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

TL;DR

This paper introduces a highly nonlinear, tunable microwave cavity device called cCPT, which combines charge and flux control, and investigates its frequency fluctuations caused by charge, flux noise, and quantum effects.

Contribution

The paper presents the design, characterization, and theoretical-experimental agreement of the cCPT device, highlighting its tunability and noise characteristics.

Findings

Excellent agreement between theory and experiment.

Frequency fluctuations are mainly due to 1/f charge and flux noise.

Quantum fluctuations induce observable frequency fluctuation signatures.

Abstract

We introduce the cavity-embedded Cooper pair transistor (cCPT), a device which 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 SQUID 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. Applied 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.