, D. Loss, Phys. Rev. Lett. **107**, 036801 (2011).

# Semaine Michael E. Fisher: 21 au 25 Mai 2012

## Colloquium: "Charms, cautions and cognition" le 21 mai 2012.

## Cours: Molecular motors: observations, experiments and theory

**Abstract:** This informal, introductory short course will explain that molecular mo- tors are proteins found in all living cells that convert chemical energy into mechanical work and motion. Thus the protein myosin together with filamentary actin underlies the operation of all muscles. Processive motor proteins such as kinesin and dynein and some types of myosin, step unidirectionally along linear tracks, specifically microtubules and actin filaments, and play crucial roles in cellular transport processes, organization, and function. How do we know about such facts ? How can we observe in vitro or even in vivo the operation of single, individual molecules ? And what sort of experiments and theory are appropriate for gaining insight into the mechanisms by which such motors operate ?

- Mercredi 23 mai : 10h-12h (pause cafe de 30 mn au milieu)
- Mercredi 23 mai : 12h-14h (pause cafe de 30 mn au milieu)
- Jeudi 24 mai : 10h-12h (pause cafe de 30 mn au milieu)

Tous les cours auront lieu dans la grande salle du Centre Blaise Pascal.

# Jeudi 17 mai 2012

## Pas de séminaire: ascension

# Jeudi 10 Mai 2012 à 14h

## Title: The nonperturbative renormalization group (NPRG): principles and some applications. (Colloquium)

**Dominique Mouhanna (LPTMC, Jussieu)**

**Abstract:** The renormalization group (RG) has been introduced in its modern form by K.G. Wilson in the seventies, based on the concept of block spin of L.P. Kadanoff. Although nonperturbative in essence the RG has been, for a long time, confined to the perturbative domain: small coupling constant, small disorder, vicinity of the upper or lower critical dimension, etc.

In the nineties, C. Wetterich has introduced a new formulation of the RG, based on the concept of running Gibbs free energy, that turns out to be the most suitable one to tackle with nonperturbative issues: strong coupling behaviour, physics far from the upper or lower critical dimensions, physics of topological excitations, bound states, disorder, etc.

In the first part of my talk I present the general principles underlying the method. In the second part I illustrate these principles in the context of various systems taken from field theory and condensed matter physics.

# Jeudi 3 mai 2012

## Title: Statistical mechanics of harmonic spheres: glass and jamming transition

**Hugo Jacquin (Laboratoire MSC, Université Denis Diderot, Paris)**

**Abstract:** When a fluid is cooled down suficiently rapidly to avoid cristallization, its dynamics slows down very rapidly, and an amorphous solid is formed. This phenomenon is called the glass transition. When piling randomly in a box more and more spheres, a density is eventually attained, where all spheres come in contact. At this density and upon further compression, the system acquires rigidity: it is the jamming transition. Jamming and glass transition are very old statistical mechanics problems that are often associated with one another, because intuition suggests that both phenomenons arise from the same physical effect, the caging of each particle by its shell of neighbours.

During my phd thesis, I studied analytically a model system of harmonic spheres (soft spheres that repel each other with finite amplitude and finite range), that allows to simultaneously study the jamming and the glass transition. I will present results obtained on the dynamics near the glass transition, as well as thermodynamics near the jamming point, with field theoretic methods and replica theory.

# Jeudi 26 Avril 2012 à 14h

## Title: Quantum phase slips in 1D Josephson junction chains

**G. Rastelli (LPMMC, Grenoble)).**

**Abstract: ** One-dimensional Josephson junction chains (1D-JJ chains) are paradigmatic systems to study the correlations between different elements in superconducting Josephson junction nanodevices. Their use has been proposed for the realization of a qubit topologically protected against decoherence [1] or for the realization of a fundamental current standard in quantum metrology [2].

The quantum ground state of the chain is ruled by the competition between the Josephson effect and the electrostatic interactions which contrast the charge transfer. This effect corresponds to an increase of the quantum fluctuations of the local phase of the condensate on the islands [3]. Indeed, in the thermodynamic limit, the theory predicted a quantum superconductor-insulator phase transition [4]. However the experimental devices designed for the above-mentioned applications are generally composed by a finite number of superconducting elements. I will discuss the effect of quantum phase fluctuations in 1D-JJ chains of finite length [5]. Some comparisons with the experiments will be also presented.

- [1] S. Gladchenko et al., Nature Physics 2009 ; B. Douçot, J. Vidal, PRL 2002 ; Douçot, M. V. Feigelman, L. B. Ioffe, PRL 2003.
- [2] W. Guichard, F.W.J.Hekking Phys. Rev. B 2010. ; J. Flowers, Science 2004.
- [3] K. A. Matveev, A.I. Larkin, L. I. Glazman, PRL 2002.
- [4] R. M. Bradley, S. Doniach, PRB 1984 ; S. E. Korshunov, Sov. Phys. JETP 1986-1989.
- [5] G. Rastelli et al., arXiv :1201.0539 (submitted to PRB).

# Jeudi 12 et 19 avril 2012

## Congés de Printemps

# Jeudi 29 Mars 2012 à 14h (amphi D)

## Title: Fluctuation-Dissipation relations for nonequilibrium systems

**Matteo Colangeli (Politechnico di Torino)**

**Abstract: TBA**

# Mardi 27 Mars 2012 à 14h (amphi H)

## Title: Holographic Correlation Functions

**Tristan McLoughlin (Albert Einstein Institute (Postdam))**

**Abstract: ** In this talk we discuss recent progress in the calculation of quantum field theory correlation functions using the AdS/CFT correspondence. In particular, we describe the strong coupling description of three-point functions in planar N=4 super Yang-Mills theory by semiclassical strings. We show that such semiclassical calculations agree with the exact known answers in special protected cases, and provide a starting point for understanding generic operators.

# Jeudi 22 Mars 2012 à 14h

## Title: Josephson parametric amplifiers for quantum information processing. (Colloquium)

**B. Huard (LPA - Ecole Normale Supérieure de Paris).**

**Abstract:** Nowadays, it is possible to control and measure the quantum state of systems with a few degrees of freedom, whether they are microscopic objects like cold atoms and single photons, or macroscopic objects like superconducting circuits. Such delicate experiments require an interface which bridges the gap of orders of magnitudes in energy between the quantum object and the data acquisition system. This problem is solved by an active device: the amplifier.

Although the amplifier is necessary to overcome the noise at the stage of data acquisition, it eventually alters the signal. We have developed an amplifier for microwave signals which adds only the minimum of noise allowed by quantum laws: the equivalent of half a photon of noise referred to the input. Our amplifier is based on a superconducting circuit of Josephson junctions in superconducting resonators.

In this talk, I will review the principles of parametric amplification with superconducting circuits and show the quantum limits specific to the different types of amplifiers. I will present applications of quantum limited amplifiers for quantum information processing using superconducting qubits and pairs of twin microwave beams. In particular, I will show that the amplifier we developed allows a direct measurement of the quantity of entanglement between two beams.

# Jeudi 15 Mars 2012 à 14h

## Title: An impurity in a Fermi sea on a narrow Feshbach resonance: A variational study.

**Christian Trefzger (ENS Paris).**

**Abstract:** We study the problem of a single impurity of mass M immersed in a Fermi sea of particles of mass m [1]. The impurity and the fermions interact through a s-wave narrow Feshbach resonance, so that the Feshbach length R* naturally appears in the system. We use simple variational ansatz, limited to at most one pair of particle-hole excitations of the Fermi sea and we determine for the polaronic and dimeronic branches the phase diagram between absolute ground state, local minimum, thermodynamically unstable regions (with negative effective mass), and regions of complex energies (with negative imaginary part). We also determine the closed channel population which is experimentally accessible. Finally we identify a non-trivial weakly attractive limit where analytical results can be obtained, in particular for the crossing point between the polaronic and dimeronic energy branches.

[1] Christian Trefzger, Yvan Castin, arXiv:1112.4364

# Jeudi 9 Mars 2012 à 14h

## Title: Ground-state phase diagram of the quantum J1-J2 model on the honeycomb lattice

**F. Mezzacapo (Max Planck Institute for Quantum optics). **

**Abstract:** Frustrated quantum antiferromagnets are a subject of current intense research. Frustration can arise either from the geometry of the system, or from competing interactions, and can lead to the stabilization of novel, exotic (magnetic and non-magnetic) phases of matter. The antiferromagnetic spin-1/2 Heisenberg Hamiltonian in presence of next nearest-neighbor coupling is a prototypical example of quantum spin model (usually referred as the J1-J2 model ) featuring interaction induced frustration. Such a model is of relevance for experimentally accessible compounds and has been proposed as an effective description to characterize the spin-liquid phase of the half-filled Hubbard model on the honeycomb lattice [Z. Y. Meng et al., Nature (London) **464**, 847 (2010)]. For these reasons it has been recently investigated by means of various computational approaches, however, different studies have yielded conflicting physical scenarios. For example it is not clear if the model features a disordered ground state (GS) for any value of J2/J1 and even the nature of the ordered phases remains controversial.

In this talk I will present results of a variational study on the GS phase diagram of the quantum J1-J2 model on the honeycomb lattice. Values of energy and relevant order parameters are computed, in the range 00.4 (collinear). In the intermediate region, the GS is disordered. The results discussed here also show that the reduction of the half-filled Hubbard model to the J1-J2 one does not yield accurate predictions.

# Jeudi 1 Mars 2012 à 14h

## Title: Holographic fluids, vorticity and analogue gravity

**M. Petropoulos (CPhT - Ecole Polytechnique).**

**Abstract:** In vew of the recent interest in reproducing holographically various properties of conformal fluids, I will analyze the emergence of rotation and vortices in the framework of AdS/CFT. The boundary backgrounds involved in this study turn out to exhibit interesting relationships with sailing in drift currents or sound wave propagation in moving media. The latter opens the way to the holographic description of analogue-gravity models.

# Jeudi 23 février 2012

## Title: The group field theory description of quantum spacetime

**Daniele Oriti (Postdam)**

**Abstract:** We present an introduction to group field theories and tensor models, as a framework for the dynamics of quantum spacetime. They are a generalization of matrix models for 2d quantum gravity, incorporating insights from other approaches like spin foam models and simplicial quantum gravity, as well as tools from non-commutative geometry. We review briefly recent developments, and, if time allows, discuss in more detail some of them.

# Jeudi 9 Février 2012 à 14h

## Title: Perturbative quantum gravity with Immirzi parameter.

**Simone Speziale (CPT Marseille)**

**Abstract: ** If one uses a first order action principle for general relativity, there is a third fundamental coupling constant that appears, next to Newton's and the cosmological constants. It enters the coupling of the gravitational field to fermions, and it is oftent referred to as the Immirzi parameter, for historical reasons. In this talk, I will review this formulation of general relativity and the meaning of the parameter. I will then present recent results on the 1-loop quantum effective action which leads to a non-trivial running of the Immirzi parameter, and discuss possible implications for non-perturbative quantum gravity.

# Jeudi 2 Février 2012 à 14h

## Title: Random matrix ensembles for quantum spin decoherence.

**F. David (SPhT - CEA Saclay).**

**Abstract: ** I present a class of random matrix ensembles relevant for the study of quantum decoherence for quantum spins. These ensembles generalize the standard GUE ensemble. For a single spin j, they lead to exact solutions for the dynamics of decoherence and for quantum diffusion. I discuss the general non-Markovian case, the Markovian limits and the quantum-to-classical transition.

# Lundi 30 janvier 2012 à 16h (salle 117)

## Title: A Geometric Classification of Supersymmetric Solutions in String Theory

**Alessandro Tomasiello (Université de Milano-Bicocca)**

**Abstract:** Supersymmetry is a conjectural new symmetry of the universe, which would help solve some of its puzzles. In this talk, we will consider supersymmetric solutions in type II string theory; we will describe a system of equations which reformulates supersymmetry in terms of differential forms, without any need to resort to spinors. This extends to any spacetime a similar method already available for vacuum compactifications, i.e. manifolds of the type AdS4 x M6 or Minkowski4 x M6, where M6 is compact, which led to constraints on M6 involving Hitchin's "generalized complex geometry". In our new, more general setup no factorization of spacetime is assumed.

# Jeudi 26 Janvier 2012 à 14h (salle 115)

## Title: Quantum Critical Magnetization Behaviors of the Kagome- and Triangular- Lattice Antiferromagnets

**T. Sakai (Condensed MAtter Theory Group, Spring-8, Harima).**

**Abstract: ** Magnetization process of the S=1/2 isotropic Heisenberg antiferromagnets on the Kagome and triangular lattices are studied. Data from numerical- diagonalization method up to 39-spin systems, are reexamined from the viewpoint of the derivative of the magnetization with respect to the magnetic field. We find that the behavior of the derivative around the 1/3 height of the magnetization saturation is quite different from the cases of typical magnetization plateaux for the Kagome-lattice antiferromagnet. This new phenomenon is called the *magnetization ramp* [1]. We also compare it with the 1/3 magnetization plateau of the triangular antiferromagnet. The critical exponent analysis indicates a clear difference between the magnetization plateau and ramp [2]. In addition using the numerical diagonalization up to 42-spin systems we suggest that the kagome-lattice antiferromagnet has a gapless singlet- triplet excitation in the thermodynamic limit [3].

[1] H. Nakano and T. Sakai: J. Phys. Soc. Jpn. **79** (2010) 053707.

[2] T. Sakai and H. Nakano: Phys. Rev. B **83** (2011) 100405(R).

[3] H. Nakano and T. Sakai: J. Phys. Soc. Jpn. **80** (2011) 053704.

