Investigating the mechanisms of charge-parity switching in offset-charge-sensitive transmons

Charge-parity switches in superconducting qubits contribute to decoherence and limit qubit performance. In the past, such decoherence was exclusively attributed to pre-existing non-equilibrium quasiparticles tunneling across Josephson junctions and exchanging energy with the qubit. However, it was recently predicted that high-frequency photons can be efficiently absorbed at transmon Josephson junctions and cause charge-parity switches. This process requires no pre-existing quasiparticles, but in fact generates two quasiparticles and can likewise change the qubit state. These two types of charge-parity switches are distinguishable by their relative rates of qubit excitation and relaxation, which have been measured in single-junction offset-charge-sensitive transmons. The transition rates were found to be inconsistent with a thermal distribution of quasiparticles in the superconductor tunneling across the junction, but may be explained by photon-assisted tunneling events. Here, we will present experimental results demonstrating that adding flux-tunability to our device can further distinguish between these charge-parity switch-induced decoherence mechanisms.