Feedback stabilization of the resonant frequency in tunable microwave cavities with single-photon occupancy

TL;DR

This paper demonstrates a feedback stabilization method for the resonant frequency of a tunable microwave cavity with single-photon occupancy, reducing charge noise-induced fluctuations and enhancing quantum device performance.

Contribution

It introduces a feedback scheme based on Pound-Drever-Hall locking for frequency stabilization at the single-photon level in tunable microwave cavities.

Findings

Achieved frequency fluctuation reduction over 1.4 kHz bandwidth at n=10

Reduced fluctuations over 11 Hz bandwidth at n=1

Demonstrated operation of a feedback loop at the single-photon level

Abstract

We successfully demonstrate low-frequency noise suppression in the resonant frequency fluctuations of a cavity-embedded Cooper pair transistor (cCPT) driven at single-photon occupancy. In particular, we report a reduction in the resonant frequency fluctuations caused by the internal charge noise over a bandwidth of $\sim$1.4 kHz when the cavity is driven at an average photon number $n=10$, and a bandwidth of 11 Hz for average $n=1$. The gate-dependent tunability of the cCPT allows us to implement a feedback-scheme, derived from the Pound-Drever-Hall locking technique. This reduces fluctuations due to intrinsic charge-noise, which otherwise interferes with the cCPT's operation as a near quantum-limited electrometer. We believe our technique can be generalized to achieve frequency stabilization in tunable microwave resonators that play a vital role in today's quantum computing architecture, thereby moderating the limitations in detection caused by the intrinsic $1/f$-noise on such circuit devices. The work discusses the various aspects relating to the operation of a fully functional feedback loop down to the single-photon level.