Two-level Tunneling Defects in Superconducting Devices

We report theoretical results for the impact of two-level tunneling systems (TLS) on superconductivity, including Tc and the spectrum of quasiparticle excitations for conventional superconductors such as aluminum and niobium. We show that non-magnetic TLS impurities in conventional superconductors are pair-breaking or pair-enhancing defects depending on the level population and dynamics of the distribution of TLS impurities. We present results for the enhancement of superconductivity by TLS defects in thermal equilibrium with electrons and phonons. The scattering of quasiparticles by TLS impurities leads to sub-gap states below the bulk excitation gap, ∆, as well as resonances in the continuum above ∆. The energies and spectral weights of these states depend on the distribution of tunnel splittings. The spectral weights of these states are sensitive to the level occupations of the TLS impurities. Under microwave excitation a nonequilibrium level population of the TLS distribution generates subgap states near the Fermi level, leading to quasiparticle dissipation and lower performance of superconducting devices and qubits at low temperatures.