# Jeudi 19 Janvier 2012 à 14h

## Title: Coherence of Single Electron Sources from Mach-Zehnder Interferometry

**G. Haack (Université de Genève).**

**Abstract: ** A new type of single electron sources (SES) has emerged which permits to inject single particles in a controllable manner into an electronic circuit. Multiparticle exchange, two-particle interference effects, entanglement and HBT experiments have already been proposed. Here we determine the coherence length of the single-particle states analyzing the decay of Aharonov-Bohm oscillations as a function of the imbalance of a Mach-Zehnder interferometer connected to an SES. This single-particle coherence length is of particular importance as it is an intrinsic property of the source in contrast to the dephasing length.

# Jeudi 12 Janvier 2012 à 14h

## Title: Waiting times distribution of electrons flowing across mesoscopic conductors

**M. Albert (LPS, Orsay).**

**Abstract: ** Electronic transport through mesoscopic devices is known to be stochastic due to the quantum nature of the charge carriers. The noise power spectrum as well as the Full Counting Statistics (FCS) provide many important informations about the system under study as it has been shown during the past 20 years. However the distribution of waiting times (WTD) between the detection of several charge carriers has been recently investigated and shown to be very powerful to understand the short time physics and correlations between different elementary events [1,2] in the same spirit than the level spacing distribution in the spectral statistics of complex systems. In this talk we will use this quantity to discuss the short time correlations in a perfect one dimensional quantum channel with a quantum point contact. Although the system is extremely simple, the WTD reveals quite striking transport properties that can be explained using random matrix theory in a totally unexpected context. Some other quantum states, such as a train of Lorentzian pulses [3] will be also considered and the relation between the WTD and the FCS also discussed.

[1] T. Brandes, Ann. Phys. (Berlin) **17**, 477 (2008).

[2] M. Albert, C. Flindt and M. Buttiker, Phys. Rev. Lett. **107**, 086805 (2011)

[3] J. Keeling, I. Klich, L. S. Levitov, Phys. Rev. Lett. **97**, 116403 (2006)

# Lundi 9 janvier 2012 a 15h30 en salle 117

**Andres Anabalon Dupuy, Science Department, Universidad Adolfo Ibanez (Chili)**

## Title: Black Holes and Wormholes for a Self Interacting Scalar Field in asymptotically (anti) de Sitter Spacetime.

**Abstract:** TBA

# Jeudi 22 et 29 Décembre 2011

## Pas de séminaire: congés de Noël

# Mardi 13 Décembre 2011 à 14h (date et heures exceptionnels)

## Title: Renormalization Group Approach to Quasi-One Dimensional Systems.

**Samuel Moukouri (Racah Institute of Physics, Hebrew University, Jerusalem)**

**Abstract:** The advent of high performance computing and the development of sophisticated numerical techniques have opened new vistas for researchers in condensed matter physics. Exciting predictions of new quantum phases of matter in model systems can often be substantiated or falsified by numerical methods. In this talk, I will present recent development in applying the celebrated density-matrix renormalization group method to quasi-one dimensional systems. I will show the power of this technique by computing, with high accuracy, critical points in quantum phase transitions induced by small inter-chain interactions. Illustrations are made on models for magnetic frustration and for the Mott transition.

# Jeudi 8 Décembre 2011 à 14h

## Title: Non-equilibrium phase transition for a system of diffusing-coalescing particles with deposition and evaporation

**O. Zaboronsky (Université de Warwick & ENS Lyon)**

**Abstract:** In 1998 Majumdar and collaborators introduced a model of diffusing- aggregating massive particles with extra evaporation and deposition processes. Basing on mean field analysis and numerical simulations they conjectured that this model undergoes a non-equilibrium phase transition from the state with zero flux of mass toward large masses to the state with a positive asymptotically constant flux. By combining global properties of the Markov chain describing the system with the analysis of low order moments we give a rigorous proof of Majumdar's conjecture. Joint work with Roger Tribe, Colm Connaughton and R. Rajesh.

# Lundi 5 Décembre 2011 à 15h30

## Title: Title: Vacua analysis in extended supersymmetry compactifications

**G. Dibitetto (Université de Groningen)**

**Abstract:** We consider truncations of half-maximal and maximal supergravity theories in four dimensions and we analyse the landscape of vacua for those embedding tensor configurations having a string theory origin in terms of geometric flux compactifications. The full dictionary between fluxes and embedding tensor components is worked out, the gaugings are identified and the full mass spectra are computed. The search of vacua is carried out by applying algebraic geometry techniques which allows for complete analytical treatment.

We furthermore discuss the link with duality-covariant extensions of the concept of backgrounds in string theory, such as Double Field Theory and (Exceptional) Generalised Geometry.

# Jeudi 1 Décembre 2011 à 16h (salle 117)

## Title: The 1/N Expansion and The Continuum Limit in Colored Tensor Models.

**Razvan Gurau (Perimeter Institute)**

**Abstract:** Matrix models are one of the most versatile tools in theoretical physics with applications ranging from the theory of strong interaction, to string theory, critical phenomena and two dimensional quantum gravity. In higher dimensions matrix models generalize to tensor models. Ordinary tensor models do not admit a meaningful 1/N expansion, and no analytic result could be establish on their continuum limit. In this talk I will give an overview of recent results for the colored tensor models. Such models have been shown to admit a 1/N expansion dominated by graphs of spherical topology. The leading sector is summable and the tensor models undergo a phase transition to a continuum theory. I will conclude by an overview of results on the continuum limit for various specific models.

# Jeudi 1 Décembre 2011 à 14h

## Title: Discrete complex analysis and statistical physics

**H. Duminil-Copin (Université de Geneve).**

**Abstract:** Discrete harmonic and discrete holomorphic functions have been proved to be very useful in many domains of mathematics. Recently, they have been at the heart of the two dimensional statistical physics (for instance, through the works of Kenyon, Schramm, Smirnov and others...). We will present some of the connections between discrete complex analysis and statistical physics. In particular (it is a joint work with S. Smirnov), we will use discrete holomorphic functions to prove that the number a_n of self-avoiding walks of length n (starting at the origin) on the hexagonal lattice satisfies a_n^{1/n}=\sqrt{2+\sqrt 2} when n goes to infinity thus answering a conjecture made by Nienhuis in 1982.

# Jeudi 24 Novembre 2011 à 14h

## Title: Diffusion anormale d'un polymère dans un fondu non enchevêtré.

**Jean Farago (Institut Charles Sadron, Strasbourg)**

**Abstract:** Contrairement à ce qui est d'ordinaire admis, les interactions hydrodynamiques ne sont pas écrantées dans un fondu de polymères au-delà de la taille des monomères et sont importantes dans les régimes transitoires, très longs pour les polymères. Nous montrons que les interactions visco-hydrodynamiques sont responsables d'une dynamique anormale universelle qui s'étend jusqu'au temps de Rouse (~N²); la diffusion du centre de masse d'un polymère marqué est accélérée par rapport à la prédiction du modèle de Rouse d'une quantité importante, qui croît avec la taille du polymère. Nous avons développé une théorie analytique de ces effets viscoélastiques qui se compare quantitativement avec les simulations numériques, sans paramètre ajustable.

Cette théorie s'adapte aussi au cas d'une dynamique stabilisée par un thermostat de Langevin (très utilisée en simulation numérique), qui, de manière inattendue, affecte fortement et à temps longs la dynamique relaxationnelle du centre de masse, celle-ci restant néanmoins fortement accélérée. Ce problème permet également la comparaison de différentes approches théoriques, réponse linéaire, hydrodynamique fluctuante et couplage de modes, qui dans ce cas précis donnent des éclairages complémentaires sur le phénomène étudié et des résultats similaires.

# Jeudi 17 Novembre 2011 à 14h (salle 116!)

## Title: Strong back-action of a linear circuit on a single electronic quantum channel

**Anne Anthore (Laboratoire de Photonique et Nanostructures et université Denis Diderot)**

**Abstract: **How are the transport properties of a coherent conductor modified by its surrounding circuit? This fundamental question is also of practical importance for the engineering of composite quantum devices. When a coherent conductor is inserted into a circuit, its conductance is reduced due to the circuit back-action in response to the granularity of charge transfers. This phenomenon, called dynamical Coulomb blockade, has been extensively studied for a tunnel junction. However, for arbitrary short coherent conductors, it is fully understood only in the limit of small conductance reductions and low-impedance surrounding circuits.

We have investigated experimentally the strong back-action regime of a linear circuit on a single electronic conduction channel of arbitrary transmission. This was achieved by using a quantum point contact (QPC) as a test-bed for coherent conductors. The QPC was embedded in an adjustable on-chip circuit of impedance comparable to the resistance quantum RK= h/e² at microwave frequencies, leading to conductance reductions up to 90%. An in-situ short-circuit technique allows us to extract the back-action signal in the most direct way, by probing the QPC conductance in presence and in absence of the circuit back-action.

From our results, we propose a generalized expression for the conductance of an arbitrary single quantum channel embedded in a linear environment. The proposed expression is in good agreement with recent predictions derived for a purely ohmic environment

# Jeudi 3 Novembre 2011

## Title: Couplage électron-phonon dans les isolants topologiques

**S. Giraud (Dusseldorf)**

**Abstract:** Les isolants topologiques sont des matériaux isolants dans le bulk mais qui permettent le transport de charges sur leurs bords. Leur découverte récente, d'abord en dimension 2 dans des puits quantiques de HgTe, puis en dimension 3 dans une classe de matériaux tels que Bi2Se3 ou Bi2Te3, a ouvert la voie à de nombreuses applications notamment dans les domaines de la spintronique ou de l'informatique quantique. Parallèlement, plusieurs nouveaux phénomènes physiques ont pu être prédits. L?existence de ces états de bords est protégée par un invariant topologique caractéristique de la structure de bandes du bulk et une propriété essentielle de ces matériaux est l'absence totale de diffusion en dimension 2 ou une réduction drastique de l'espace des phases des états de diffusion en dimension 3. L'étude des isolants topologiques a été réalisée jusqu?à présent sans interaction. Cela était justifié par la protection topologique des états de bords. Cependant, la situation est encore mal comprise, d?autant plus que dans certains cas on s?attend à de fortes interactions. En utilisant une théorie effective de basse énergie pour l?état de surface des isolants topologiques 3D, nous avons récemment analysé les conséquences du couplage électron-phonon. Nous avons ainsi pu prédire différentes grandeurs physiques comme la durée de vie des quasi-particules ou la résistivité surfacique, qui sont en très bon accord avec les premiers résultats expérimentaux.

# Jeudi 27 Octobre 2011

## Title: Quantum Hamiltonians for the SU(2) WZW model.

**German Sierra (Institute of Theoretical Physics CSIC-UAM, Madrid)**

**Abstract:** Comment obtenir des généralisations des chaines de Haldane-Shastry par la théorie conforme des champs...

# Jeudi 20 Octobre 2011 à 14h (salle 115!)

## Title: Structures intégrables de type Calogero-Sutherland dans les théories conformes; applications à l'effet Hall quantique fractionnaire

**B. Estienne (Instituut voor Theoretische Fysica, Universiteit van Amsterdam)**

**Abstract:** Depuis son introduction il y a 40 ans, le modèle de Calogero-Sutherland a attiré beaucoup d'attention en physique théorique. Un récent développement concerne le lien entre ce système intégrable et les théories conformes en 2d. En particulier les blocs conformes permettent de construire des états propres de Calogero-Sutherland possédant des monodromies non-triviales. L'échange des particules est décrit par une représentation non-triviale du groupe de tresse: on parle alors d'anyons non-abéliens. En matière condensée, ces fonctions d'ondes apparaissent dans la description des excitations (quasi-trou) de l'effet Hall quantique fractionnaire.

Je commencerai par décrire le lien entre Calogero-Sutherland et théories conformes, ainsi que la structure intégrable sous-jacente. Dans un deuxième temps, je parlerai des conséquences pour les fonctions d'ondes de l'effet Hall quantique fractionnaire: dualité entre électrons et quasi-trous, et applications pour les simulations numériques.

# Jeudi 13 Octobre 2011 à 14h

## Title: Spin Hall effect at interfaces between topological insulators and metals

**Marine Guigou (Univ. Wurzburg)**

**Abstract:** The Spin Hall effect (SHE) is a physical phenomenon realized in nonmagnetic systems and that allows for a transverse spin current generated if an electrical charge current is driven in longitudinal direction. This can happen due to impurity scattering [Hir99], called extrinsic SHE, or due to band structure effects [Sin04], called intrinsic SHE. Both cases have been experimentally observed: the extrinsic SHE in semiconductor heterostructures and the intrinsic SHE in HgTe/CdTe heterostructures by combining the SHE with the so-called quantum spin Hall effect (QSHE) in a single device [Bru10]. The QSHE is yet another type of spin Hall effect that exists at the boundary of a two-dimensional topological insulator realized in HgTe/CdTe quantum wells [Ber06,Kon07]. In this presentation, we will first show the existence of a new type of interface SHE at junctions between Quantum Spin Hall Iinsulators and metals (normal and superconducting ones). The new type of SHE is intimately related to the coexistence of propagating and evanescent modes at the interface between a QSHI and a metal. Interestingly, this happens in the absence of structure and bulk inversion asymmetry within each subsystem. Secondly, we functionalize these findings to propose a device for all-electric spin injection into normal metal leads [Gui11].

[Ber06] B.A. Bernevig, T. L. Hughes, and S.C. Zhang, Science **314**, 1757 (2006).

[Bru10] C. Bruene, A. Roth, E.G. Novik, M. Koenig, H. Buhmann, E.M. Hankiewicz, W. Hanke, J. Sinova, and L.W. Molenkamp, Nature Physics **6**, 448 (2010).

[Hir99] J.E. Hirsch, Phys. Rev. Lett. **83**, 1834 (1999).

[Kon07] M. Koenig, S. Wiedmann, C. Bruene, A. Roth, H. Buhmann, L.W. Molenkamp, X.L. Qi, and S.C. Zhang, Science **318**, 766 (2007).

