Quasiparticle Relaxation in Superconductors Coupled to Normal Metals*

The various mechanisms by which nonequilibrium quasiparticles in a superconductor relax into the condensate over a finite length/timescale are well understood to dictate the behavior of superconducting devices at intermediate temperatures or under finite bias, where quasiparticle concentrations can be large. Recently, quasiparticle relaxation has drawn renewed interest due to the sensitivity of superconducting qubits to dephasing during quasiparticle tunneling, occurring even at zero bias and at low temperatures. Such 'quasiparticle poisoning' is potentially caused by excess quasiparticles generated by the environment or through coupling of defects to microwave drives. Promising work towards the suppression of qubit decoherence currently suggests that performance can be meaningfully improved through materials engineering, such as the prevention of lossy surface oxides via capping with noble metals or non-oxidizing superconductors. However, the impacts these strategies may have on quasiparticle relaxation, and thus decoherence, remain unknown. In this talk I present experiments performed as part of the Superconducting Quantum Materials and Systems center to better understand how coupling with normal metals through direct proximity or via a tunnel barrier impacts the charge imbalance length seen in injector-detector NIS junction pairs. By directly measuring the relaxation of quasiparticles in such systems, we hope to guide future work on the mitigation of quasiparticle poisoning.