Nonlinear Charge- and Flux-Tunable Cavity Derived From an Embedded Cooper-Pair Transistor

We introduce the cavity-embedded Cooper-pair transistor (cCPT), a device that behaves as a highly nonlinear microwave cavity whose resonant frequency can be tuned both by charging a gate capacitor and by threading flux through a superconducting loop. We characterize this device and find excellent agreement between theory and experiment. A key difficulty in this characterization is the presence of frequency fluctuations comparable in scale to the cavity linewidth, which deform our measured resonance circles in accordance with recent theoretical predictions [Brock et al., Phys. Rev. Appl. 14, 054026 (2020)]. By measuring the power spectral density of these frequency fluctuations at carefully chosen points in parameter space, we find that they are primarily a result of the 1/f charge and flux noise common in solid-state devices. Notably, we also observe key signatures of frequency fluctuations induced by quantum fluctuations in the cavity field via the Kerr nonlinearity.

Automated summary

In this study, a new device called the cavity-embedded Cooper-pair transistor (cCPT) is introduced, which can tune its resonant frequency through different methods. The research finds excellent agreement between theory and experiment, but faces challenges in dealing with frequency fluctuations comparable to the cavity linewidth. By measuring the power spectral density of these fluctuations, it is discovered that they are primarily caused by 1/f charge and flux noise common in solid-state devices.