Quasiparticles in superconducting qubits with asymmetric junctions

Abstract

Designing the spatial profile of the superconducting gap -gap engineering -has long been recognized as an effective way of controlling quasiparticles in superconducting devices. In aluminum films, their thickness modulates the gap; therefore, standard fabrication of Al/AlOx/Al Josephson junctions, which relies on overlapping a thicker film on top of a thinner one, always results in gapengineered devices. Here we reconsider quasiparticle effects in superconducting qubits to explicitly account for the unavoidable asymmetry in the gap on the two sides of a Josephson junction. We find that different regimes can be encountered in which the quasiparticles have either similar densities in the two junction leads, or are largely confined to the lower-gap lead. Qualitatively, for similar densities the qubit's excited state population is lower but its relaxation rate higher than when the quasiparticles are confined; therefore, there is a potential trade-off between two desirable properties in a qubit.

Mitigation of quasiparticle loss in superconducting qubits by phonon scattering

“…Finally, we highlight that phonon traps are also relevant for transmons realized with conventional Al/AlOx Josephson junctions [44], as well as for other types of superconducting qubits, such as fluxoniums [57] and novel protected qubit designs [58].…”

“…1(g)]. The resulting phonons of energy E ≤ 2∆ trap can no longer excite quasiparticles in the qubit structures, for which the superconducting gaps of the islands ∆ NbTiN ≥ 1500 µeV and of the nanowires ∆ nw = 270 µeV are larger than that of the traps ∆ trap = 180 µeV [30,[43][44][45].…”