Quasiparticle dynamics in Andreev bound states part 1: phonon interactions

Non-equilibrium quasiparticles in superconducting circuits can be a major source of error in both traditional and hybrid qubit platforms. In order to suppress these quasiparticles, it is necessary to learn more about their behavior. Quasiparticle dynamics in Andreev Bound States (ABS) are particularly interesting, as these provide a promising hybrid qubit platform. We present measurements of the dynamics of quasiparticles trapped in the ABS of a fully superconducting aluminum nanobridge Josephson junction. We focus on electron-phonon interactions leading to relaxation and excitation between the ABS and the bulk continuum of quasiparticle states. We find evidence for thermal processes and low-temperature saturation to multiple non-equilibrium processes.