Dark counts initiated by macroscopic quantum tunneling in NbN   superconducting photon detectors
  We perform measurements of the switching current distributions of three w = 120 nm wide, 4 nm thick NbN superconducting strips which are used for single-photon detectors. These strips are much wider than the diameter the vortex cores, so they are classified as quasi-two-dimensional (quasi-2D). We discover evidence of macroscopic quantum tunneling by observing the saturation of the standard deviation of the switching distributions at temperatures around 2 K. We analyze our results using the Kurkijarvi-Garg model and find that the escape temperature also saturates at low temperatures, confirming that at sufficiently low temperatures, macroscopic quantum tunneling is possible in quasi-2D strips and can contribute to dark counts observed in single photon detectors. 
A. Murphy, A. Semenov, A. Korneev, Yu. Korneeva, G. Gol'tsman and A.   Bezryadin
cond-mat.supr-con