Qubit cloaking and readout*

Cavity and circuit quantum electrodynamics (QED) explore light-matter interaction at its most fundamental level, providing the tools to control the dynamical evolution of single atoms and photons in a deterministic fashion. Qubit cloaking is a simple technique to engineer a situation where, in the presence of a cavity drive, the atom decouples from the classical part of the cavity field, perceiving the cavity as if it was in the vacuum state. This is achieved by sending a cancellation drive to the qubit that destructively interferes with the effective drive coming from the filled cavity. Under cloaking, the atom and cavity undergo Rabi oscillations in the strong coupling regime even when the resonator is loaded with a coherent state. This can be a resource for non-classical cavity state preparation. In the dispersive regime, qubit cloaking allows to unconditionally arm the cavity with a coherent state in preparation for qubit readout. When measuring the qubit, the cancellation drive is switched off. We show experimental results of transmon readout using this approach. Moreover, we theoretically show that by modulating the cavity drive once the cancellation drive is switched off, one can speed up dispersive readout for any ratio of dispersive interaction over cavity linewidth and for any measurement integration time.