Quasiparticle tunneling in a single-junction transmon qubit*

The recent increase in transmon qubit quality factor into the million range [1] makes non-equilibrium quasiparticle tunneling a potentially limiting mechanism for qubit coherence. We investigate the dynamics of quasiparticle tunneling in a single-junction transmon qubit with relaxation time $T_1=85~\mathrm{\mu s}$ ($Q=2.6$ million). The qubit operates at moderate ratio of Josephson to charging energy, $E_J/E_C\sim30$, where charge parity in the qubit islands is encoded in the qubit transition frequency. Using Ramsey-type and stimulated echo experiments, we investigate quasiparticle tunneling across the qubit junction on time scales short and long compared to $T_1$. We observe that the quasiparticle tunneling time for the single-junction qubit is at least as long as $T_1$, but shorter than the $1~\mathrm{ms}$ repetition rate. This result is consistent with recent theory and qualitatively different from the two-junction transmon. The dephasing time $T_2^{\ast}=10~\mathrm{\mu s}$ is limited by slow background charge fluctuations and extended to $T_2=95~\mathrm{\mu s}$ using dynamical decoupling. \\[4pt] [1] Paik et al. arXiv:1105.4652v4