Picosecond superconducting single-photon optical detector

We experimentally demonstrate a supercurrent-assisted, hotspot-formation mechanism for ultrafast detection and counting of visible and infrared photons. A photon-induced hotspot leads to a temporary formation of a resistive barrier across the superconducting sensor strip and results in an easily measurable voltage pulse. Subsequent hotspot healing in ;30 ps time frame, restores the superconductivity ~zero-voltage state!, and the detector 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 efficiency of ;20% for 0.81 mm wavelength photons and negligible dark counts. © 2001 American Institute of Physics. @DOI: 10.1063/1.1388868#