Spencer Diamond

Department of Applied Physics, Yale University

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

Single-electron tunneling across Josephson junctions in hybrid and conventional superconducting qubits contributes to decoherence and limits qubit performance. In several architectures, including superconducting transmons and proximitized semiconductor nanowires, single-electron tunneling rates have been measured, and mitigating efforts are ongoing. In the past, such decoherence was exclusively attributed to a density of non-equilibrium quasiparticles of mysterious origin. However, it was recently predicted that high-frequency photons can be efficiently absorbed at transmon Josephson junctions and induce single-electron tunneling. This process requires no pre-existing quasiparticles, but in fact generates two quasiparticles and can likewise change the qubit state. We attempt to distinguish between these two types of single-electron tunneling by measuring charge-parity switching rates in offset-charge-sensitive transmons. We found that the relative rates of qubit excitation and relaxation caused by single-electron tunneling were inconsistent with a thermal distribution of quasiparticles in the superconductor tunneling across the junction and consistent with photon-assisted tunneling events. Additionally, we have calculated the anticipated rate of single-electron tunneling for both mechanisms as a function of the flux threading a dc-SQUID implementation of the offset-charge-sensitive transmon. Finally, we have recently measured a flux-tunable device, gaining insights into the causes of charge-parity switches. These results add to our understanding of this decoherence source and may lead to more effective mitigation techniques.