SQUID-Readout for the Superconducting Nanowire Single-Photon Detector

Superconducting single-photon detectors from thin niobium nitride nanostrips exhibit a near-infrared cut-off of the wavelength-independent quantum efficiency along with a moderate energy resolution in the wavelength range around the cut-off. For the wavelength range before the cut-off, we have developed the electro-thermal model that describes the initial growth, diminution and disappearance of the normal domain created by a photon in the superconducting nanostrip. The response parameters extracted from the model fit were used to optimize the signal-to-noise ratio of the SQUID-based readout and to achieve in the temperature range from 1 to 4 K a photon counting frequency of 20 MHz and a dark count rate less than four in one hour.