Feedback stabilization of low frequency noise in tunable microwave cavities with a single photon occupancy.

We successfully demonstrate suppression of low frequency noise over an approximately 1.4 kHz bandwidth in the resonance frequency fluctuations of a cavity-embedded Cooper pair transistor (cCPT) driven at an average photon number n≤10. The gate-dependent tunability of the cCPT allows us to implement a feedback technique derived from the well-established method of Pound-Drever-Hall locking, thus diminishing the intrinsic charge noise which interferes with its 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 samples. The work discusses the various aspects relating to the operation of a fully functional feedback loop at the single photon level, such as circuit bench-marking leading to optimized response, bias-dependence and non-linear characteristics determining the bandwidth of the noise reduction, and the interference of quasi-particle poisoning typically observed in similar devices that can affect the feedback parameters.