Vanishing quasiparticle density in a hybrid Al/Cu/Al single-electron transistor

TL;DR

This paper reports on the extremely low density of residual quasiparticles in a hybrid Al/Cu/Al single-electron transistor, achieved through improved shielding and relaxation, enhancing device fidelity.

Contribution

It provides the first upper bounds on quasiparticle density and Dynes parameter in such devices, surpassing previous measurements by an order of magnitude.

Findings

Residual quasiparticle density n_qp < 0.033 μm^-3

Dynes parameter γ < 1.6×10^-7

Quasiparticle number in leads effectively zero

Abstract

The achievable fidelity of many nanoelectronic devices based on superconducting aluminum is limited by either the density of residual nonequilibrium quasiparticles n_qp or the density of quasiparticle states in the gap, characterized by Dynes parameter \gamma. We infer upper bounds n_qp < 0.033 um^-3 and \gamma < 1.6*10^-7 from transport measurements performed on Al/AlOx/Cu single-electron transistors, improving previous results by an order of magnitude. Owing to efficient microwave shielding and quasiparticle relaxation, typical number of quasiparticles in the superconducting leads is zero.