Agenda de l'ENS de Lyon

Effect of the environment on Fluxonium qubits and thermodynamics of quantum measurement

lun 27 sep 2021



Soutenance de thèse de M. STEVENS Jéremy sous la Direction de M. HUARD Benjamin

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Description générale

Superconducting qubits are at the heart of many experiments exploring elementary quantum mechanics and are one of the principal candidates for use in a future quantum computer. In both cases, a high level of control over both the quantum system and its environment is necessary. The first chapters of this thesis describe several techniques used to engineer the effect of the environment in circuit QED with a focus on the promising Fluxonium qubit. We give a detailed treatment of the theoretical basis for protected qubits and analyse device design and dilution refrigerator wiring from the perspective of environment noise reduction. 
As an application of these methods, we reproduce state of the art conditions for Fluxonium experiments in order to study the effect of the environment on the qubit. We show how photons in the cavity used to measure the quantum bit and a high temperature of the dilution refrigerator in which the device is placed can have detrimental effects on the stability of the quantum state. Critically, this shows that there remain several problems still to solve regarding the dispersive readout of Fluxoniums before using them as the basis for a quantum computer.
Theoretically, the decoherence of a qubit can be broken down into many coherent exchanges with the qubit environment about which the observer has no information. In the last part of the thesis, we show the results of an experiment providing insight into the thermodynamics of quantum measurement and operations by observing the coherent energy exchange between a propagating field and a qubit under measurement. We provide an interpretation of the preparation of a qubit state by a coherent light pulse as a weak measurement of the pulse by the qubit.


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