Simple quantum feedback of a solid-state qubit

TL;DR

This paper proposes a simplified quantum feedback control experiment for a solid-state qubit, using detector current quadratures to maintain coherent oscillations with high fidelity, achievable with current technology.

Contribution

It introduces a simpler feedback method for solid-state qubits that requires less bandwidth and maintains oscillations with about 95% fidelity.

Findings

Feedback can sustain qubit oscillations over long times

The method is experimentally verifiable through detector current phase

Fidelity limited to approximately 95%

Abstract

We propose an experiment on quantum feedback control of a solid-state qubit, which is almost within the reach of the present-day technology. Similar to the earlier proposal, the feedback loop is used to maintain the coherent (Rabi) oscillations in a qubit for an arbitrary long time; however, this is done in a significantly simpler way, which requires much smaller bandwidth of the control circuitry. The main idea is to use the quadrature components of the noisy detector current to monitor approximately the phase of qubit oscillations. The price for simplicity is a less-than-ideal operation: the fidelity is limited by about 95%. The feedback loop operation can be experimentally verified by appearance of a positive in-phase component of the detector current relative to an external oscillating signal used for synchronization.