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

TL;DR

This study investigates quasiparticle tunneling in bandgap-engineered transmon qubits, revealing that quasiparticle dynamics occur rapidly and do not significantly affect qubit relaxation times, highlighting the need for further mitigation strategies.

Contribution

It demonstrates real-time quasiparticle tunneling detection in bandgap-engineered transmons and shows that this tunneling does not impact relaxation times under current conditions.

Findings

Quasiparticle tunneling occurs faster than 10 μs.

Qubit relaxation time T₁ is insensitive to bandgap engineering.

Reducing quasiparticle tunneling is necessary for longer T₁.

Abstract

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 $\mu$s, indicating quasiparticle-induced relaxation has to be reduced to achieve $T_1$ much longer than 100 $\mu$s.