Ultimate low system dark count rate for superconducting nanowire single-photon detector

The dark count rate (DCR) is a key parameter of single-photon detectors. By introducing a bulk optical band-pass filter mounted on a fiber-to-fiber optical bench cooled at 3 K and blocking down to 5 micrometer, we suppressed the DCR of a superconducting nanowire single-photon detector by more than three orders of magnitude. The DCR is limited by the blackbody radiation through a signal passband of 20 nm bandwidth. The figure of merit, system detection efficiency, and DCR were 2.7 x 10^11, 2.3 %, and 0.001 Hz, respectively. Narrowing the bandwidth to 100 GHz suppresses the DCR to 0.0001 Hz and the figure of merit increases to 1.8 x 10^12.