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

Abstract

The achievable fidelity of many nanoelectronic devices based on superconducting aluminum is limited by either the density of residual nonequilibrium quasiparticles ${n}_{\mathrm{qp}}$ or the density of quasiparticle states in the gap, characterized by Dynes parameter $\ensuremath{\gamma}$. We infer upper bounds ${n}_{\mathrm{qp}}<0.033\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}3}$ and $\ensuremath{\gamma}<1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ from transport measurements performed on Al/Cu/Al single-electron transistors, improving previous results by an order of magnitude. Owing to efficient microwave shielding and quasiparticle relaxation, a typical number of quasiparticles in the superconducting leads is zero.