We evaluate experimentally the intrinsic detection efficiency (IDE) of superconducting NbN nanowire single-photon detectors in the range of wire thicknesses from 4 to 12 nm. The study is performed in the broad spectral interval between near-ultraviolet (wavelength 400 nm) and near-infrared (wavelength 2000 nm) light with plane waves at normal incidence. For visible light the IDE of the thinnest detectors reaches 70%. We use numerically computed absorptance of the nanowire-structures for the analysis of the experimental data. Variations in the detection efficiency with both the wire thickness and the wavelength evidence the red boundary of the hot-spot photon-detection mechanism. We explain the detection at larger wavelengths invoking thermal excitation of magnetic Pearl vortices over the potential barrier at the edges of the wire.

The critical current density, j C , in Ba(Fe,Co) 2 As 2 thin-film microbridges was evaluated from current-voltage characteristics measured using a standard four-probe technique. The 90-nm-thick films were deposited by pulsed laser deposition on heated (La,Sr)(Al,Ta)O 3 substrates and patterned by means of photolithography and ionmilling techniques. The resulting microbridges show a good long-term stability and only minor degradation of the superconducting properties with respect to as-deposited films. The self-field j C at T = 4.2 K reaches a value of about 3 MA/cm 2 . The temperature dependence of j C is described by (1 − T /T C ) 1.5 , which is identical to the Ginzburg-Landau theory for the depairing critical current, in the wide temperature range 0.4 < T /T C < 1. Expulsion of the magnetic vortices is considered to be the mechanism responsible for overcoming Likharev's limit, where the width of the microbridge must be smaller than 4.4ξ GL (T ) to observe the depairing critical current.

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Ultra-thin films of superconducting tantalum nitride are deposited by reactive magnetron sputtering on heated sapphire substrates. The critical temperature T C=10.25 K is reached for films thicker than 10 nm. A superconducting nanowire single-photon detector in the form of a meander line with a width of 110 nm was made from 5 nm thick TaN film. The detector had a transition temperature of 8.3 K and a critical current density of 4 MA/cm2 at 4.2 K. A photon detection efficiency of 20% has been obtained for the detector with a filling factor of 0.55 at wavelengths up to 700 nm.

We demonstrate the transfer of single photon triggered electrical pulses from a superconducting nanowire single photon detector (SNSPD) to a single flux quantum (SFQ) pulse. We describe design and test of a digital SFQ based SNSPD readout circuit and demonstrate its correct operation. Both circuits (SNSPD and SFQ) operate under the same cryogenic conditions and are directly connected by wire bonds. A future integration of the present multi-chip configuration seems feasible because both fabrication process and materials are very similar. In contrast to commonly used semiconductor amplifiers, SFQ circuits combine very low power dissipation (a few microwatts) with very high operation speed, thus enabling count-rates of several gigahertz. The SFQ interface circuit simplifies the SNSPD readout and enables large numbers of detectors for future compact multi-pixel systems with single photon counting resolution. The demonstrated circuit has great potential for scaling the present interface solution to 1,000 detectors by using a single SFQ chip.