Constriction-limited detection efficiency of superconducting nanowire   single-photon detectors
  We investigate the source of large variations in the observed detection effiiencies of superconducting nanowire single-photon detectors between many nominally identical devices. Through both electrical and optical measurements, we infer that these variations arise from "constrictions:" highly localized regions of the nanowires where the effective cross-sectional area for superconducting current is reduced. These constrictions limit the DC bias current density to well below its critical value over the remainder of the wire, and thus prevent the detection efficiency from reaching the high values that occur in these devices only when they are biased near the critical current density. 
Andrew J. Kerman, Eric A. Dauler, Joel K.W. Yang, Kristine M.   Rosfjord, Vikas Anant, Karl K. Berggren, G. Gol'tsman and B....