TLS Fluctuations in Superconducting Circuits - Part 2*

At the single-photon regime and millikelvin temperatures, two-level systems (TLS) present in amorphous dielectric materials remain the main source of loss and noise in superconducting quantum devices. Superconducting microwave resonators, due to their low relaxation and decoherence rates and high sensitivity to noise, have long been used to investigate the physics of TLSs. In addition, their ease of fabrication and simpler measurement requirements make them a convenient experimental platform.

In this work, we study the time-fluctuations of four lumped-element resonators. We attribute these stochastic fluctuations to interacting TLSs, as explained by the generalized tunneling model (GTM). We measure the resonator quality factor and resonance frequency over 60-hour periods and perform in-depth statistical analyses (the power spectral density and the Allan deviations) to characterize the noise. These analyses reveal that the dominant process is the TLS-induced 1/f, or near-1/f, noise.

For comparison, we also perform comprehensive simulations of the loss and frequency time-fluctuations of resonators according to the GTM. The simulations are found to be in excellent agreement with the experimental data and therefore provide additional evidence for the correctness of the TLS-TLS interaction hypothesis of the GTM.