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 T1 of the qubits is shown to be insensitive to this bandgap engineering. Operating at relatively low EJ/EC 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 μs, indicating quasiparticle-induced relaxation has to be reduced to achieve T1 much longer than 100 μs.
@article{arxiv.1112.2621,
title = {Measurements of Quasiparticle Tunneling Dynamics in a Bandgap-Engineered Transmon Qubit},
author = {L. Sun and L. DiCarlo and M. D. Reed and G. Catelani and Lev S. Bishop and D. I. Schuster and B. R. Johnson and Ge A. Yang and L. Frunzio and L. I. Glazman and M. H. Devoret and R. J. Schoelkopf},
journal= {arXiv preprint arXiv:1112.2621},
year = {2012}
}