Coherent Feedback Improved Qubit Initialization in the Dispersive Regime

TL;DR

This paper demonstrates that measurement-based qubit initialization in circuit QED can be improved by applying coherent feedback to correct measurement-induced rotations, enhancing the fidelity of qubit state preparation.

Contribution

It introduces a correction method using coherent feedback to mitigate measurement-induced rotations, improving qubit initialization fidelity in the dispersive regime.

Findings

Measurement only non-demolition in the infinite time limit.

Formation of dressed coherent states causes effective qubit rotation.

Coherent feedback corrects the rotation, enhancing initialization fidelity.

Abstract

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.