[Sin04] J. Sinova, D. Culcer, Q. Niu, N.A. Sinitsyn, T. Jungwirth and A.H. MacDonald, Phys. Rev. Lett. **92**, 126603 (2004).

[Gui11] M. Guigou, P. Recher, B. Trauzettel, and J. Cayssol, arXiv:1102.5066.

# Mercredi 12 octobre a 14h en salle 115 (exceptionnel)

## Title: Generalised geometry and E11

**Peter WEST (King's College, London)**

**Abstract:** I will explain how generalized geometry was contained in the E11 proposal and discuss some recent applications.

# Lundi 10 octobre a 13h30 en salle 115 (exceptionnel)

## Long-lived qubits in atomic systems

**Hui-Khoon NG (Center for Quantum Technologies, National University of Singapore) **

**Abstract:** In this talk, I will present a scheme for qubits stored in clusters of three atoms that are long-lived against decoherence from fluctuating magnetic fields, a limiting source of noise in many experiments. Each qubit is stored in a rotationally invariant subsystem of the total angular momentum states of the three atoms, and can persist for time-scales on the order of hours, compared to milliseconds for an unprotected qubit. I will first present the theoretical scheme of rotationally invariant subsystems in atomic systems, and then move on to discuss current work at CQT on an experimental scheme to demonstrate the persistence of the qubit. This includes methods for state preparation via Rydberg blockade, state tomography via light scattering, as well as novel techniques for state estimation with sparse data.

# Lundi 10 octobre a 15h en salle 115 (exceptionnel)

## Title: The classical integrable structure of AdS/CFT

**Benoit VICEDO (DESY, Hamburg)**

**Abstract:** I will present the classical integrable structure which underlies all know integrable superstring theories on AdS-spaces with CFT duals. It is described by a standard r-matrix on the underlying twisted loop algebra. However, the latter is equipped with a non-standard inner product which encodes the non-ultralocality of the sigma-model.

# Jeudi 6 Octobre 2011 à 14h

## Title: Textures généralisées pour l'effet Hall quantique entier et modes collectifs

**Benoit Doucot (LPTHE, université Pierre et Marie Curie)**

**Abstract: ** Il existe plusieurs systèmes bidimensionnels dans lesquels les électrons portent, en plus de leur spin, un autre degré de liberté interne auquel est associé un nombre fini d'états quantiques. On peut penser par exemple à des bicouches de gaz d'électrons bidimensionnels, ou encore au graphène, avec ses deux branches d'excitations linéaires au bord de la première zone de Brillouin. En présence d'un fort champ magnétique, l'Hamiltonien cinétique est quantifié en niveaux de Landau fortement dégénérés. Lorsque la densité électronique correspond au remplissage exact du plus bas de ces niveaux, la répulsion Coulombienne induit des processus d'échange qui sélectionnent un état fondamental ferromagnétique. Mais si le facteur de remplissage diffère légèrement de l'unité, cet ordre magnétique est remplacé par un réseau de textures appelées Skyrmions. Je montrerai comment étendre la construction de tels états lorsque le spin 1/2 de l'électron est remplacé par un degré de liberté interne avec $d$ états distincts. Je montrerai ensuite comment on peut obtenir les modes d'excitations collectives au voisinage de ces textures généralisées dans une approche de Hartree-Fock dépendant du temps. Ce travail est issu d'une collaboration avec Roderich Moessner et Dmitri Kovrizhin (MPI Dresden).

# Jeudi 22 Septembre 2011 à 14h (salle 115!)

## Title: New results inspired by quantum gravity: topological models and statistical systems coupled to random lattices

**Valentin Bonzom (Perimeter Institute)**

**Abstract: ** In this talk, I will present an overview on results which have appeared this year. While they are inspired from quantum gravity issues, they have a pretty large spectrum. I first focus on a new form of a generalized Kitaev Hamiltonian for topological order, which led to a new way of solving 2+1 quantum gravity as well as to new semi-classical formulae for re-coupling of quantum angular momenta. Those systems are based on fixed lattices. The second part is instead dedicated to statistical systems coupled to random lattices in dimension three and higher, an exciting field which generalizes matrix models.

# 2010-2011

# Jeudi 30 Juin 2011 à 14h

## Soutenance de thèse: Optique quantique électronique

**Charles Grenier (ENS Lyon)**

**Abstract:** Les progrès des techniques de nanofabrication des dix dernières années ont permis la mise en place de protocoles visant à manipuler les charges uniques dans les nanostructures. Ces nouvelles techniques permettent d'envisager la réalisation d'expériences d'optique quantique avec des électrons. Cette thèse s'inscrit dans ce contexte.

Le but de ce travail a été la construction d'un formalisme adapté à la description de telles expériences. Ce formalisme, construit en analogie avec la théorie de la cohérence quantique du champ électromagnétique de Glauber, souligne les similitudes et différences entre les photons se propageant dans le vide, et le transport électronique dans des conducteurs balistiques unidimensionnels. En particulier, il rend compte de la décohérence et de la relaxation en énergie des excitations électroniques en présence d'interactions. Un autre aspect de cette thèse a été la proposition de protocoles permettant de mesurer des quantités directement reliées aux propriétés de cohérence décrites par le formalisme de l'optique quantique électronique. En particulier, un protocole de tomographie quantique reposant sur l'effet Hanbury Brown et Twiss a été proposé pour reconstruire la cohérence à un corps d'une source d'excitations mono-électroniques. Ce protocole peut aussi être envisagé pour obtenir des informations sur les mécanismes de décohérence.

**Jury:** Markus Buttiker, Pascal Degiovanni, Christian Glattli, Frank Hekking, Peter Holdsworth.

# Mardi 28 juin (salle 117) à 14h

## Title: The AdS(5) x S(5) semi-symmetric space sine-Gordon theory

**Speaker: J. Luis MIRAMONTES (Universidad de Santiago de Compostela, Espagne)**

**Abstract:** The motion of strings on (semi)symmetric space target spaces underlies the integrability of the AdS/CFT correspondence. Although the relevant theories, whose excitations are giant magnons, are non-relativistic, they are classically equivalent, via the Pohlmeyer reduction to a family of 2-d relativistic integrable field theories known as (semi-)symmetric space sine-Gordon (S-SSSG) theories. Moreover, it has been conjectured that this equivalence could extend into the full quantum theory. In this talk I will review the main features of the AdS_5 x S^5 S-SSSG theory, including the semiclassical quantization of their soliton spectrum which has been recently sorted out in ArXiv:1104.2429. It exhibits supersymmetry and leads to a natural conjecture for their S-matrix formulation.

# Jeudi 23 Juin 2011 à 14h

## Title: Quench Dynamics in Interacting one-dimensional systems

**Aditi Mitra (New York University)**

**Abstract:** Due to experiments in cold-atomic gases, the problem of quench dynamics which is the unitary time evolution of interacting quantum systems arising due to sudden change in system parameters has become a topic of great current interest. Fundamental questions such as whether systems thermalize at long times after a quench, what is the time-scale associated with thermalization, and the role played by integrability and system size are still largely open and a frontier of current research.

In this talk I will present results for the time-evolution of some 1-dimensional models and first show how various non-thermal steady states can arise at long times, at least for simple integrable models such as the XX spin-chain and the Luttinger liquid. Next I will address the issue of how stable these non equilibrium steady-states are to non-trivial interactions or mode-coupling. Employing analytic approaches such as perturbative renormalization group and random-phase-approximation, I will show that even infinitesimally weak interactions or mode-coupling generate a dissipation (and hence a finite lifetime for the modes), and also a finite temperature. However the notion of the temperature even with interactions can be quite subtle as it can depend in a non-trivial way on both the frequency as well as the momenta of the modes.

# Jeudi 16 Juin 2011 à 14h

## Soutenance d'Habilitation à diriger des recherches: STATISTICAL MECHANICS OF SYSTEMS WITH LONG RANGE INTERACTIONS AND OF GEOPHYSICAL TURBULENT FLOWS

**Freddy Bouchet**

**Abstract:** Devising theories for the dynamics of the largest scales of turbulent flows, and their applications to geophysical problems is a current challenge in statistical mechanics. It turns out that vortices in two dimensional flows, planetary atmospheres, and large scale ocean flows interact non locally. These long range interactions lead to peculiarities for the statistical mechanics of those systems: exotic phase transitions, negative heat capacity (the temperature increase when the energy is decreased), self-organized dynamical behavior leading to anomalous diffusion and long relaxation times, and so on. During the last ten years, the foundations of the statistical mechanics of systems with long range interactions have been precised and completed through joint efforts of a large community. We will present contributions to this domain including a classification of phase transitions, prediction and observations of generic anomalous diffusion, explanation of generic ergodicity breaking, construction of microcanonical measures for the 2D Euler and Vlasov equations.

Those theoretical progresses have had some applications in astrophysics, plasma physics, cold atom physics. We will emphasize applications to two dimensional and geophysical turbulence: predictions and experimental observation of non-equilibrium phase transitions and statistical ensemble inequivalence in two-dimensional turbulence, explanation of the structure and the drift properties of ocean vortices, and characterization of the long time inviscid dissipation of two-dimensional flows. We will also discuss challenges in this domain, mainly the development of the non equilibrium statistical mechanics of far from equilibrium turbulent flows.

**Jury:** Bernard Castaing, Cristel Chandre, Henk Dijkstra, Krzysztof Gawedzki, David Mukamel, Sergey Nazarenko, Cédric Villani

# Mardi 14 Juin 2011 à 14h (salle 116)

## Title: Towards the quantum S-matrix of the Pohlmeyer reduced form of AdS_5 x S^5 superstring theory

**Benjamin HOARE (Imperial College, Londres).**

**Abstract:** The Pohlmeyer reduced form of AdS_5 x S^5 superstring theory is a classical reformulation of the original superstring theory. Of particular interest is whether this equivalence extends beyond the classical level -- one way of investigating this is to construct the S-matrix of the reduced theory and compare to that of the superstring theory. In this talk we will review the construction of the reduced theory and using perturbation theory, integrability, symmetry and analogies with various truncations propose an exact quantum S-matrix for the Lagrangian-field excitations.

# Mercredi 8 Juin 2011 à 14h (colloquium de théorie, horaire exceptionnel)

## Title: Extreme Events in Turbulence, Populations and Finance

**M.H. Jensen (Niels Bohr Institute, University of Copenhagen)**

**Abstract:** Many phenomena in nature and society are governed by extreme events. Extreme events refer to very abrupt changes with a statistics that often do not follow normal distributions. Instead, they show "heavy/fat" tails indicating that the large and extreme events have much higher probability than one would expect from normal statistics [1]. We study extreme events in turbulence, populations and finance by mean forward statistics, measuring typical systemic changes over a specified time, resulting in the heavy tails. We further invoke threshold dynamics or "inverse" statistics by pre-describing a specific size an event and estimates the length of time before the first occurrence of this event [2]. In case of population dynamics the extreme events cause increased species competition and shorter fixation times [3]. For financial time series, inverse statistics leads to an observation of an optimal investment horizon and an asymmetry in the market between gains and losses [4].

[1] T. Bohr, M.H. Jensen, G. Paladin and A. Vulpiani, "*Dynamical Systems Approach to Turbulence*", Cambridge University Press (1998).

[2] M.H. Jensen, "*Multiscaling and Structure Functions in Turbulence: An Alternative Approach*", Phys. Rev. Lett. **83**, 76 (1999).

[3] S. Pigolotti, R. Benzi, M.H. Jensen and D.R. Nelson, "*Population genetics in compressible flows*", preprint (2010).

[4] R. Donangelo, M.H. Jensen, I. Simonsen and K. Sneppen, "*Synchronization Model for Stock Market Asymmetry*", J. Stat. Mech. **11**, L11001 (2006).

# Mercredi 1er Juin 2011 à 14h (colloquium de théorie, date exceptionnelle)

## Title: Corrélations fortes dans les niveaux de Landau du graphène -- du ferromagnétisme à l'effet Hall quantique fractionnaire

**Marc Goerbig (Université Paris Sud)**

**Abstract:** La découverte d'un effet Hall quantique "relativiste" dans le graphène en 2005 a montré que les électrons de basse énergie dans ce matériau bidimensionnel (2D) sont décrits par une équation de Dirac (pour des fermions sans masse) plutôt que par une équation de Schrödinger. Une question naturelle à être posée est la suivante : quid de l'effet Hall quantique fractionnaire ? ou bien plus généralement : quid des interactions dans les niveaux de Landau ? Est-ce qu'elles réflète également le comportement ultra-relativiste des électrons dans le graphène ? Ce seminaire tente d'illustrer ces questions du point de vue d'un théoricien, tout en tenant compte des récents progrés expérimentaux sur le sujet.

# Jeudi 19 Mai 2011 à 14h

## Title: Quantum Hall transitions and conformal restriction

**Ilya Gruzberg (Univ. Chicago)**

**Abstract:** Disordered electronic systems exhibit continuous quantum phase transitions between insulating and conducting phases (Anderson transitions). The nature of the critical state at and the critical phenomena near such a transition are of great current interest. A famous example is the integer quantum Hall (IQH) plateau transition. In spite of much effort over several decades, an analytical treatment of most of the critical conducting states in disordered electronic systems has been elusive. We propose to use the recently developed rigorous theory of conformal restriction and Schramm-Loewner evolutions to study the IQH and other Anderson transitions in two dimensions, assuming conformal invariance at these critical points. We consider the so-called point contact conductances (PCC) and obtain, for the first time, exact analytical results for PCC's in the presence of a variety of boundary conditions at the IQH and similar critical points.

