Soutenance d'Antoine Essig

When a two-level system – a qubit – is used to probe a larger system, it naturally leads to answering a single yes-no question about the system state. Identifying what is the state of a system thus comes down to ask a series of binary questions iteratively to refine our knowledge. However, this approach leads to long measurement times for large systems, such as a resonator containing a large number of photons. In this thesis, we propose a new approach which enable us to make a measurement in a time independent of the system size. This new measurement uses the qubit as an encoder of information about the system state into the many propagating modes of a transmission line.

Assuming an ideal detector, we show that photon counting can then be implemented in a time independent of the number of photons. We demonstrate the practicality of this approach by counting the number of photons in a microwave resonator coupled dispersively to a single superconducting qubit. We observe the qubit fluorescence dependence on the resonator photon number when the qubit is driven by a microwave monochromatic tone. Using the backaction of this dispersive measurement and post-selection, we evidence the photon counting ability of the measurement. The dephasing rate between two Fock states induced by the photon number measurement is measured and compared to theory. The latter allows us the study the non-linear dependence of the dephasing rate on the microwave drive amplitude.

In a second time, the qubit fluorescence is probed using a frequency comb. Multiplexed heterodyne detections are simultaneously performed at each comb frequency and allow us to measure the photon number in the microwave resonator. This multiplexed measurement benefits from the recent bandwidth improvements of near quantum limited amplifiers. The limited cavity lifetime and detector efficiency prevented us from reaching single shot readout of the photon number in this proof-of-principle experiment. However, unlike in sequential measurement schemes, a single run of our experiment does provide, in parallel, partial information about the occupancy of each Fock state. Besides, we manage to observe the multiplexed measurement backaction on the resonator using direct Wigner tomography, which allowed us to measure the decoherence rate of the resonator induced by the measurement. We evidence an optimal qubit drive amplitude for information extraction, which matches the expected dynamics of a qubit under a multifrequency drive.