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 μm wavelength photons and negligible dark counts.

We report our studies on spectral sensitivity of meander-type, superconducting NbN thin-film single-photon detectors (SPDs), characterized by GHz counting rates of visible and near-infrared photons and negligible dark counts. Our SPDs exhibit experimentally determined quantum efficiencies ranging from ∼0.2% at the 1.55 μm wavelength to ∼70% at 0.4 μm. Spectral dependences of the detection efficiency (DE) at the 0.4 to 3.0-μm-wavelength range are presented. The exponential character of the DE dependence on wavelength, as well as its dependence versus bias current, is qualitatively explained in terms of superconducting fluctuations in our ultrathin, submicron-width superconducting stripes. The DE values of large-active-area NbN SPDs in the visible range are high enough for modern quantum communications.

We have measured the quantum efficiency (QE), GHz counting rate, jitter, and noise-equivalent power (NEP) of nanostructured NbN superconducting single-photon detectors (SSPDs) in the visible to infrared radiation range. Our 3.5-nm-thick and 100- to 200-nm-wide meander-type devices (total area 10×10μm2), operating at 4.2K, exhibit an experimental QE of up to 20% in the visible range and ∼10% at 1.3 to 1.55μm wavelength and are potentially sensitive up to midinfrared (∼10μm) radiation. The SSPD counting rate was measured to be above 2GHz with jitter <18ps, independent of the wavelength. The devices’ NEP varies from ∼10−17W∕Hz1∕2 for 1.55μm photons to ∼10−20W∕Hz1∕2 for visible radiation. Lowering the SSPD operating temperature to 2.3K significantly enhanced its performance, by increasing the QE to ∼20% and lowering the NEP level to ∼3×10−22W∕Hz1∕2, both measured at 1.26μm wavelength.

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