Coherent Feedback Improved Qubit Initialization in the Dispersive Regime

Understand

Readout of the state of a superconducting qubit by homodyne detection of the output signal from a dispersively coupled microwave resonator is a common technique in circuit quantum electrodynamics, and is often claimed to be quantum non-demolition (QND) up to the same order of approximation as in the dispersive approximation.

• However, in this work we show that only in the limit of infinite measurement time is this protocol QND, as the formation of a dressed coherent state in the qubit-cavity system applies an effective rotation to the qubit state.
• We show how this rotation can be corrected by a coherent operation, leading to improved qubit initialization by measurement and coherent feedback.