Optical-to-electrical conversion, which is the basis of the operation of optical detectors, can be linear or nonlinear. When high sensitivities are needed, single-photon detectors are used, which operate in a strongly nonlinear mode, their response being independent of the number of detected photons. However, photon-number-resolving detectors are needed, particularly in quantum optics, where n-photon states are routinely produced. In quantum communication and quantum information processing, the photon-numberresolving functionality is key to many protocols, such as the implementation of quantum repeaters 1 and linear-optics quantum computing 2 . A linear detector with single-photon sensitivity can also be used for measuring a temporal waveform at extremely low light levels, such as in longdistance optical communications, fluorescence spectroscopy and optical time-domain reflectometry. We demonstrate here a photon-number-resolving detector based on parallel superconducting nanowires and capable of counting up to four photons at telecommunication wavelengths, with an ultralow dark count rate and high counting frequency.Among the approaches proposed so far for photon-numberresolving (PNR) detection (Table 1) are detectors based on charge integration or field-effect transistors 3-5 , which are, however, affected by long integration times, leading to bandwidths of ,1 MHz. Transition edge sensors 6 operate at 100 mK and show long response times (several microseconds). Approaches based on photomultipliers 7 and avalanche photodiodes, such as the visiblelight photon counter 8,9 , two-dimensional arrays of avalanche photodiodes 10,11 and time-multiplexed detectors 12,13 are not sensitive or are plagued by high dark count rates (DKs) and long dead times in the telecommunication spectral windows. Arrays of single-photon detectors (SPDs) also involve complex readout schemes 11 or separate contacts, amplification and discrimination 14. The parallel nanowire detector (PND) presented here significantly outperforms these approaches in terms of simplicity, sensitivity, speed and multiplication noise.The basic structure of the PND comprises the parallel connection of N superconducting nanowires, each connected in series to a resistor R 0 (Fig.

We have measured the ultrafast reset time of NbN superconducting single photon detectors (SSPDs) based on a design consisting of N parallel superconducting stripes. Compared to a standard SSPD of identical active area, the parallel SSPD displays a similar detection efficiency and a kinetic inductance, which is divided by N2. For N=12, the duration of the voltage detection pulse is reduced by nearly two orders of magnitude down to 200ps. The timing jitter associated with the rising front is only 16ps. These results open a way to efficient detectors with ultrahigh counting rate exceeding 1 GHz.

It was investigated the possibility of creating NbN superconducting single-photon detectors with saturated dependence of quantum efficiency versus normalized bias current. It was shown that the saturation increases for the detectors based on finer films with a lower value of R s300 /R s20 . The decreasing of R s300 /R s20 related to increasing influence of quantum corrections to conductivity of superconductors and, in its turn, to decreasing electron diffusion coefficient. The best samples has constant value of system quantum efficiency 94% at I b /I c~0 .8 and wavelength 1310 nm.

No abstract

We present a new photon number resolving detector (PNR), the Parallel Nanowire Detector (PND), which uses spatial multiplexing on a subwavelength scale to provide a single electrical output proportional to the photon number. The basic structure of the PND is the parallel connection of several NbN superconducting nanowires (≈100 nm-wide, few nm-thick), folded in a meander pattern. Electrical and optical equivalents of the device were developed in order to gain insight on its working principle. PNDs were fabricated on 3-4 nm thick NbN films grown on sapphire (substrate temperature T S =900°C) or MgO (T S =400°C) substrates by reactive magnetron sputtering in an Ar/N 2 gas mixture. The device performance was characterized in terms of speed and sensitivity. The photoresponse shows a full width at half maximum (FWHM) as low as 660ps. PNDs showed counting performance at 80 MHz repetition rate. Building the histograms of the photoresponse peak, no multiplication noise buildup is observable and a one photon quantum efficiency can be estimated to be η∼3% (at 700 nm wavelength and 4.2 K temperature). The PND significantly outperforms existing PNR detectors in terms of simplicity, sensitivity, speed, and multiplication noise.