Origin of dark counts in nanostructured NbN single-photon detectors

We present our study of dark counts in ultrathin (3.5 to 10 nm thick), narrow (120 to 170 mn wide) NbN superconducting stripes of different lengths. In experiments, where the stripe was completely isolated from the outside world and kept at temperature below the critical temperature T-c, we detected subnanosecond electrical pulses associated with the spontaneous appearance of the temporal resistive state. The resistive state manifested itself as generation of phase-slip centers (PSCs) in our two-dimensional superconducting stripes. Our analysis shows that not far from T-c, PSCs have a thermally activated nature. At lowest temperatures, far below T-c, they are created by quantum fluctuations.