Drive-induced excitation of subgap quasiparticles in superconducting circuits

Quasiparticles (QPs) in superconducting circuits can be trapped in subgap states. This trapping may be deliberate to prevent them from tunneling across Josephson elements or for use in Andreev bound state (ABS) qubits. It may also be accidental due to spurious junctions, unwanted ABSs, or material defects. Strong driving can excite these trapped QPs into the continuum, causing errors in Josephson-based qubits or destroying ABS qubits. We report investigations of the mechanisms behind low-frequency drives exciting trapped quasiparticles. We trap QPs in the ABSs of nanobridge Josephson junctions incorporated into a superconducting resonator. We drive with a tunable microwave tone to inject controlled energy and clear the quasiparticles out of trap states and into the continuum. By sweeping the ABS energy and the clearing tone frequency and power, we show the impact of both multi-photon processes and consecutive single-photon processes using intermediate states.