# Jeudi 12 Mai 2011 à 14h

## Title: Mélanges Bose-Fermi dans un potentiel 1D désordonné

**Francois Crépin (LPS Orsay)**

**Abstract:** Les systèmes unidimensionnels permettent une étude approfondie des effets conjoints du désordre et des interactions sur la localisation d'un gaz de fermions ou de bosons [1]. Alors que des fermions sans interactions sont invariablement localisés par un potentiel aléatoire, une transition vers une phase superfluide se produit pour des interactions suffisamment attractives. De manière analogue, de fortes interactions répulsives provoquent la localisation d'un gaz de bosons [1,2].

Lors de ce séminaire je présenterai l'étude d'un mélange de bosons et de fermions sans spin en interaction dans un potentiel aléatoire. Les corrélations superfluides sont augmentées par les interactions inter-espèce, alors que le potentiel de désordre essaye d'accrocher chacune des composantes du gaz. En utilisant les méthodes du groupe de renormalisation, complétées par un calcul variationnel, nous avons mis en évidence l'existence de plusieurs phases, localisées ou superfluides, incluant une nouvelle phase isolante, analogue au verre de Bose, où les deux espèces sont localisées et en interaction [3]. Le calcul du facteur de structure dynamique, pour des paramètres expérimentaux typiques de certaines expériences d'atomes froids, a permis de mettre en évidence des signatures de chacune de ces phases, pouvant typiquement être observés par diffusion de Bragg.

[1] T. Giamarchi and H.J. Schulz, Phys. Rev. B **37**, 325 (1988)

[2] M.P.A Fisher et al, Phys. Rev. B **40**, 546 (1989)

[3] F. Crépin, G. Zaránd, P. Simon, Phys. Rev. Lett. **105**, 115301 (2010)

# Jeudi 21 Avril 2011 à 14h

## Title: Quantum spin glass at T=0

**Alexei Andreanov (ICTP de Trieste)**

**Abstract:** Quantum spin glasses are glassy systems where both thermal and quantum fluctuations could drive the transition into a glassy phase. Since classical spin glasses turned are complicated and interesting models the questions of interest are How quantum fluctuations affect the glass phase: can they destroy it ? How the properties of the glass are affected ? What's the spectrum of excitations like in the glass phase ? We have studied a simple quantum spin glass, a transverse field Ising spin glass [2,3], where the strength of quantum fluctuations is tuned by a transverse field. There are a paramagnetic and a glassy phases [2,3]. The quantum phase transition at T=0 is well established however little is known about the glass phase dominated by quantum fluctuations. We have proved that the entire glass phase is critical and gapless giving rise to specific low lying excitations which can be interpreted as collective oscillators. For small transverse fields we have also discovered a fixed point in the Parisi flow identical to that unveiled by Pankov in classical spin glasses[3].

[1] A. Bray and M. Moore, J. Phys. C **13**, L655 (1980).

[2] J. Miller and D. Huse, Phys. Rev. Lett. **70**, 20 (1993) 3147.

[3] M.J. Rozenberg and D.R. Grempel, Phys. Rev. Lett. **81**, 12 (1998) 2550.

[4] S. Pankov, Phys. Rev. Lett. **96**, 197204 (2006..

# Jeudi 14 Avril 2011 à 14h

## Title: Relaxation rates of hot electrons in quantum wires

**Zoran Ristivojevic (ENS Paris)**

**Abstract:** Due to constraints imposed by the conservation laws, relaxation of hot electrons in quantum wires is suppressed if one considers two-body processes. Finite relaxation time can be achieved by three-body collisions. We will consider the cases of screened and unscreened Coulomb interaction and derive the corresponding relaxation rates, that behave as power laws of temperature. We will also discuss the role of three-body collisions for interaction induced corrections to conductance and thermopower in quantum wires.

# Jeudi 7 avril 2011 à 14h

## Title: Projective symmetry group approach of chiral phases

**Laura Messio (EPFL)**

**Abstract:** Quantum spin liquids are generally opposed to classical Néel states. In the Schwinger boson mean-field theory (SBMFT), the spin S is a continuous parameter and one can study connections between spin liquids and Néel states. Mean-field Ansätze respecting the lattice symmetries can be selected using projective symmetry groups (PSG) (Wen, PRB 2002 and Wang et al., PRB 2006). They lead to phases which can either be spin liquids if the gap is non zero (small S), or magnetized states (large S) with Goldstone modes. These Ansätze are labelled by gauge invariant quantities called fluxes.

We present an adaptation of the PSG approach to the classical spin limit and apply it to triangular and Kagome lattices. This helps us in sorting spin rotational symmetry breaking states. We show that well known chiral classical states are excluded in the orignal approach of Wen and Wang. But relaxing constraints, we are able to display three dimensional chiral Néel states wich under the effects of quantum fluctutions transpose in chiral spin liquids, breaking the time reversal symmetry. We propose models where these states could be ground states.

# Jeudi 31 Mars 2011 à 14h

## Title: Ultrastrong coupling cavity QED in solid-state systems

**Cristiano Ciuti, (Matériaux et Physique Quantique, Université Denis Diderot, Paris) **

**Abstract:** This talk will be devoted to cavity and circuit quantum electrodynamics (QED) in the ultrastrong coupling regime. Such an unconventional limit is achieved when the vacuum Rabi frequency (quantifying the light-matter interaction) is comparable or larger than the two-level transition frequency coupled to the bosonic field of a resonator. The ultrastrong coupling regime is being explored both theoretically and experimentally in semiconductor and superconducting systems[1]. After an introduction, here we will describe theoretically the quantum properties of a chain of Josephson atoms in a transmission line resonator, both in the case of inductive [2] and capacitive [3] coupling with the resonator field. Predictions and constraints will be presented for the occurrence of quantum phase transitions with the appearance of a doubly degenerate vacuum (ground state). The robustness and protection of the vacuum degeneracy and the manipulation of quantum information [4] stored in a ?vacuum? qubit will be presented.

[1] For semiconductors, see C. Ciuti, G. Bastard, I. Carusotto, Phys. Rev. B **72**, 115303 (2005); C. Ciuti, I. Carusotto, Phys. Rev. A. **74**, 033811 (2006); G. Günter et al. Nature **458**, 178 (2009); Y. Todorov et al., Phys. Rev. Lett. **105**, 196402 (2010).

In the case of superconducting circuits, see e.g. : M.H. Devoret, S.M. Girvin, R.J. Schoelkopf, Ann. Phys. **16**, 767 (2007); T. Niemczyk et al., Nature Physics **6**, 772-776 (2010).

[2] P. Nataf, C. Ciuti, *Vacuum degeneracy of a circuit-QED system in the ultrastrong coupling regime*, Phys. Rev. Lett. **104**, 023601 (2010) and references therein. [3] P. Nataf, C. Ciuti, *No-go theorem for superradiant quantum phase transitions in cavity QED and counter-example in circuit-QED*, Nat. Commun **1**, 72 (2010) and references therein. [4] P. Nataf, C. Ciuti, *Protected quantum computation with multiple resonators in ultrastrong coupling circuit QED*, submitted.

# Mercredi 23 Mars 2011 à 14 h

## Title: From Rotating Atomic Rings to Quantum Hall States

**Matteo Rizzi (Garching)**

**Abstract:** Considerable efforts are currently devoted to the preparation of ultracold neutral atoms in the emblematic strongly correlated quantum Hall regime. The routes followed so far essentially rely on thermodynamics, i.e. imposing the proper Hamiltonian and cooling the system towards its ground state. In rapidly rotating 2D harmonic traps the role of the transverse magnetic field is played by the angular velocity. The needed angular momentum is very large and it can be obtained only for spinning frequencies extremely near to the deconfinement limit; consequently, the required control on experimental parameters turns out to be far too stringent.

Here we propose to follow instead a dynamic path starting from the gas confined in a rotating ring. The large moment of inertia of such geometry facilitates the access to states with a large angular momentum, corresponding to a giant vortex. The trapping potential is then adiabatically transformed into a harmonic confinement, which brings the interacting atomic gas in the desired quantum Hall regime. We provide clear numerical evidence that for a relatively broad range of initial angular frequencies, the giant vortex state is adiabatically connected to the bosonic $\nu=1/2$ Laughlin state, and we discuss the scaling to many particles and the robustness against experimental defects.

# Jeudi 10 Mars 2010 à 14h

## Title: Interaction induced hierarchy of non-equilibrium locking

**Masud HAQUE (MPIPKS-Dresde)**

**Abstract:** In 1D interacting lattice systems, I will present geometry-induced structures in the energy specturm. A dramatic series of dynamics-suppression effects arise due to these spectral structures.

I will show versions of the phenomenon for three classic condensed-matter models: (1) the Bose-Hubbard model; (2) the spinless fermion model with nearest-neighbor repulsion; (3) the XXZ spin chain.

# Jeudi 24 Février 2011 à 14h

## Title: Fractionalization in three components fermionic atomic gases in a one dimensional optical lattice.

**Patrick Azaria (LPTMC, Université Pierre & Marie Curie) **

**Abstract:** We study a three component fermionic gas loaded in a one-dimensional optical trap at half filling. We find that the system is fully gapped and may order into 8 possible phases: four 2kF density wave and spin-Peierls phases with all possible Pi phase shifts between the three species. We find that trionic excitations are unstable towards the decay into pairs of kinks carrying a fractional number (3/2) of atoms. These sesquions eventually condense upon small doping and are described by a Luttinger liquid. We finally discuss the phase diagram of a three component mixture made of three hyperfine levels of Li6 as a function of the magnetic field.

# Jeudi 17 Février 2011 à 14h

## Title: Non-equilibrium dynamics of interacting fermions: From quantum spin chains to fermions in optical lattices

**Fabian Heidrich-Meisner (Ludwig-Maximilians-Universitaet, Munich)**

**Abstract:** The non-equilibrium properties of interacting fermions are attracting many theorists' attention. Besides the interest in thermalization and relaxation processes in quantum quenches and the study of steady-state problems, there is a group of experiments in which net currents are finite while a stationary regime is typically not reached. I will discuss two examples: first, the energy and spin dynamics in quantum spin chains and ladders and second, the sudden expansion of interacting fermions. In the former example we study the expansion of energy or spin-density wave packets aiming at classifying the dynamics as either ballistic or diffusive. We further determine the range of validity of a effective low-energy descriptions to the non-equilibrium dynamics of lattice models. In the second example, we study the expansion of interacting fermions in an optical lattice, after quenching the trapping potential to zero. Here we are particularly interested in the dependence on initial conditions such as filling [2] and the behavior of correlations during the expansion [3].

- [1] Langer, HM, Gemmer, McCulloch, Schollwoeck, Phys. Rev. B
**79**, 214409 (2009) - [2] HM, Rigol, Muramatsu, Feiguin, Dagotto, Phys. Rev. A
**78**, 013620 (2008) - [3] HM, Manmana, Rigol, Muramatsu, Feiguin, Dagotto Phys. Rev. A
**80**, 041603(R) (2009)

# Jeudi 10 Février 2011 à 14h

## Title: Quantum deformations of spin foam models

**Winston Fairbairn (Université de Hambourg) **

**Abstract:** Invariants of topological manifolds that are based on the representation theory of quantum groups play an important role in mathematical physics. In particular, they are of interest to quantum gravity, where they are known under the name of spin foam models. Compared to the `classical´ models which are based on the representation theory of Lie groups, the q-deformed models constructed upon the representation categories of quantum groups offer several advantages, the most important one being the improvement of the convergence properties.

In this talk, I will discuss q-deformations of spin foam models in three and four space-time dimensions. I will firstly review the derivation of the Ponzano-Regge model of 3d quantum gravity from a physical perspective and present the Turaev-Viro invariant as a natural regulator of its divergences. I will then discuss analogue constructions in four dimensions and present recent results concerning the q-deformation of a certain constrained topological model.

# Jeudi 3 Février 2011 à 14h (Colloquium)

## Title: Cavity QED in solid state physics: new insights and potential for quantum devices.

**Alexia Auffèves (Institut Néel, Grenoble) **

**Abstract:** Thanks to technological progresses in the field of solid-state physics, a wide range of quantum optics experiments previously restricted to atomic physics can now be implemented using solid-state emitters and cavities. Still, so-called artificial atoms are subjected to intrinsic decoherence processes that broaden the emission linewidth, making them very different from isolated atoms. At the same time, very high quality factors are achieved for state of the art cavities. These new conditions open an unexplored regime for cavity quantum electrodynamics (CQED) so far, where the emitter(s)'s linewidth can be of the same order of magnitude, or even broader than the cavity mode one. In this talk, I will focus on two different realizations of this situation.

First, I will consider the coupling of a high Q cavity to a single emitter homogeneously broadened, which can safely model a quantum dot (QD) coupled to a semiconducting optical cavity. In that case, unusual phenomena can happen. In particular, we have shown [1] that photons spontaneously emitted by a QD coupled to a detuned cavity can efficiently by emitted at the cavity frequency, even if the detuning is large; whereas if the QD is continuously pumped, decoherence can induce lasing [2]. These effects clearly show that decoherence, far from being a drawback, is a fundamental resource in solid-state cavity quantum electrodynamics, offering appealing perspectives in the context of advanced nano-photonic devices.

In a second part, I will present recent results where we investigated theoretically the coupling of a cavity mode to a continuous distribution of emitters, paying attention to the influence of inhomogeneous broadening on the existence and the coherence properties of the polaritonic peaks. We found that their coherence depends crucially on the shape of the distribution and not only on its width. Under certain conditions the coupling to the cavity protects the polaritonic state from inhomogeneous broadening, resulting in a longer storage time for a quantum memory based on emitters ensembles. When two different ensembles of emitters are coupled to the resonator, they support a peculiar collective dark state, also very attractive for the storage of quantum information.

[1] "*Pure emitter's dephasing : a resource for advanced single photon sources*", A. Auffèves, J.M. Gérard and J.P. Poizat, PRA **79**, 053838 (2009).

