  The Parallel Nanowire Detector (PND) is a photon number resolving (PNR)
detector 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. PNDs were fabricated
on 3-4 nm thick NbN films grown on MgO (TS=400C) substrates by reactive
magnetron sputtering in an Ar/N2 gas mixture. The device performance was
characterized in terms of speed and sensitivity. PNDs showed a counting rate of
80 MHz and a pulse duration as low as 660ps full width at half maximum (FWHM).
Building the histograms of the photoresponse peak, no multiplication noise
buildup is observable. Electrical and optical equivalent models of the device
were developed in order to study its working principle, define design
guidelines, and develop an algorithm to estimate the photon number statistics
of an unknown light. In particular, the modeling provides novel insight of the
physical limit to the detection efficiency and to the reset time of these
detectors. The PND significantly outperforms existing PNR detectors in terms of
simplicity, sensitivity, speed, and multiplication noise.
