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The achievable fidelity of many nanoelectronic devices based on superconducting aluminum is limited by either the density of residual nonequilibrium quasiparticles nₐ or the density of quasiparticle states in the gap, characterized by Dynes parameter. We infer upper bounds nₐ<0. 0334pt{0ex}m^-3 and <1. 610^-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.
Saira et al. (2012) studied this question.