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.
@article{arxiv.2011.06298,
title = {A Nonlinear Charge- and Flux-Tunable Cavity Derived from an Embedded Cooper Pair Transistor},
author = {B. L. Brock and Juliang Li and S. Kanhirathingal and B. Thyagarajan and William F. Braasch and M. P. Blencowe and A. J. Rimberg},
journal= {arXiv preprint arXiv:2011.06298},
year = {2021}
}