Quasiparticle Generation & Dissipation from Embedded TLS Impurities in Superconductors under Microwave Excitation

Two-level system (TLS) and nonequilibrium quasiparticle losses are ubiquitous in superconducting devices operating under microwave drive. But, the microscopic origin of TLS and nonequilibrium quasiparticle populations, as well as the loss mechanisms, are not fully understood. We report on recent theoretical work on the interaction of an ensemble of TLS impurities with electrons and Cooper pairs in superconducting devices under microwave drive [1]. A key prediction of our theory is that embedded TLS impurities are strong pair-breakers when their level populations are driven out of equilibrium. Inelastic scattering by the nonequilibrium TLS population generates sub-gap quasiparticle states near the Fermi level which contribute Ohmic dissipation and thus degrade the performance of superconducting devices at low temperatures. We report calculations for the sub-gap quasiparticle spectrum generated by nonequilibrium TLS impurities, and the frequency and temperature dependent microwave conductivity of superconductors for realistic distributions of embedded TLS impurities.

[1] J. He, Wei-Ting Lin and J. A. Sauls, Theory of Two-Level Tunneling Systems in Superconductors, Prog. Theor. Expt. Phys., 2025, 6, 063I01 (2025)