Photon-assisted charge-parity switches in superconducting qubits*

Here we identify a mechanism by which stray photons, with energy large enough to break Cooper pairs, can cause decoherence in superconducting qubits. Similar to decoherence induced by steady-state nonequilibrium quasiparticles, these photon-assisted tunneling (PAT) processes are associated with a change in the charge parity of the qubit. This enables the separation of these quasiparticle-related processes from other contributions to decoherence. Our theory predicts similar rates of PAT-induced relaxation and excitation of the qubit, in agreement with recent experiments. This talk will detail the similarities and differences between PAT processes and decoherence induced by nonequilibrium quasiparticle tunneling.