Abstract: Absorption of an infrared photon in an ultrathin film (such as 10-nm NbN) creates a localized nonequilibrium hotspot on the submicron length scale and sub-ns time scale. If a strip /spl sim/1 /spl mu/m wide is biased in the middle of the superconducting transition, this hotspot will lead to a resistance pulse with amplitude proportional to the energy of the incident photon. This resistance pulse, in turn, can be converted to a current pulse and inductively coupled to a SQUID amplifier with a digitized output, operating at 4 K or above. A preliminary design analysis indicates that this data can be processed on-chip, using ultrafast RSFQ digital circuits, to obtain a sensitive infrared detector for wavelengths up to 10 /spl mu/m and beyond, with bandwidth of 1 GHz, that counts individual photons and measures their energy with 25 meV resolution. This proposed device combines the speed of a hot-electron bolometer with the single-photon-counting ability of a transition-edge microcalorimeter, to obtain an infrared detector with sensitivity, speed, and spectral selectivity that are unmatched by any alternative technology.

Abstract: A new type of ultra-high-speed single-photon counter for visible and near-infrared wavebands based on an ultrathin NbN hot-electron photodetector (HEP) has been developed. The detector consists of a very narrow superconducting stripe, biased close to its critical current. An incoming photon absorbed by the stripe produces a resistive hotspot and causes an increase in the film’s supercurrent density above the critical value, leading to temporary formation of a resistive barrier across the device and an easily measurable voltage pulse. Our NbN HEP is an ultrafast (estimated response time is 30 ps; registered time, due to apparatus limitations, is 150 ps), frequency unselective device with very large intrinsic gain and negligible dark counts. We have observed sequences of output pulses, interpreted as single-photon events for very weak laser beams with wavelengths ranging from 0.5 /spl mu/m to 2.1 /spl mu/m and the signal-to-noise ratio of about 30 dB.

Abstract: Fabrication of NbN superconducting single-photon detectors, based on the hotspot effect is presented. The hotspot formation arises in an ultrathin and submicrometer-width superconductor stripe and, together with the supercurrent redistribution, leads to the resistive detector response upon absorption of a photon. The detector has a meander structure to maximally increase its active area and reach the highest detection efficiency. Main processing steps, leading to efficient devices, sensitive in 0.4-5 /spl mu/m wavelength range, are presented. The impact of various processing steps on the performance and operational parameters of our detectors is discussed.

Abstract: We present our recent achievements in the development of superconducting nanowire single-photon detectors (SNSPDs) integrated with optical waveguides on a chip. We demonstrate both single-photon counting with up to 90% on-chipquantum-efficiency (OCDE), and the heterodyne mixing with a close to the quantum limit sensitivity at the telecommunication wavelength using single device.

Abstract: We present our studies on quantum efficiency (QE), dark counts, and noise equivalent power (NEP) of the latest generation of nanostructured NbN superconducting single-photon detectors (SSPDs) operated at 2.0 K. Our SSPDs are based on 4 nm-thick NbN films, patterned by electron beam lithography as highly-uniform 100÷120-nm-wide meander-shaped stripes, covering the total area of 10x10 μm² with the meander filling factor of 0.7. Advances in the fabrication process and low-temperature operation lead to QE as high as  30-40% for visible-light photons (0.56 μm wavelength)-the saturation value, limited by optical absorption of the NbN film. For 1.55 μm photons, QE was  20% and decreased exponentially with the wavelength reaching  0.02% at the 5-μm wavelength. Being operated at 2.0-K temperature the SSPDs revealed an exponential decrease of the dark count rate, what along with the high QE, resulted in the NEP as low as 5x10^-21 W/Hz^-1/2, the lowest value ever reported for near-infrared optical detectors. The SSPD counting rate was measured to be above 1 GHz with the pulse-to-pulse jitter below 20 ps. Our nanostructured NbN SSPDs operated at 2.0 K significantly outperform their semiconducting counterparts and find practical applications ranging from noninvasive testing of CMOS VLSI integrated circuits to ultrafast quantum communications and quantum cryptography.