Measurement backaction and feedback on a superconducting qubit
Philippe Campagne-Ibarcq (LPA)

Abstract

In quantum physics, a measurement corresponds to the interaction of a system with an observer, who is part of the environment. In general, this measurement disturbs the system state in a an effect known as the quantum backaction. This perturbation is stochastic and cannot be predicted a priori. However, if the observer efficiently extracts the information from the measurement, he can know about the back action a posteriori, and thus keep track of the system evolution.

As flexible quantum machines, whose collective behavior follows the laws of quantum physics, superconducting circuits are promising systems to investigate this subject. A particular superconducting qubit, the 3D transmon, which follows a recently developed architecture, has been shown to reach coherence times over 100 microseconds. Combined with the development of near quantum limited parametric amplifiers, also based on superconducting circuits, it is possible to coherently control, measure and react on the 3D transmon before it loses its coherence.

In this thesis, we describe several experiments performing such tasks on a 3D transmon. In particular, the fluorescence signal is used to unravel quantum jumps during relaxation. When averaged conditionnally to a final projective measurement outcome, the signal displays weak values out of range for unconditionnal average. It is also used to implement continuous analog feedback in order to stabilize an arbitrary state of the qubit. A high fidelity non demolition measurement in a single shot is also demonstrated and is used to implement stroboscopic digital feedback, which also stabilizes an arbitrary state.