Abstract
The parallel nanowire detector (PND) is a photon number resolving (PNR) detector that 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, a few nm thick), folded in a meander pattern. PNDs were fabricated on 3–4 nm thick NbN films grown on MgO (TS = 400 °C) 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 660 ps 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 into 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.
Highlights
The structure of parallel nanowire detector (PND) is the parallel connection of N superconducting nanowires (N-PND), each of which can be connected in series to a resistor R0 (N-PND-R, figure 1(b))
In superconducting single photon detector (SSPD), if a superconducting nanowire is biased close to its critical current, the absorption of a photon causes the formation of a normal barrier across its cross section, so almost all the bias current is pushed to the external circuit
We found that the leakage current significantly affects only the PND detection efficiency, whereas it has a marginal effect on its signal to noise ratio (SNR)
Summary
The structure of PNDs is the parallel connection of N superconducting nanowires (N-PND), each of which can be connected in series to a resistor R0 (N-PND-R, figure 1(b)). In SSPDs, if a superconducting nanowire is biased close to its critical current, the absorption of a photon causes the formation of a normal barrier across its cross section, so almost all the bias current is pushed to the external circuit. Δ Ilk limits the maximum bias current allowed for the stable operation of the device and the detection efficiencies of the sections. The leakage current depends on the ratio between the impedance of a section ZS and Rout and it can be reduced by engineering the dimensions of the nanowire ( its kinetic inductance) and of the series resistor (see section 6). The design without series resistors simplifies the fabrication process, but, as ZS is lower, δ Ilk significantly limits the detection efficiency of the device
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