Measurements of Quasiparticle Tunneling Dynamics in a Bandgap-Engineered Transmon Qubit

We have engineered the bandgap profile of transmon qubits by combining oxygen-doped Al for tunnel junction electrodes and clean Al as quasiparticle traps to investigate energy relaxation due to quasiparticle tunneling. The relaxation time $T_1$ of the qubits is shown to be insensitive to this bandgap engineering. Operating at relatively low $E_J/E_C$ makes the transmon transition frequency distinctly dependent on the charge parity, allowing us to detect the quasiparticles tunneling across the qubit junction. Quasiparticle kinetics have been studied by monitoring the frequency switching due to even/odd parity change in real time. It shows the switching time is faster than 10 $μ$s, indicating quasiparticle-induced relaxation has to be reduced to achieve $T_1$ much longer than 100 $μ$s.

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