Real-time measurement of quasiparticle tunneling in a single-junction transmon qubit using feedback*

With coherence times of superconducting qubits now exceeding $100~\mathrm{\mu s}$, the contribution of quasiparticle (QP) tunneling to qubit relaxation and dephasing becomes potentially relevant. We report the real-time measurement of QP tunneling across the single junction of a 3D transmon qubit. We integrate recent developments in projective qubit readout with $99\%$ fidelity and feedback-based reset to transform the qubit into a charge-parity detector with $6~\mathrm{\mu s}$ resolution. We detect a symmetric random telegraph signal matching a QP tunneling time of $0.8~\mathrm{ms}$. By measuring the correlation function of charge parity conditioned on specific initial and final qubit states, we determine that most QP tunneling does not induce qubit transitions, in contradiction with recent theory [1]. We extract a QP-induced qubit relaxation time $T_1^{\mathrm{qp}} \sim 3~\mathrm{ms}$, decidedly not limiting the measured $T_1 = 0.14~\mathrm{ms}$.\\[4pt] [1] G. Catelani et al., Phys. Rev. B 84, 064517 (2011).