[2] "*Controlling the dynamics of a coupled atom-cavity system by pure dephasing : basics and applications in nanophotonics*", A. Auffèves, D. Gerace, J.M. Gérard, M.P. França Santos, L.C. Andreani and J.P. Poizat, PRB **81**, 245419 (2010).

[3] " *Strongly coupling a cavity to inhomogeneous ensembles of emitters : potential for long lived solid-state quantum memories*", I. Diniz, S. Portolan, R. Ferreira, J.M. Gérard, P. Bertet and A. Auffèves, **arXiv:0176967**.

# Lundi 31 janvier 2011, 14h00, salle 117

## Title: Fusion of line operators and quantum integrability in conformal sigma models on supergroups.

**Raphael BENICHOU (Vrije Universiteit Brussel et The International Solvay Institutes)**

**Abstract:** I will present recent progress in the understanding of two-dimensional sigma-models on the supergroup PSl(n|n). I will emphasize the relevance of these models to study quantum integrability in the AdS/CFT correspondence. In particular i will explain the computation of the fusion of some line operators, the transfer matrices, that encode an infinite number of conserved charges. This computation leads to a first-principle, perturbative derivation of the Hirota equation, which has been argued to provide a solution to the spectrum problem in N=4 SYM.

# Jeudi 27 Janvier 2011 à 14h

## Title: Probing quasiparticle states in strongly interacting atomic gases

**Tung-Lam Dao (Institut d'Optique, Orsay) **

**Abstract:** We investigate a momentum-resolved Raman spectroscopy technique which is able to probe the one-body spectral function and the quasi-particle states of a gas of strongly interacting ultracold atoms [1]. This technique is inspired by Angle-Resolved Photo-Emission Spectroscopy, a powerful experimental probe of electronic states in solid-state systems. A very good agreement is found with recent experimental data for the study of the BEC-BCS crossover [2,3]. . We discuss also direct applications of the Raman spectroscopy technique for recent experiments of interacting fermionic atoms loaded into an optical lattice. This technique is applicable to detect the temperature of weakly interacting Fermi gas in the experimentally relevant temperature regimes. Additionally, we show that a similar spectroscopy scheme can be used to obtain information on the quasiparticle properties and Hubbard bands of the metallic and Mott-insulating states of interacting fermionic spin mixtures. These two methods provide experimentalists with novel probes to accurately characterize fermionic quantum gases confined to optical lattices [4].

- [1] T-L. Dao et al., Phys. Rev. Lett.
**98**, 240402 (2007) - [2] T-L. Dao et al., Phys. Rev. A.
**80**, 023627 (2009) - [3] J. T. Stewartet al., Nature
**454**, 774 (2008). - [4] J-S. Bernier et al., to be published in Phys. Rev. A.

# Jeudi 20 Janvier 2011 à 14h (Colloquium)

## Title: Dark matter : from astrophysics and cosmology to the LHC

**Geneviève Bélanger (LAPPTH, Annecy) **

**Abstract:** There is strong evidence from astrophysics and cosmological measurements that most of the matter that constitutes the universe is dark. Early indications of the presence of dark matter from observations of galaxies were confirmed in the last few years with in particular a precise extraction of the relic density of dark matter from WMAP measurements. This has stimulated numerous direct and indirect searches for dark matter and has fuelled theoretical speculations on the nature of dark matter. The favoured explanation for dark matter is to postulate a new neutral stable and weakly interacting particle such as the ones found in extensions of the standard model.

In this talk I will present the current status for dark matter searches and will discuss how the LHC will further probe various dark matter candidates.

# Jeudi 13 Janvier 2011 à 14h

## Title: The Bose-glass phase transition in the 1-D mean-field limit

**Vincenzo Savona (Laboratory of Theoretical Physics of Nanosystems, EPFL, Switzerland) **

**Abstract:** A one-dimensional system of noninteracting bosons in presence of disorder is always in an insulator state at zero temperature. Interactions can induce a quantum phase transition to a superfluid state, characterized by quasi long-range order. This Bose-glass to superfluid transition has been the subject of intense theoretical studies and of several recent experiments carried out on ultracold atomic clouds.

I will present a theoretical study of the quantum phase transition of the 1D disordered Bose gas in the mean field regime, based on the extended Bogolyubov model for a quasicondensate. In this context, I will derive the phase diagram on the interaction-disorder plane (U,D), by inspecting the long-range behaviour of the one-body density matrix as well as the drop in superfluid fraction. It turns out that the phase boundary between the quasicondensed and the Bose-glass phases is marked by two different algebraic relations. These can be analytically explained as two limiting behaviours of the Bose gas in the white noise limit - where D ~ U^(3/4) and in the Thomas-Fermi regime - where D ~ U.

The in-situ density profile is perhaps the feature of an ultracold atomic cloud that can be most easily measured in an experiment. I will show that a direct link exists between the fragmentation of the density profile and the occurrence of the phase transition. This link is given by the probability distribution of the gas density. In particular, the appearance of a superfluid fraction coincides with a vanishing probability distribution in the limit of zero density, namely with the disappearance of fragmentation. This analysis sets the intuitive relation between fragmentation and insulating behaviour into a more rigourous framework, and opens the way to the experimental detection of the phase transition.

# Jeudi 16 Décembre 2010

## Journée Gravité Quantique

### Thèse de Maïté Dupuis (10h00)

### Titre: Spin foam models for quantum gravity and semi-classical limit

**Abstract**: The spin foam framework is a proposal for a regularized path integral for quantum gravity, to define transition amplitudes between quantum geometry states. This covariant approach is based on the following fact. General relativity can be seen as a topological theory (i.e. with non-local degrees of freedom) plus constraints (the so-called simplicity constraints which reintroduce local degrees of freedom). The issue is then to implement consistently the constraints at the quantum level. I will first recall the spin foam quantization procedure and focus more particularly on the step consisting in implementing the simplicity constraints. Then, I will present an original way using harmonic oscillators to impose the simplicity constraints in the context of 4d Euclidean. Another key-issue is to extract semi-classical information from a given spin foam model. I will present new techniques and new results that allow to compute semiclassical asymptotic expressions for the transition amplitudes of 3d quantum gravity.

### Habilitation à diriger des recherches d'Etera Livine (14h00)

### Titre: The Spinfoam Framework for Quantum Gravity

**Abstract:** Spinfoam models provide a path integral formalism for quantum gravity. They define quantum space-time structures, which describe the evolution in time of the spin network states for quantum geometry derived from loop quantum gravity.These models are inspired mainly from topological field theory and Regge calculus for discretized general relativity. Beyond the mere constuction of such models, this framework turns out to be relevant for quantum gravity phenomenoloy. Indeed, it is possible to recover the leading orders of the graviton propagator (that's Newton law fro gravity!) and to compute the quantum gravity effects on the matter dynamics, which can be interpreted in term of non-commutative geometry.

# Jeudi 9 Décembre 2010 à 14h

## Title: Duality invariance and non-renormalisation theorems in supergravity.

**Guillaume BOSSARD (CPHT, Ecole Polytechnique)**

**Abstract:** E7 duality is a symmetry of the equations of motion of N=8 supergravity. Using a non-manifestly diffeomorphism invariant formalism one can write an action which is manifestly E7 and super- diffeomorphism invariant, although the super-diffeomorphism are realised in an unconventional way. I will show the consistency of this formalism by exhibiting its quantum equivalence with the conventional formulation, defining a regularisation scheme consistent with the quantum action principle, and proving the absence of anomaly. With the result that the theory can be renormalised such that it is E7 invariant, its logarithm divergences must therefore be both E7 and super-diffeomorphism invariant. I will then prove that the candidate counter-terms for putative divergences at 3, 5 and 6 loops are all breaking E7. However, one can show the existence of a non-vanishing E7 invariant counter-term associated to a 7-loop divergence; which suggests that the theory might diverge logarithmically at 7-loop, if no further hidden symmetry was to be discovered.

# Jeudi 25 Novembre 2010 à 14h

## Title: Recent trends in the AdS/CFT

**Eoin O Colgain (Korea Institute for Advanced Study, Seoul)**

**Abstract:** Whether one is searching for geometries dual to supersymmetric surface operators or those with potential condensed matter application, special tools are required in higher-dimensional supergravity. We review the use of consistent truncations and G-structures in finding new solutions and comment on properties of these solutions.

# Mercredi 1er Décembre 2010, 14h, salle 115

## Title: Aspects of Gauge-Strings Duality

**Carlos NUNEZ (Swansea University)**

**Abstract:** I will make a general discussion on the duality between gauge fields and Strings. The focus of the talk will be to review some well established results and comment on some new aspects recently discovered.

# Jeudi 18 Novembre 2010 à 14h

## Title: Light-induced gauge potentials for cold atoms

**Gediminas JUZELIUNAS (Institute of Theoretical Physics and Astronomy of Vilnius University)**

**Abstract:** In the initial part of the talk we shall review schemes enabling to produce the artificial magnetic field for cold atoms using several light beams. We discuss the possibilities to create both Abelian and also non-Abelian gauge potentials. Subsequently we shall talk on some recent studies of the effects due to the non-Abelian gauge potentials for cold atoms including their quasi-relativistic behaviour and negative reflection. We also talk about about a scheme of generating the non-Abelian gauge potentials for cold atoms containing three degenerate dressed states, so their centre of mass motion is described by a three-component spinor.

# Jeudi 4 Novembre 2010 à 14h

## Title: Phase transitions in the quantum transport problem.

**Vivo Pierpaolo (ICTP Trieste, Italy)**

**Abstract:** Linear statistics on ensembles of random matrices occur frequently in many applications. We present a general method to compute probability distributions of linear statistics for large matrix size N. This is applied to the calculation of full probability distribution of conductance and shot noise for ballistic scattering in chaotic cavities, in the limit of large number of open electronic channels. The method is based on a mapping to a Coulomb gas problem in Laplace space, displaying phase transitions as the Laplace parameter is varied. As a consequence, the sought distributions generally display a central Gaussian region flanked on both sides by non-Gaussian tails, and weak non-analytical points at the junction of the two regimes.

References:

[1] Phys. Rev. B **81**, 104202 (2010)

[2] Phys. Rev. Lett. **101**, 216809 (2008).

# Vendredi 22 Octobre 2010 à 14h (salle commune du laboratoire)

## Title: Accuracy of the Quantum Capacitor as a Single Electron Source

**Mathias Albert (Université de Genève) **

**Abstract:** Controllable single electron sources are at the forefront of current research on nano-scale electronics. Systems that generate quantized electrical currents, for example quantum capacitors and quantum pumps, are of great interest due to their potential applications in metrology and quantum information processing as well as in basic research on single- and few-electron physics in mesoscopic structures. Despite the experimental and theoretical advances, the accuracy at which the quantum capacity emits electrons is still not well understood. Here we consider a conceptually simple model of a quantum capacitor and find analytically the noise spectrum as well as the full distribution of emitted electrons (full counting statistics) and the waiting time distribution. We find that the failure rate of the capacitor can be arbitrarily small when operated under favorable conditions. Our predictions may be tested in future experiments..

# Jeudi 14 Octobre 2010 à 14h

## Title: Scaling in Single-Chain Magnets

**Alessandro Vindigni, (ETH Zurich, Suisse) **

**Abstract:** It is well-known that long-range order cannot occur in 1d magnetic systems with short-range interactions. A remanent magnetization may, however, be obser ved in some anisotropic spin chains due to slow dynamics. The physics of such systems ? named Single-Chain Magnets ? is mainly dictated by the temperature dependence of the relaxation time (tau) and the correlation length (xi). A simple random-walk argument relates these two quantities with each other : within a time tau a domain wall performs a random walk over a distance xi. Depending on the relative strength of the exchange interaction and the single-ion anisotropy, the relevant excitations consist either of sharp (extending over just one lattice spacing) or broad (extending over several lattice constants) domain walls. By combining time-quanti?ed Monte-Carlo simulations with transfer-matrix and renormalization-group calculations, we highlighted that the broad- and sharp-wall regimes are associated with different temperature dependences i) of the correlation length and ii) of the diffusion coef?cient of domain-wall motion. These ?ndings allow us to explain the different relationship between tau and xi reported for broad- and sharp-wall Single-Chain Magnets.

# Jeudi 7 Octobre 2010 à 14h

## Title: Fluctuation relations in mesoscopic transport

**Alessandro De Martino, (Univ. Cologne) **

**Abstract:** In this talk we will discuss the concept of fluctuation relations in application to mesoscopic transport. Fluctuation relations connect the statistics of pairs of time-reversed evolutions of physical observables (e.g., heat, work, current, etc.) in nonequilibrium systems and thereby establish rigorous identities for their averages. We will derive general functional FRs for the current flow induced by time-varying forces and illustrate their utility in the description of transport through systems of mesoscopic size. We will then show that under nonequilibrium conditions rare realizations of transport observables are crucial and imply strong stochastic fluctuations around the exact averages established by the FRs. We will illustrate our general results on the paradigmatic example of a mesoscopic RC circuit driven by time-dependent voltage.

# Jeudi 30 Septembre 2010 à 14h

## Title: Localisation d'Anderson et transition metal-isolant d'Anderson dans les gaz atomiques froids

**Dominique Delande, (Laboratoire Kastler Brossel) **

**Abstract:** En presence d'un potentiel aléatoire, le mouvement classique d'une particule est typiquement diffusif, avec une constante de diffusion dépendant du degré de desordre. Quand les interferences quantiques entre differents chemins sont pris en compte, la situation peut être radicalement differente, conduisant en particulier à une transition diffusif-localise, i.e. métal-isolant, - appelee transition d'Anderson - quand on modifie l'amplitude du désordre. Ces effets sont très sensibles à la préservation de la cohérence de phase quantique et donc difficiles à observer experimentalement. En utilisant des ondes de matière atomiques à très basse température, manipulées par des champs lasers dépendant du temps, on peut construire des systèmes dont la dimensionnalité effective et les paramètres du désordre peuvent être ajustées à volonté. On a ainsi pu observer la localisation d'Anderson et la transition métal-isolant. On a en particulier pu mesurer finement les exposants critiques, les lois d'échelle au point critique de la transition et les fluctuations géantes en son voisinage. Ces études ouvre des perspectives nouvelles pour l'étude du transport quantique, en presence de désordre et/ou d'interactions.

