Power dependence of resonator quality factors in a quasiparticle-dominated dissipation regime

Determine the microscopic origin of the power dependence of internal quality factors in superconducting resonators operated at millikelvin temperatures and low photon number, and establish whether this power dependence arises from the dynamics or saturation of nonequilibrium quasiparticles localized by disorder-induced superconducting-gap fluctuations rather than from dielectric two-level systems at surfaces and interfaces, thereby reconciling the observed behavior with a quasiparticle-based dissipation mechanism.

Background

The paper reports a universal scaling between microwave dissipation and the superfluid density across superconducting materials and device geometries, arguing for a bulk conductive loss mechanism tied to nonequilibrium quasiparticles trapped in disorder-induced sub-gap states rather than conventional dielectric two-level systems (TLS).

However, many experiments exhibit a characteristic increase of resonator quality factor with drive power that is often attributed to saturation of dielectric TLS baths. This trend appears in materials where dielectric participation does not control losses, raising a tension with the quasiparticle-based interpretation and motivating a targeted investigation of the power dependence within a nonequilibrium quasiparticle framework.

The authors note that trapped quasiparticles can, in some circumstances, behave as TLS and could thus mimic the traditional TLS-like power dependence, but a definitive microscopic account distinguishing TLS from quasiparticle mechanisms is not yet settled.