Single-photon detectors (SPDs) are the foundation of all quantum communications (QC) protocols.
Among different classes of SPDs currently studied, NbN superconducting SPDs (SSPDs) are established as the
best devices for ultrafast counting of single photons in the infrared (IR) wavelength range. The SSPDs are
nanostructured, 100 μm2
in total area, superconducting meanders, patterned by electron lithography in ultra-thin
NbN films. Their operation has been explained within a phenomenological hot-electron photoresponse model.
We present the design and performance of a novel, two-channel SPD receiver, based on two fiber-coupled NbN
SSPDs. The receivers have been developed for fiber-based QC systems, operational at 1.3 μm and 1.55 μm
telecommunication wavelengths. They operate in the temperature range from 4.2 K to 2 K, in which the NbN
SSPDs exhibit their best performance. The receiver unit has been designed as a cryostat insert, placed inside a
standard liquid-heliumstorage dewar. The input of the receiver consists of a pair of single-mode optical fibers,
equipped with the standard FC connectors and kept at room temperature. Coupling between the SSPD and the
fiber is achieved using a specially designed, precise micromechanical holder that places the fiber directly on top
of the SSPD nanostructure. Our receivers achieve the quantum efficiency of up to 7% for near-IR photons, with
the coupling efficiency of about 30%. The response time was measured to be < 1.5 ns and it was limited by our
read-out electronics. The jitter of fiber-coupled SSPDs is < 35 ps and their dark-count rate is below 1s-1. The
presented performance parameters show that our single-photon receivers are fully applicable for quantum correlation-type QC systems, including practical quantum cryptography.
We have fabricated fiber-coupled superconducting single-photon detectors (SSPDs), designed for quantum-correlationtype
experiments. The SSPDs are nanostructured (~100-nm wide and 4-nm thick) NbN superconducting meandering
stripes, operated in the 2 to 4.2 K temperature range, and known for ultrafast and efficient detection of visible to nearinfrared
photons with almost negligible dark counts. Our latest devices are pigtailed structures with coupling between
the SSPD structure and a single-mode optical fiber achieved using a micromechanical photoresist ring placed directly
over the meander. The above arrangement withstands repetitive thermal cycling between liquid helium and room
temperature, and we can reach the coupling efficiency of up to ~33%. The system quantum efficiency, measured as the
ratio of the photons counted by SSPD to the total number of photons coupled into the fiber, in our early devices was
found to be around 0.3 % and 1% for 1.55 &mgr;m and 0.9 &mgr;m photon wavelengths, respectively. The photon counting rate
exceeded 250 MHz. The receiver with two SSPDs, each individually biased, was placed inside a transport, 60-liter
liquid helium Dewar, assuring uninterrupted operation for over 2 months. Since the receiver's optical and electrical
connections are at room temperature, the set-up is suitable for any applications, where single-photon counting capability
and fast count rates are desired. In our case, it was implemented for photon correlation experiments. The receiver
response time, measured as a second-order photon cross-correlation function, was found to be below 400 ps, with
timing jitter of less than 40 ps.
We report on our progress in research and development of ultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbN nanostructures. Our SSPDs were made of the 4-nm-thick NbN films with Tc ~11 K, patterned as meander-shaped, 100-nm-wide strips, and covering an area of 10×10 μm2. The detectors exploit a combined detection mechanism, where upon a single-photon absorption, a hotspot of excited electrons and redistribution of the biasing supercurrent, jointly produce a picosecond voltage transient signal across the superconducting nanostripe. The SSPDs are typically operated at 4.2 K, but their sensitivity in the infrared radiation range can be significantly improved by lowering the operating temperature from 4.2 K to 2 K. When operated at 2 K, the SSPD quantum efficiency (QE) for visible light photons reaches 30-40%, which is the saturation value limited by the optical absorption of our 4-nm-thick NbN film. With the wavelength increase of the incident photons,the QE of SSPDs decreases significantly, but even at the wavelength of 6 μm, the detector is able to count single photons and exhibits QE of about 10-2 %. The dark (false) count rate at 2 K is as low as 2x10-4 s,-1 which makes our detector essentially a background-limited sensor. The very low dark-count rate results in a noise equivalent power (NEP) below 10-18 WHz-1/2 for the mid-infrared range (6 μm). Further improvement of the SSPD performance in the mid-infrared range can be obtained by substituting NbN for another, lower-Tc materials with a narrow superconducting gap and low quasiparticles diffusivity. The use of such superconductors should shift the cutoff wavelength below 10 μm.
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.
We report on a study of S-N and N-S current switching in high quality YBaCuO films deposited onto ZrO2 and NdGaO3 substrates. The films 60-120 nm thick prepared by laser ablation were structured into single strips and were provided with gold contacts. We monitored the time dependence of the resistance upon application of the voltage step on the film. Experiment performed within certain ranges of voltage amplitudes and temperatures showed the occurrence of the fast stage both in S-N (shorter than 300 ps) and N-S transition. We discuss the mechanism of switching taking into account the hot electron phenomena in YBaCuO. The contributions of various thermal processes in the subsequent stage of the resistance dynamic are also discussed. The basic limiting characteristics (average dissipated power, minimum work done for switching, maximum repetition rate) of a picosecond switch which is proposed to be developed are estimated.