# Jeudi 16 septembre 2010 à 14h

## Title: Charge order and quantum critical behavior in layered organic conductors

**Simone Fratini (Institut Néel, Grenoble)**

**Abstract:** Low-dimensional organic conductors show a variety of electronic phases that are believed to originate from the presence of strong electronic interactions. After a brief overview on the phase diagrams of these materials, I will focus on the charge ordering that occurs in the family of the two-dimensional theta-ET2 salts with quarter-filled electronic bands. These systems are ideally located halfway between the strongly correlated oxides (dominated by Mott physics close to integer fillings) and the two-dimensional electron gas (2DEG), that exhibits Wigner crystallization at low density. I will present results based on a model that accounts for both local electronic correlations and longer range Coulomb interactions responsible for charge ordering, and discuss the emergence of unconventional phases and their possible relevance to experiments.

# 2009-2010

# Semaine du jeudi 24 Juin 2010

## Pas de séminaire (soutenances de thèses).

# Mercredi 23 juin 2010

## Soutenance de thèse de Arnaud Le Diffon

# Mardi 22 juin 2010 à 14h

## Soutenance de thèse de Guillaume Paulin: Transport Electronique et Verres de Spins

**Résumé:** Les travaux décrits dans cette thèse apportent une contribution à la physique de la matière condensée des systèmes désordonnés, à la physique mésoscopique d?une part et à la physique des verres de spins d?autre part. La première partie de cette thèse étudie de manière numérique le trans- port électronique cohérent dans un metal non magnétique dopé par des impuretés magnétiques gelées (un verre de spins à basse température). A l?aide d?un code récursif de calcul de la conductance à deux terminaux du système, nous étudions en détail le régime métallique de conduction (car- actérisé par une conductance élevée) ainsi que le régime isolant (faible con- ductance). Dans ces deux régimes, des comportements universels du système sont mis en évidence. En outre, une étude des corrélations de conduc- tance pour deux con?gurations di?érentes des spins des impuretés permet de relier ces corrélations aux corrélations entre con?gurations de spins (ap- pelées recouvrement). Cette étude ouvre la voie à la première détermination expérimentale du recouvrement par des mesures de transport. Une deuxième partie de cette thèse consiste à étudier le modèle de champ moyen de Sherrington-Kirkpatrick, qui décrit la phase à basse température d?un verre de spins d?Ising. Nous nous intéressons ici à la généralisation au cas de spins d?Ising quantiques (i.e en champ magnétique transverse) de ce modèle classique très étudié ces trente dernières années. Nous déduisons analytiquement des équations du mouvement dans le cas semi-classique où l?in?uence des ?uctuations quantiques est faible, que nous comparons au cas classique. Ces équations sont résolues numériquement par une méthode pseudo-spectrale.

# Jeudi 27 Mai 2010 (salle 116)

## Title: Fermions ultrafroids et problèmes à quelques corps.

**Xavier Leyronas (LPS, ENS Paris)**

**Abstract:** Je présenterai mon travail sur le problème de fermions ultrafroids en interaction. Après une introduction sur ce que l'on appelle le "crossover BEC-BCS", je montrerai les résultats d'expériences déterminant l'équation d'état d'un tel système, en particulier celle récente du groupe Li6 du Labotatoire Kastler Brossel. Enfin, j'exposerai nos travaux théoriques visant à calculer cette équation d'état, où les problèmes à trois et quatre corps apparaissent.

# Jeudi 8 Avril 2010 (salle 116)

## Titre: Mesure des fluctuations de courant d?une source d?électrons : une preuve de l?émission contrôlée d'électrons uniques

**Gwendal Fève (Laboratoire Pierre Aigrain, ENS Paris)**

Le transport électronique dans les conducteurs quantiques balistiques présente de nombreuses analogies avec le transport de photons dans le vide. Sur des longueurs microniques aux températures cryogéniques, les électrons se propagent sans subir de collisions et la phase de leur fonction d?onde reste bien définie. Ces analogies ont pu être spectaculairement mises en valeur par la réalisation, par exemple, d'interféromètres de type Mach-Zehnder [1]. La manipulation contrôlée de quelques uniques électrons dans un circuit permettrait de pousser l'analogie jusqu'à l'optique quantique. En particulier, certaines de ses expériences fondatrices pourraient être reproduites avec des électrons, comme les expériences de Hanbury-Brown et Twiss ou Hong-Ou-Mandel dans lesquelles un ou deux photons sont partitionnés sur une lame séparatrice. La nature non-classique des faisceaux incidents peut alors être mise en évidence par la mesure des corrélations entre les courants en sortie de la lame. La reproduction de ces expériences avec des électrons repose donc sur la possibilité d?émettre à la demande des électrons uniques dans un circuit et de mesurer les corrélations de courants mono-électroniques.

Dans cet exposé, je présenterai la mesure des fluctuations du courant (autocorrélation) généré par une source d'électrons périodiquement déclenchée. Cette source permet l'émission contrôlée d'une unique charge d'une boîte quantique réalisée dans un gaz bidimensionnel d'électrons à l'interface d?une hétérostructure AlGaAs/GaAs [2]. Par une soudaine variation du potentiel de la boîte, un seul électron peut être émis par couplage tunnel de la boîte vers le reste du gaz sur un temps caractéristique subnanoseconde. Lorsque ce régime d'émission d'électrons uniques est atteint, les fluctuations du courant se réduisent à l?incertitude quantique sur le temps tunnel d?émission d?une unique charge. L'observation de ce bruit irréductible permet alors de démontrer l?émission contrôlée de particules uniques et d?envisager la réalisation de futures expériences d'**optique quantique électronique**.

[1] Y. Ji et al., Nature **422**, 415 (2003) .

[2] G. Fève et al., Science **316**, 1169 (2007).

# Vendredi 23 avril 2010 à 13h30 (salle 116)

## De Sitter vacua of supergravity and supersymmetry breaking branes.

**David Andriot (LPTHE Jussieu)**

Finding a de Sitter vacuum of supergravity seems to be rather difficult, as many four-dimensional studies have shown. In this talk I will study this question from a ten-dimensional point of view. A starting point to find such solutions is to consider as an ansatz a deformation of a known supersymmetric solution. Nevertheless, this is often not sufficient to obtain a positive cosmological constant, and then one usually adds extra ingredients. Here I will rather break in addition the supersymmetry of the sources. I will make a proposal for such sources, and provide a concrete example of such a de Sitter solution obtained by compactifying on a solvmanifold. This proposal may open the door to first order equations generalizing the supersymmetry ones. The four-dimensional stability of the solution remains to be studied, but the four-dimensional potential admits at least a minimum in the dilaton and the volume moduli.

Based on arXiv:1003.3774.

# Jeudi 29 Avril 2010 (salle 116)

## Title: Universal Resistances of the Quantum RC circuit.

**Christophe Mora (Laboratoire Pierre Aigrain, ENS Paris)**

We discuss the capacitance and the resistance, usually called the charge relaxation resistance, of a quantum coherent RC circuit driven by a low-frequency AC voltage.

This circuit is the quantum analogue of the classical RC circuit: it comprises a dot capacitively coupled to a nearby gate and connected to a single reservoir lead. As a result of phase coherence and electronic interactions, the quantum circuit behaves quite differently and Kirchhoff's law is violated.

Here we show that the charge relaxation resistance is perfectly quantized, regardless of the single lead transmission and for an arbitrary strength of the interaction. Its low-frequency value is h/2e^2. When the driving frequency exceeds the dot level spacing, we predict a transition to a metallic regime with a doubled quantized resistance h/e^2. The novel quantized resistance h/e^2 is connected to the Korringa-Shiba relation of the Kondo model, thereby revealing the physics behind these universal charges.

# Jeudi 6 Mai 2010 (salle 116)

## Title: Time-dependent theory of non-linear response and current fluctuations.

**Inès Safi (Laboratoire de Physique des Solides, Orsay)**

A general non-linear response theory is derived for an arbitrary time-dependent Hamiltonian, not necessarily obeying time-reversal symmetry. This allows us to obtain a greatly generalized Kubo type formula. Applied to a mesoscopic system with any type of interactions, and coupled to multiple probes and gates with arbitrarily time-dependent voltages, we derive current-conserving differential conductance and current fluctuation matrices obeying a generalized Fluctuation-Dissipation Theorem. This relation provides a common explanation for asymmetries of the excess noise in several non-linear mesoscopic systems, as well as of its surprising negative sign.

# Jeudi 25 Mars 2010 (salle 116)

## Title: Phase-Diagram of Quasi-Two-Dimensional Trapped Bose Gases.

**Markus Holtzmann (LPTMC, UMPC Paris & LPMMC, UJF Grenoble)**

discuss quasi-two-dimensional Bose gases in harmonic traps at temperatures close to the Kosterlitz-Thouless transtion. Using Quantum Monte Carlo calculations for experimentally relevant geometries and interparticle interactions, we have studied density profiles, superfluid and condensate fractions, single-particle coherence, and pair correlations. Quantitative comparisions with mean-field, and effective (classical) field theory allows us to study universal two-dimensional correlations in the fluctuation region, and to characterize the cross-over from Kosterlitz-Thouless to Bose-Einstein behavior for small particle numbers.

# Jeudi 18 Mars 2010 (salle 116)

## Title: Description du mode de respiration d'un système de particules en interaction.

**Alain Olivetti (université de Nice)**

Considérant un ensemble de particules piégées, l'étude des modes d'oscillation de ce système donne accès à un grand nombre d'informations, et en particulier les effets collectifs qui interviennent. Ici, nous nous intéresserons plus précisément au mode dit de respiration, où le système de particules alterne des phases de compression et de dilatation. Notre travail se base sur les équations de la hiérarchie BBGKY, couplées à un opérateur de Fokker-Planck. À l'aide d'un ansatz, nous décrivons le mode de respiration pour une grande gamme de systèmes, et ce quelles que soient la température, la dimension de l'espace, que l'on soit en régime linéaire ou non linéaire ... Par la suite nous généraliserons nos résultats au cas où la friction et/ou la diffusion peuvent être des quantités dépendant de l'espace, une situation rencontrée dans les pièges magnéto-optiques et qui peut être à l'origine d'instabilités.

# Jeudi 11 Mars 2010 (salle 116)

## Title: On the microscopic origin of excess low frequency flux 1/f noise in qubits and SQUIDs.

**Lara Faoro (LPTHE, Paris)**

At millikelvin temperatures, superconducting flux and phase qubits and SQUIDs (Superconducting QUantum Interference Devices) both suffer from intrinsic magnetic flux noise. The noise power spectrum scales as 1/fa, where f is frequency and b is approximately unity. Low-frequency flux noise enhances decoherence in qubits and reduces flux resolution in SQUIDs. Remarkably, all devices show approximately the same level of noise, a few ?-?0 per square root of Hz at 1 Hz (?0 is the flux quantum). The magnitude of the flux noise scales only weakly with the area of the device, and is independent of temperature T.

In this talk I will illustrate our theoretical picture for the excess low frequency flux noise consistent with data in which the noise is due to the spins at the Superconductor Insulator interface coupled via RKKY interaction. In contrast to the alternative models, this mechanism explains many puzzling features of the flux noise: its apparent temperature independence down to 20 mK, its persistence to at least 20 MHz and the rough SQUID loop area independence. This mechanism generates roughly 1/f noise in a broad frequency. I will report on some recent experimental results that support our theoretical conjecture. I will also discuss some very recent puzzling results by McDermott group at University Madison Wisconsin that indicate a large and previously overlooked source of noise: fluctuations of the kinetic inductance in the superconducting wires which can be masked as a flux noise in some cases.

# Jeudi 4 Mars 2010 (salle 116)

## Title: Physique statistique et systèmes sociaux complexes.

**Sébastien Grauwin (ENS de Lyon)**

Un problème récurrent au sein des sciences sociales, connu comme le 'micro-to-macro problem', concerne notre capacité à expliquer la relation entre les éléments constitutifs des sciences sociales (les individus) et les phénomènes collectifs émergeant résultant de leurs interactions (ola, émeutes, ségrégations urbaines, institutions, société, économie...). Ces dernières années, les physiciens se sont attaqués à ce problème avec des outils issus de la physique statistique. Le double pari implicite est que les outils et le regard 'neuf' des physiciens peut aider les chercheurs en sciences sociales à mieux comprendre leurs systèmes, mais aussi que les spécificités des systèmes sociaux permettent de développer des outils de modélisation qui peuvent intéresser la physique et d'autres disciplines. Deux approches sont tentées au niveau mondial pour mener à bien ce pari.

- 1/ La modélisation de sociétés virtuelles simples où l'on peut comprendre les causalités. Cette approche sera illustrée par un modèle analytique de ségrégation urbaine que nous avons développé dans un récent article [1]. Ce modèle présente notamment une extension du concept d'énergie libre pour les systèmes sociaux.
- 2/ L'analyse de données réelles par des méthodes sophistiquées. Nous avons rassemblé une base de données de 200000 articles rattachés à des thématiques 'systèmes complexes'. L'examen de cette grande base de données nous a amené à développer des d'outils d'analyse spécifiques qui apportent un éclairage nouveau par rapport aux études plus traditionnelles.

Je présenterais enfin les grandes lignes d'une troisième démarche mixte qui tente de relier ces deux approches restées jusqu'à aujourd'hui étanches, les modèles étant trop simplistes pour rendre compte de la richesse des données réelles.

