Abstract: We present the design and performance of a novel, two-channel single-photon receiver, based on two fiber-coupled NbN superconducting single-photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders covering an area of 100 μm² and are known for ultrafast and efficient counting of single, visible-to-infrared photons. Their operation has been explained within a phenomenological hot-electron photoresponse model. Our receiver is intended for fiber-based quantum cryptography and communication systems, operational at near-infrared (NIR) telecommunication wavelengths, λ = 1.3 μm and λ = 1.55 μm. Coupling between the NbN detector and a single-mode optical fiber was achieved using a specially designed, micromechanical photoresist ring, positioned directly over the SSPD active area. The positioning accuracy of the ring was below 1 μm. The receiver with SSPDs was placed (immersed) in a standard liquid-helium transport Dewar and kept without interruption for over two months at 4.2 K. At the same time, the optical fiber inputs and electrical outputs were kept at room temperature. Our best system reached a system quantum efficiency of up to 0.3 % in the NIR radiation range, with the detector coupling efficiency of about 30 %. The response time was measured to be about 250 ps and was limited by our read-out electronics. The measured jitter was close to 35 ps. The presented performance parameters show that our NIR single photon detectors are suitable for practical quantum cryptography and for applications in quantum-correlation experiments.

Abstract: We have developed a new class of superconducting single-photon detectors (SSPDs) for ultrafast counting of infrared (IR) photons for secure quantum communications. The devices are operated on the quantum detection mechanism, based on the photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-wide superconducting stripe. The detectors are fabricated from 3.5-nm-thick NbN films and they operate at 4.2 K inside a closed-cycle refrigerator or liquid helium cryostat. Various continuous and pulsed laser sources have been used in our experiments, enabling us to determine the detector experimental quantum efficiency (QE) in the photon-counting mode, response time, time jitter, and dark counts. Our 3.5-nm-thick SSPDs reached QE above 15% for visible light photons and 5% at 1.3 – 1.5 μm infrared range. The measured real-time counting rate was above 2 GHz and was limited by the read-out electronics (intrinsic response time is <30 ps). The measured jitter was <18 ps, and the dark counting rate was <0.01 per second. The measured noise equivalent power (NEP) is 2 x 10^-18 W/Hz^1/2 at λ = 1.3 μm. In near-infrared range, in terms of the counting rate, jitter, dark counts, and overall sensitivity, the NbN SSPDs significantly outperform their semiconductor counterparts. An ultrafast quantum cryptography communication technology based on SSPDs is proposed and discussed.

Abstract: NbN detectors, formed into meander-type, 10×10-μm2 area structures, based on ultrathin (down to 3.5-nm thickness) and nanometer-width (down to below 100 nm) NbN films are capable of efficiently detecting and counting single photons from the ultraviolet to near-infrared optical wavelength range. Our best devices exhibit QE >15% in the visible range and ∼10% in the 1.3–1.5-μm infrared telecommunication window. The noise equivalent power (NEP) ranges from ∼10−17 W/Hz1/2 at 1.5 μm radiation to ∼10−19 W/Hz1/2 at 0.56 μm, and the dark counts are over two orders of magnitude lower than in any semiconducting competitors. The intrinsic response time is estimated to be <30 ps. Such ultrafast detector response enables a very high, GHz-rate real-time counting of single photons. Already established applications of NbN photon counters are non-invasive testing and debugging of VLSI Si CMOS circuits and quantum communications.

Abstract: We have measured the quantum efficiencysQEd, GHz counting rate, jitter, and noise-equivalentpowersNEPdof nanostructured NbN superconducting single-photon detectorssSSPDsdin thevisible to infrared radiation range. Our 3.5-nm-thick and 100- to 200-nm-wide meander-typedevices(total area 10310mm2), operating at 4.2 K, exhibit an experimental QE of up to 20% inthe visible range and,10% at 1.3 to 1.55mm wavelength and are potentially sensitive up tomidinfrareds,10mmdradiation. The SSPD counting rate was measured to be above 2 GHz withjitter,18 ps, independent of the wavelength. The devices’ NEP varies from,10−17W/Hz1/2for1.55mm photons to,10−20W/Hz1/2for visible radiation. Lowering the SSPD operatingtemperature to 2.3 K significantly enhanced its performance, by increasing the QE to,20% andlowering the NEP level to,3310−22W/Hz1/2, both measured at 1.26mm wavelength.

Abstract: The transparency of a film/substrate interface for thermal phonons was investigated for YBa2Cu3O7 thin films deposited on MgO, Al2O3, LaAlO3, NdGaO3, and ZrO2 substrates. Both voltage response to pulsed-visible and to continuously modulated far-infrared radiation show two regimes of heat escape from the film to the substrate. That one dominated by the thermal boundary resistance at the film/substrate interface provides an initial exponential decay of the response. The other one prevailing at longer times or smaller modulation frequencies causes much slower decay and is governed by phonon diffusion in the substrate. The transparency of the boundary for phonons incident from the film on the substrate and also from the substrate on the film was determined separately from the characteristic time of the exponential decay and from the time at which one regime was changed to the other. Taking into account the specific heat of optical phonons and the temperature dependence of the group velocity of acoustic phonons, we show that the body of experimental data agrees with acoustic mismatch theory rather than with the model that assumes strong diffusive scattering of phonons at the interface.