Revealing the role of photon-assisted quasiparticle tunneling and superconducting gap differences in transmon qubits

Tunneling of nonequilibrium quasiparticles (QPs) is a source of decoherence in transmon qubits. This tunneling may occur either by pre-existing QPs tunneling across the Josephson junction or with the generation of QPs at the junction due to a photon-assisted tunneling process. In order to distinguish the contribution of photon-assisted QP tunneling to the overall QP tunneling rate, we measure the QP tunneling rate in a flux-tunable charge-parity-sensitive transmon. We observe a peak in the flux-dependence of the QP tunneling rate which we attribute to a difference in the superconducting gaps of the two aluminum films comprising the device. By examining qubit-state dependence of this peak, we infer that QPs rapidly relax in energy to a cold distribution with excess number in the lower gap film.  With a model that accounts for gap difference and photon-assisted tunneling and generation, we determine that the photon-assisted tunneling process is responsible for approximately half of all QP tunneling. Furthermore, by enhancing the rate of the photon-assisted tunneling process with a thermal photon source, we show that photon-assisted generation at the Josephson junction accounts for approximately half the QP density in the device.