We present our progress on the research and development of niobium-nitride, superconducting single-photon detectors (SSPDs) for ultrafast counting of near-infrared photons for secure quantum communications. The SSPDs operate in the quantum detection mode, based on photon-induced hotspot formation and subsequent development of a transient resistive barrier across an ultrathin and submicron-width superconducting stripe. The devices are fabricated from 4-nm-thick NbN films and kept at cryogenic (liquid helium) temperatures. The detector experimental quantum efficiency in the photon-counting mode reaches above 40% in the visible range of radiation and up to 30% at the 1.3 to 1.55 μm infrared range. The dark counts are below 0.01 per second. The measured real-time counting rate is above 2 GHz and is limited by our readout electronics. The SSPD jitter is below 18 ps, and the best-measured value of the noise-equivalent power (NEP) is 5 x 10^-21 W/Hz^1/2 at 1.3 μm. In terms of the photon-counting efficiency and speed, our NbN SSPDs significantly outperform semiconductor avalanche photodiodes and photomultipliers at the 1.3 to 1.55 μm infrared range.

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1. Superconducting hot-electron bolometer based on NbN nanostructures as THz mixer, direct detector and IR single-photon counter