Superconducting single-photon ultrathin NbN film detector

Superconducting single-photon ultrathin NbN film detectors are studied. The development of manufacturing technology of detectors and the reduction of their operating temperature down to 2 K resulted in a considerable increase in their quantum efficiency, which reached in the visible region (at 0.56 μm) 30%–40%, i.e., achieved the limit determined by the absorption coefficient of the film. The quantum efficiency exponentially decreases with increasing wavelength, being equal to ~20% at 1.55 μm and ~0.02% at 5 μm. For the dark count rate of ~10^-4s^-1, the experimental equivalent noise power was 1.5 × 10^-20 W Hz^-1/2; it can be decreased in the future down to the record low value of 5 × 10^-21 W Hz^-1/2. The time resolution of the detector is 30 ps.