Investigating the mechanism of single-electron tunneling in charge-parity-sensitive transmons*

Single-electron tunneling across Josephson junctions in superconducting qubits contributes to decoherence and limits qubit performance. In the past, such decoherence was exclusively attributed to pre-existing non-equilibrium quasiparticles that tunnel across junctions and exchange energy with the qubit. However, it was recently predicted that high-frequency photons can be efficiently absorbed in transmon Josephson junctions and induce single-electron tunneling. This process requires no pre-existing quasiparticles; in fact it generates two quasiparticles and, in doing so, can change the qubit state. Past measurements of single-electron tunneling-induced excitation and relaxation in charge-parity-sensitive transmons were consistent with photon-assisted tunneling. Here, we will present theoretical and experimental results demonstrating that adding flux-tunability to a charge-parity-sensitive transmon can distinguish the contributions of different single-electron tunneling processes in our devices.