Quasiparticle Tunneling Suppression in Gap-Engineered Transmons

For many superconducting qubits, non-equilibrium quasiparticles tunneling across the Josephson junction is suspected to be one of the dominant sources of energy relaxation. This loss mechanism is expected to be suppressed devices with junctions that are made from electrodes with sufficiently different superconducting gaps [1]. We have designed and fabricated an asymmetric Al/AlOx/Al/Ti transmon coupled to a 3D Al cavity. To do this, we deposited one pure Al electrode, oxidized it, and then deposited an Al/Ti bilayer as the counter-electrode. This process forms a standard AlOx tunnel barrier, but gives a ~100 μeV difference in superconducting gaps due to the Ti proximitizing the Al of the top layer. Fabrication of these devices, Giaever tunneling measurements of the small Al/AlOx/Al/Ti junction, as well as coherence measurements of the device will be discussed.

[1] Zhang, PhD Thesis DOI: 10.13016/c516-gizd (2020)