[1] S. Grauwin et al, Proc. Natl. Acad. Sci. USA **106**, 20622-20626 (Dec 2009)

# Jeudi 25 Février 2010 (salle 116)

## Title: Towards coherent spintronics (with Carbon Nanotubes).

**Takis Kontos (Laboratoire Pierre Aigrain, ENS Paris)**

L'asymétrie de diffusion entre les spins + et les spins - à l'interface entre un métal ferromagnétique et un métal non-magnétique est au coeur du principe de fonctionnement des jonctions tunnel ou des multicouches magnétiques qui ont valu le prix Nobel à A. Fert et P. Grünberg en 2007. Bien que ces dispositifs utilisent l'effet tunnel et le spin de l'électron, ils n'exploitent pas un degré de liberté crucial autorisé par la mécanique quantique : la phase de la fonction d'onde. En effet, le plus souvent, cet aspect reste confidentiel et le transport électronique à travers de tels objets est très bien décrit par des lois essentiellement classiques. Dans le travail que je vais présenter, nous avons effectué des mesures de transport dépendant du spin dans des nanotubes de carbone à plusieurs contacts. Nous observons des signaux non-locaux de type vanne de spin contrôlables par un champ électrique appliqué à l'aide d'électrodes de grille. Ceci révèle que le spin ainsi que la phase de la partie orbitale de la fonction d'onde sont conservés dans de tels dispositifs. Ces observations réalisent un pont entre la physique mésoscopique et l'électronique de spin et ouvrent la voie vers la réalisation de composants de la nano-électronique utilisant ces deux degrés de liberté quantiques sur un pied d'égalité.

# Jeudi 11 Fevrier 2010 (salle 116)

## Title: Quasi-particules anyoniques et transition de phase topologique (le code torique en champ magnétique)

**Sébastien Dusuel (Lycée Saint Louis, Paris)**

Je commencerai par décrire qualitativement les propriétés élémentaires des systèmes quantiques bidimensionnels possédant des excitations anyoniques (particules de statistique fractionnaire, n'étant ni des bosons ni des fermions), en relation avec les domaines récents des qubits topologiquement protégés et du calcul topologique quantique.

Ensuite, je donnerai une introduction pédagogique du modèle de spins 1/2 le plus simple qui présente de tels excitations exotiques et de l'ordre topologique, à savoir le code torique de Kitaev avec anyons émergents Z_2 [1].

Finalement, la robustesse de la phase topologique du code torique à la perturbation locale la plus simple (un champ magnétique) sera estimée, en donnant une image physique en termes de quasi-particules anyoniques [2,3]. En fonction de la direction du champ magnétique : - l'ordre topologique est détruit par une transition de phase quantique du premier ou du second ordre, d'où un diagramme de phase riche - le système peut posséder une pléthore d'états liés

- [1] Kitaev, Ann. Phys
**303**, 2 (2003) - [2] Vidal, Dusuel & Schmidt, Phys. Rev.
**B 79**, 033109 (2009) - [3] Vidal, Thomale, Schmidt & Dusuel, Phys. Rev.
**B 80**, 081104(R) (2009)

# Jeudi 21 Janvier 2010 (salle 116)

## Title: Edge-State Spectroscopy and Energy Exchanges in the Integer Quantum Hall Regime

**Carles Altimiras (LPN Marcoussis)**

The quantum Hall regime is a state of matter where quantum phenomena manifest themselves on a macroscopic scale. Its most salient feature is a dissipationless charge propagation along one-dimensional edge channels, whose similarity with light beams has inspired electronic analogues of quantum optics experiments. Yet, the microscopic physics of edge states is poorly understood, as vividly illustrated by the on-going debate to interpret the recent electronic Mach-Zehnder experiments.

Two important issues are particularly acute. The first one concerns the expected reconstruction of the edges due to Coulomb repulsion in realistic smooth confining potentials: the edge channels acquire a finite width and additional acoustic modes of density oscillations across the width are predicted. Second, the interaction between co-propagating edge channels may deeply modify the nature of edge excitations: for strong enough interactions, the excitations are predicted to delocalize among the co-propagating channels. At filling factor 2 (with two co-propagating edge channels of opposite spin), this effect results in a spin-charge separation of the edge dynamics.

I will present a setup that permits us to extract the energy distribution f(E) in an edge channel driven out-of-equilibrium. This novel spectroscopy provides a stringent test of whether the predicted additional acoustic modes capture part of the injected energy [1]. Moreover, by measuring f(E) for various propagation lengths, we can test the inelastic mechanisms at work and the nature of the pertinent electronic excitations. Our results for two co-propagating edge channels reveal complete energy current equilibration, over a few micrometers. This strongly suggests that the dynamics is governed by collective edge excitations delocalized over both channels [2].

- [1] C. Altimiras et al., Nature Physics
**6**, 34-39 (2009) - [2] P. Degiovanni et al., arXiv:0910.2642 (2009)

# Jeudi 14 Janvier 2010 (Amphi Schrodinger)

## Title: Systèmes dynamiques loin de l'equilibre: que peut on dire de la fonction susceptibilité ?

**David Ruelle (IHES)**

# Jeudi 26 Novembre à 14h

## Title: Localization of BECs in quasiperiodic lattices

**Michele Modugno (LENS, Florence, Italie)**

**Abstract:** I will discuss the localization behavior of a Bose-Einstein condensate in a one-dimensional bichromatic optical lattice, by considering both static and dynamical properties. In particular, I will report on the recent experiments performed at LENS, where a Bose-Einstein condensate with tunable interactions has been employed to explore the delocalization transition induced by interactions. I will also consider the quantum spreading of a wave-packet in the quasiperiodic potential, by discussing the interplay of quasi-disorder and nonlinearity and the role of initial conditions.

# Jeudi 3 décembre 2009 à 14h

## Title: AC conductance of quantum chaotic cavities: semiclassical approach

**Cyrille Petitjean (Université de Regensburg)**

**Abstract:** Due to progress in the control and manipulation of mesoscopic structures driven by high frequency periodic voltages, the ac regime has recently been experimentally investigated [1] and consequently theoretical interest in it has been renewed.

We consider a quantum chaotic cavity that is coupled via tunnel barriers and gates to a macroscopic circuit which contains ac-sources. For the transparent barrier, our semiclassical techniques permit us to include the Ehrenfest time in the weak-localization correction to the screened conductance, previously obtain by the random matrix theory [2].

Then by extending the recent semiclassical theory in presence of tunnel barriers [3] to the ac-transport, we investigate the effect of dephasing on the relaxation resistance of a chaotic capacitor in the linear low frequency regime. This last investigation is in principle relevant to the recent measurements of the admittance at zero magnetic flux of a mesoscopic capacitor [1,4].

Works in collaboration with D. Waltner, J. Kuipers, I. Adagideli and K. Richter:

C. Petitjean et al. Phys. Rev. B **80**, 115310 (2009).

[1] J. Gabelli et al., Science **313**, 499 (2006). [2] P.W. Brouwer and M. Buttiker, Europhys. Lett. **37**, 441 (1997). [3] R.S. Whitney, Phys. Rev. B **75**, 235404 (2007). [4] S. Nigg and M. Buttiker, Phys. Rev. B **77**, 085312 (2008).

# Jeudi 10 décembre 2009

## Pas de séminaire!

Le GDR Physique Quantique Mésoscopique co-organise une mini école de trois jours sur le thème **Les isolants topologiques** du Mercredi 9 au Vendredi 11 décembre 2009. Elle est organisée à l'Ecole Normale Supérieure de Lyon par D. Carpentier et J. Cayssol. Pour en savoir plus, visitez le site Web.

# Jeudi 12 Novembre à 14h

## Title: Universal detector efficiency of a mesoscopic capacitor

**Simon Nigg (université de Genève)**

**Abstract:** In this talk I will discuss a novel type of high frequency quantum detector based on the mesoscopic capacitor recently realized by Gabelli et al., [1], which consists of a quantum dot connected via a single channel quantum point contact to a single lead. I will show that the state of a double quantum dot charge qubit capacitively coupled to this detector can be read out in the GHz frequency regime with near quantum limited efficiency. To leading order, the quantum efficiency is found to be universal owing to the universality of the charge relaxation resistance of the single channel mesoscopic capacitor.

References: [1] J. Gabelli et al., Science **313**, 499 (2006)

# Jeudi 5 Novembre à 15h

## Title: Self duality and supersymmetry

**M. Konyushikhin (Subatech, Université de Nantes)**

**Abstract:**

# Jeudi 29 Octobre 2009 à 14h

## Title: "Financial market fluctuations and predictability: asynchronous models and statistical mechanics"

**Damien Challet (Université de Fribourg)**

**Abstract:** Starting from the minority game, this talk discusses how to design and solve agent-based models of financial markets that focus on information ecology. This yields a powerful picture of how real markets operate and allows one to link price volatility to predictability. This talk then makes a case for asynchronous models of speculation and focuses on two of them. The difficulties of tackling them analytically will be discussed.

# Jeudi 22 Octobre 2009 à 14h

## Title: The extremal black hole/CFT correspondence

**G. Compère (University of California, Santa Barbara)**

**Abstract: ** Universal properties of black holes such as the first law can be derived from generic properties of Killing horizons and diffeomorphism covariance. We show that the entropy of extremal black holes can be derived universally as well, suggesting a conformal field theory description of extremal black holes. The linear perturbations in the Kerr throat and the greybody factors will be surveyed in the perspective of the existence of a CFT.

# Jeudi 15 Octobre 2009 à 14h

## Title: Magnétorésistance tunnel et symétries électroniques

**David Halley, (IPCMS Strasbourg) **

**Abstract:** La magnétorésistance géante dans les systèmes entièrement métalliques a été découverte il y a une vingtaine d?années par A. Fert et P. Grünberg. De nombreuses applications en ont découlé notamment pour l?enregistrement magnétique ou les systèmes de capteurs.

Les études des années 90 ont montré que l'effet de magnéto-résistance géante était plus important dans des systèmes incluant une barrière isolante amorphe, que les électrons franchissent par effet tunnel (jonctions magnétiques tunnel). Cependant, jusqu'à une date récente les valeurs de magnéto-résistance tunnel demeuraient inférieures à 100% ce qui limitait encore le nombre d?applications envisageables pour de tels systèmes.

Depuis quelques années, l'étude de tels systèmes, cette fois-ci monocristallins, a permis d'accroître considérablement les valeurs de magnéto-résistance (jusqu?à 500%), les rendant très attractifs pour l?industrie. Nous montrerons comment les effets de filtrage en symétrie par une barrière monocristalline permettent d?obtenir une forte polarisation en spin du courant d?électrons et donc une très forte valeur de magnéto-résistance. Nous montrerons en outre comment ces effets de filtrage par la barrière peuvent modifier le comportement de métaux « normaux » placés à proximité de celle-ci, les rendant isolants au regard du transport électronique. Nous illustrerons ce phénomène dans le cas de « puits quantiques » sélectifs en symétrie observés dans le système Fe/Cr/Fe/MgO/Fe.

# 2008-2009

# Jeudi 2 Juillet 2009 à 14h

## Title: Macroscopic quantum dynamics in 3D beyond mea field - the case of colliding BECs

**Piotr Deuar, LPTMS, Universite Paris-sud / CNRS, Orsay, France**

**Abstract:** During a supersonic collision of Bose-Einstein condensates, like in experiments with metastable Helium, a complex dynamics develops among the halo of scattered atoms. I will show some of the interesting phenomena that occur, and explain why they are completely missed by a mean field approach.

Nevertheless, the full quantum dynamics can be simulated ab-initio by sampling the positive-P representation of the boson field. This method, deveolped originally in quantum optics, is successful when single-particle interactions are weak but collective effects are strong - even in 3D.

To understand the behaviour of the halo particles, one can dissect the Bogoliubov quasiparticle dynamics to see which processes are responsible for what. With a dynamically evolving background condensate, finding the Bogoliubov modes exactly in 3D is intractable, but this has been successfully circumvented by sampling their positie-P representation.

# Vendredi 26 juin 2009

## Title: Charting the phase diagram of a disordered quantum spin chain with state fidelity

**Toby Jacobson, University of Southern California, Los Angeles**

**Abstract:** The phase diagram of a quantum XY spin chain with Gaussian-distributed random anisotropies and transverse fields is investigated, with focus on the fidelity susceptibility, a recently introduced quantum information theoretical measure. Monitoring the finite-size scaling of the probability distribution of this quantity as well as its average and typical values, we detect a disorder-induced disappearance of criticality and the emergence of Griffiths phases in this model.

References:

Phys. Rev. Lett. 102, 057205 (2009)

Phys. Rev. B 79, 184427 (2009)

# Jeudi 28 mai 2009

## Titre: Anyons, fermions, et ordre topologique dans un système de spins 1/2: le modèle de Kitaev

**J. Vidal (Laboratoire de Physique Théorique de la Matière Condensée, Université Pierre et Marie Curie)**

**Abstract:** En 1977, Leinaas et Myrheim [1] suggéraient l'existence de statistiques quantiques différentes de celles de Bose-Einstein et de Fermi-Dirac. Quelques décennies plus tard, les particules obéissant à ces statistiques, les anyons, n'ont toujours pas été observées expérimentalement.

Le modèle de Kitaev [2] qui décrit des spin 1/2 en interaction sur réseau hexagonal est sans aucun doute un des meilleurs candidats à la detection de tels objets. En effet, le spectre de ce système contient des excitations anyoniques, abéliennes et non-abéliennes, intimement liées à l'ordre topologique sous-jacent. Ces anyons sont localisés dans l'espace ce qui permet d'envisager simplement les manipulations indispensables à la mise en évidence expérimentale de leur statistiques [3]. Cependant, le spectre contient également des excitations fermioniques susceptibles de polluer les processus de détection.

