Picosecond superconducting single-photon optical detector

We experimentally demonstrate a supercurrent-assisted, hotspot-formation me chanism for ultrafast detection and counting of visible and infrared photon s. A photon-induced hotspot leads to a temporary formation of a resistive b arrier across the superconducting sensor strip and results in an easily mea surable voltage pulse. Subsequent hotspot healing in similar to 30 ps time frame, restores the superconductivity (zero-voltage state), and the detecto r is ready to register another photon. Our device consists of an ultrathin, very narrow NbN strip, maintained at 4.2 K and current-biased close to the critical current. It exhibits an experimentally measured quantum efficienc y of similar to 20% for 0.81 mum wavelength photons and negligible dark cou nts. (C) 2001 American Institute of Physics.