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

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 mu m(-3) and gamma < 1.6 x 10(-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.