Power dependence of superconducting resonator quality factors

Determine the physical origin of the power dependence of internal quality factors in superconducting microwave resonators—specifically, ascertain whether the commonly observed increase of Q_i with drive power arises from saturation of dielectric two-level-system (TLS) baths or can be reconciled with dissipation dominated by nonequilibrium quasiparticles, including trapped quasiparticles that may behave as TLS.

Background

The paper argues that a universal scaling of microwave dissipation with the superfluid density points to a bulk conductive loss mechanism involving nonequilibrium quasiparticles, challenging the prevailing attribution of losses to dielectric TLS baths. In strongly disordered films, the observed dependencies on power resemble TLS behavior yet show no correlation with electric-field participation ratios, suggesting a different dissipation origin.

The authors discuss evidence that trapped quasiparticles can manifest TLS-like behavior and note recent experiments where quality factors increase with power without corresponding participation-ratio dependence, indicating quasiparticle-driven mechanisms might underlie both dissipation and its power dependence. They highlight that this reinterpretation has received limited experimental attention and that ongoing theoretical work on quasiparticle kinetics supports considering microwave-assisted processes such as overheating or ionization.