Je présenterai une analyse perturbative de ce modèle autour de la limite de dimères isolés permettant de comprendre les problèmes liées à la coexistence de ces deux types de particules, anyons et fermions, au sein d'un même spectre [4].

- [1] J. M. Leinaas et J. Myrheim, Nuovo Cimento Soc. Ital. Fis. B 37, 1 (1977).
- [2] A. Kitaev, Ann. Phys. 321, 2 (2006).
- [3] L. Jiang et al., Nature Physics 4, 482, (2008).
- [4] J. Vidal, K. P. Schmidt et S. Dusuel, Phys. Rev. Lett. 100, 177204 (2008).

# 12-15 mai 2009

## Joint Euro-Japnaese Conference: Frustration in condensed matter

# Jeudi 9 avril 2009

## Challenges for String Inflation

**Marcus BERG (Stockholm University)**

**Abstract:** : Abstract: There are by now many models that claim to achieve inflationary cosmology in string theory (warped brane inflation, kaehler inflation, axion monodromy, etc.). I will explain the motivation for constructing such models, and give some details about the three aforementioned examples. I will summarize some concrete challenges concerning the consistency of these models, and how to make them more predictive.

# Vendredi 20 mars 2009 (Colloquium)

## Dynamics of a Nonlinear Luttinger Liquid.

**L. Glazman (Yale University and Inst. Néel)**

**Abstract:** : Dynamics of one-dimensional quantum many-body systems is usually described within the Luttinger Liquid paradigm. In that paradigm, the generic nonlinear dispersion relation of particles is replaced by a linear one. That allows one to solve the dynamics problem exactly, at the expense of introducing Lorentzian symmetry which was absent in the generic system. We investigate the dynamic responses without the linearization of the particle spectrum and find new universal singular behavior of the response functions.

# Jeudi 12 mars 2009 (Colloquium)

## Continuously monitoring the quantum oscillations of an electrical circuit.

**P. Bertet (Quantronics group, SPEC/CEA Saclay)**

**Abstract:** Superconducting circuits based on Josephson junctions can be used to realize artificial atoms, with coherence times sufficient to perform interesting atomic physics experiments. They can be strongly coupled to the electromagnetic field of an on-chip superconducting resonator, allowing to realize cavity quantum electrodynamics experiments with electrical circuits, giving rise to a new field called Circuit Quantum Electrodynamics (Circuit QED) [1,2]. We have studied the interplay between quantum dynamics and measurement in a Circuit QED setup. In our experiment, we use a ?transmon?, a modified Cooper-Pair Box coupled to a coplanar waveguide cavity which protects it from the environment and allows to reach long enough coherence times. An electromagnetic mode of the cavity is used to measure the qubit state. The photons stored in the cavity progressively extract information about the quantum state of the qubit, and correlatively dephase it. This information is carried by the phase of the electromagnetic field leaking out of the cavity that is measured by homodyne detection. By continuously applying the measuring field during Rabi oscillations of the circuit, we revisit the quantum measurement problem of a mesoscopic quantum electrical circuit [3]. By increasing the average number of photons in the cavity, we observe the transition between the weak measurement and Zeno regimes, both in the time and frequency domains. In the latter case, we discuss how far the experimental results provide a proof of the quantum behavior of the circuit.

[1] A. Blais et al., Phys. Rev. A 69, 062320 (2004)

[2] A. Wallraff et al., Nature 431, 162 (2004)

[3] A. Korotkov and D. Averin, Phys. Rev. B 64, 165310 (2001)

# Jeudi 5 mars 2009

## Transition from a one-dimensional to a quasi-one-dimensional state in interacting quantum wires

**Julia S. Mayer (Ohio state university)**

**Abstract:** In experiment, signatures of one-dimensional (1D) behavior have been observed in quantum wires and carbon nanotubes as well as cold atomic gases. While the 1D aspects make the above mentioned systems so fascinating, the real world is three-dimensional and, therefore, even in these confined geometries, features pertaining to deviations from one-dimensionality may remain. My interest is in identifying how the one-dimensional effects are modified in realistic situations and exploring the novel phenomena that arise.

Upon increasing the density of electrons in a quantum wire, the system undergoes a transition from a one-dimensional to a quasi-one-dimensional state. In the absence of interactions between electrons, this corresponds to filling up the second subband of transverse quantization. On the other hand, strongly interacting one-dimensional electrons form a Wigner crystal, and the transition corresponds to it splitting into two chains (zigzag crystal).

We study the evolution of the system and the electronic excitation modes in the vicinity of the transition as a function of the interaction strength. In particular, we establish that, for spin-polarized electrons, only one gapless mode exists on either side of the transition at any interaction strength. In the strongly interacting regime, the effective Hamiltonian is represented by two weakly coupled modes given by a Luttinger liquid and a transverse field Ising model. Performing a renormalization group analysis, we show that the critical fixed point is Lorentz invariant. However, the critical velocity vanishes due to marginally irrelevant operators.

# Jeudi 26 février 2009

## Excitation spectrum of the light-atom field in a periodic ultracold atomic gas.

**M. Antezza (Laboratoire Kastler-Brossel, ENS paris)**

**Abstract:** We study the excitation spectrum of the light-atom field in a periodic system of atoms located in the lowest vibrational state of an optical lattice. To this purpose the eigenmodes of the atomic gas interacting via the electromagnetic field are analytically investigated by taking into account both the vectorial character of the light and the quantum atomic motion. We show the problems of models which assume the atoms as point-like scatterers at rest at periodical positions, and that, on the contrary, the inclusion of the quantum atomic motion naturally leads to a well defined and divergency free model. We finally predict a gapless photonic spectrum.

# Jeudi 5 février 2009

## Anderson localization in ultracold atomic gases.

**L. Sanchez-Palencia (Laboratoire Charles Fabry, Institut d'Optique, Orsay)**

**Abstract:** We present our recent theoretical and experimental works on the expansion of a Bose-Einstein condensate in a disordered potential. We show that a such a system can exhibit single-particle Anderson localization under conditions that we will discuss. We determine analytically the localization and find that experimental data are in very good agreement. In addition, we show that the one-dimensional speckle potentials used in the experiments are very peculiar as they exhibit an effective mobility edge.

We also investigate the effects of disorder in a Bose-Einstein condensate at equilibrium in a regime where the interaction energy dominates over the kinetic energy. While the ground state is extended owing to the strong interactions, we show that the elementary excitations of the condensate (Bogolyubov quasi-particles) are localized. This constitutes an exemple of many-body Anderson localization in a system with strong meanfield interactions. We present a general formalism to determine analytically the localization lengths and compare them to numerical calculations in 1D.

# Jeudi 29 janvier 2009

## Interaction effects on transport in disordered d-wave superconductors: a study of several universality classes.

**L. Dell Anna (SISSA, Trieste)**

**Abstract:** We study the localization properties of disordered d-wave superconductors by means of fermionic replica trick method, deriving the effective non-linear sigma-model for the spin diffusive modes. According to the presence of certain symmetries and the range of imputity potential, we provide a detailed classification for the behavior of some physical quantities, like the density of states, the spin and the quasiparticle charge conductivities. Following the Finkel'stein approach, we finally extend the effective functional method to include residual quasiparticle interactions, at all orders in the scattering amplitudes, obtaining the complete RG equations for the full set of couplings of the theory.

# Jeudi 22 janvier 2009

## Branching random walks : effect of the selection on the survival and the genealogies.

**Damien Simon (université de Cologne)**

**Abstract:** Many simple biological models for the evolution of species are based on branching random walks. In particular, the influence of the environment (limitied resources...) on a population of such walks can lead either to a extinction of the population or to a saturation of its size. We present here some properties of these branching random walks from the point of view of statistical physics. First, we will present the phase transition between survival and extinction which occurs when the boundary conditions of the domain change. In a second part, we will characterize the genealogical trees in different selection regimes and establish some links with other classical models of statistical physicsm such as the directed polymers and voter models.

# Jeudi 8 janvier 2009 (Colloquium)

## Conducteurs balistiques: inductance cinétique, temps de vol et interactions

**B. Plaçais, Laboratoire Pierre Aigrain, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris cedex 5**

**Abstract:** Au-delà des propriétés de transmission électronique moyenne et de probabilité de transmission sondées par le transport basse fréquence et le bruit, le régime de transport quantique dynamique apporte des informations nouvelles sur les temps de transit électroniques dans les conducteurs. Pour l'expérimentateur, le transport dans un conducteur peut être décrit par un réseau d'impédances qui fait intervenir, selon sa géométrie, des résistances mais aussi des capacités et des inductances quantiques. Ces dernières ont une importance particulière dans les conducteurs quantiques qui tient au caractère fini de la densité d'états.

Dans l'exposé, nous étudierons l'inductance des fils quantiques chiraux et non-chiraux en présence d'interactions et d'écrantage [1]. Nous nous appuierons sur la théorie de diffusion développée par Christen et Büttiker [2]. Nous présenterons des mesures d'admittance GHz réalisées sur une barre de Hall [1] et des nanotubes de carbone [3]. Enfin, nous discuterons les implications de ces résultats pour la dynamique des nanotransistors mésoscopiques, qui sont des dispositifs prometteurs pour la détection au vol d'électrons unique.

[1] Relaxation time of a chiral quantum R-L circuit, J. Gabelli, et al., Phys. Rev. Lett. 98, 166806 (2007)

[2] Low-frequency admittance of quantized Hall conductors, T. Christen, M. Büttiker, Phys. Rev. B 53, 2064 (1996)

[3] Single carbon nanotube transistor at GHz frequency, J. Chaste, et al., Nano Letters 8, 525 (2008)

# Jeudi 4 décembre 2008

## Nicolas Regnault (Laboratoire Pierre Aigrain, ENS Paris)

**Titre:**Peindre l'effet Hall quantique fractionnaire

**Abstract:** Bien qu´il ait été découvert il y a plus de vingt ans et couronné d´un prix Nobel en 1998, l'effet Hall quantique fractionnaire reste un sujet vivant. Parmi les problématiques qui ont émergé récemment, on peut citer la possibilité de trouver une physique similaire dans les gaz atomiques ultrafroids en rotation et les prédictions concernant l'existence de statistiques exotiques au delà des statistiques fermioniques, bosoniques ou même fractionnaires, du nom de statistiques non abéliennes. Ces dernières suscitent un vif intérêt : il a été montré qu´elles pouvaient être utilisées comme base pour le calcul quantique en étant robuste par construction à la décohérence, actuellement le principal obstacle au développement de l´ordinateur quantique.

Avoir une vision de l´effet Hall quantique fractionnaire permettrait d´appréhender l´intégralité des phénomènes en jeu reste un objectif à atteindre. Ce système est l´archétype même de ceux où physique quantique et interactions entre particules sont si imbriquées, que les schémas usuels d´approximations ne sont plus valides. Suivant le nombre de particules et l´intensité du champ magnétique, des modèles différents permettaient d´appréhender la nature des phénomènes en jeu jusqu´à ce jour. Les plus célèbres sont le liquide de Laughlin, le modèle des fermions composites de Jain, ou encore l´état de Moore-Read. Pour autant, aucune de ces approches n´est ¨universelle¨.

Nous présenterons une approche reposant sur des liquides quantiques colorés. Chaque groupe de particules avec une couleur donnée forme un liquide de Laughlin. L´utilisation de couleurs est une manière artificielle de distinguer les particules alors qu´elles sont toutes identiques. En réalité, elles sont en quelques sorte toutes ¨grises¨. Nous proposons une procédure qui permet de rendre les particules de nouveau ¨grises¨. Cette méthode est l´analogue d´une photographie d´une peinture couleur avec un appareil photo noir et blanc. L´image en noir et blanc révèle de façon surprenante la structure interne du liquide quantique : il est constitué de gouttelettes formées à partir d´un nombre de particules égal au nombre de couleurs utilisées initialement. Les résultats de calculs numériques menés pour valider cette approche, semblent indiquer que cette structure est une caractéristique commune des différentes approches de l´effet Hall quantique fractionnaire.

# Jeudi 27 novembre 2008

## Sylvain Capponi (Université Paul Sabatier)

**Titre:** One-dimensional multicomponent fermionic cold atoms

# Jeudi 13 novembre 2008

## Christian MAES (Leuven)

**Titre:** How to understand the minimum entropy production principle

# Jeudi 6 novembre 2008

## Guiliano Orso (LPTMS, Universit Paris Sud)

**Titre:** Exotic Fermi superfluids and Bose glasses: exact results for one dimensional correlated systems

**Abstract:** Recent experiments have shown that atomic quantum gases can be used as model systems to investigate t raditional problems in Condensed Matter Physics.

In this talk I present my recent theoretical works on interacting one dimensional gases, where some exact results can be obtained.

In the first part I discuss the problem of attractive fermions undergoing supeerfluid pairing. I will focus on the interesting situation where the number of spin-up and spin-down fermions is u nequal, the population imbalance acting in the same way as the magnetic field for usual superconductors. Starting from the Bethe's ansatz solution, I have derived the exact quantum phase diagram for one dimensional gases with contact in teractions. Moreover I also showed that, in the presence of a shallow longitudinal trap, the gas phase-separates in tw o shells, with a partially polarized core. The latter is a direct realization of the celebrated Fulde-Ferrell-Larkin-O vchinnikov (FFLO) superfluid.

In the second part of my seminar, I will present some recent results on strongly interacting lattice bosons in the pre sence of disorder. I will consider the response of the gas when the tunneling rate between neighboring sites is modulated periodically in time. Using exact Bose-Fermi mappings, I have calculated the energy absorption rate as a function of the driven frequency, disorder strength and a toms filling. I will point out the differences in the response of the gas in the disordered Bose glass phase with resp ect to the Mott insulator, which occurs at integer filling.

# Mardi 28 octobre 2008 à 14 h

## Gregory FALKOVICH (Weizmann Institute)

**Titre: Inverse cascades and condensates**