Ultrafast superconducting single-photon detectors for near-infrared-wavelength quantum communications

The paper reports progress on the design and development of niobium-nitride, superconducting single-photon detectors (SSPDs) for ultra-fast 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 appearance of a transient resistive barrier across an ultrathin and submicron-width superconducting stripe. The devices are fabricated from 3.5 nm thick NbN films and kept at cryogenic ( liquid helium) temperatures inside a cryostat. The detector experimental quantum efficiency in the photon-counting mode reaches above 20% in the visible radiation range and up to 10% at the 1.3-1.55 mm infrared range. The dark counts are below 0.01 per second. The measured real-time counting rate is above 2GHz and is limited by readout electronics ( the intrinsic response time is below 30 ps). The SSPD jitter is below 18 ps, and the best-measured value of the noise-equivalent power (NEP) is 2 x 10(-18) W/Hz(1/2) at 1.3 mum. In terms of photon-counting efficiency and speed, these NbN SSPDs significantly outperform semiconductor avalanche photodiodes and photomultipliers.