Ultra-low dark count rate and high system efficiency single-photon detectors with 50 nm-wide superconducting wires

Both system detection efficiency (DE) and dark count rate (DCR) are critical parameters of single-photon detectors for practical applications. For a superconducting nanowire single-photon detector (SNSPD), DE is always achieved with high bias current, which also produces high dark count rate. In this paper, we analyzed the DE of SNSPD with 50 nm NbN nanowires patterned on the films with a thickness of 4 and 6 nm. The maximum DE for communication wavelengths is 10.3% at 1310 nm with a 50 nm-wide and 4 nm-thick device. The chip with 6 nm thickness shows high DE at ultra-low dark count rate. For example, DEs of 32% for 404 nm photons and 21% for 660 nm photons were achieved at a low dark count rate of 0.01 Hz. Comparing with the SNSPD with 4 nm-thick films, the improved DE for visible photons with the 6 nm-thick SNSPD is caused by the increased absorbance of NbN films, which also reduced the dark count rate produced by thermal activation around film defects. By optimizing the micro-fabrication process and further decreasing the nanowire width while retaining uniformity, the wavelength response of the 6 nm-thick SNSPD may be extended to infrared photons.