Measuring Quasi-particle Tunneling Rates in Transmon Qubits*

The tunneling of non-equilibrium quasiparticles (QPs) across the Josephson junction of superconducting qubits is a source of energy loss and dephasing, resulting in a reduced coherence time. To measure changes in the charge parity rate between the two pads of a transmon qubit, we have designed transmons with a charge dispersion approximately 6 MHz between the first excited |e> and the second excited states |f>. The designed transmons allow fast mapping of the charge parity rate while maintaining a good charge-noise protection between the |g> and |e> states. Using a modified Ramsey pulse sequence on the |e> to |f> states, we monitor temporally the effective charge parity of the qubits. For some Al qubits, we compare the measured charge parity rates to T₁ with and without a direct galvanic connection to the ground plane.