Superconducting nanowire single photon detector (SNSPD) has been widely used in many fields such as quantum computing, quantum key distribution and laser radar, due to its high detection efficiency, low dark count rate, high counting rate, and low timing jitter. In most cases, SNSPD works under the DC-bias mode that can detect single photons at any time they arrive. In cases like satellite laser ranging and singlephoton laser radar where the light pulses arrive regularly, the AC-bias mode enable SNSPD work with higher counting rates and lower background dark counts, which however require complicated readout due to the low signal-to-noise ratio of the photon response. In this paper, we report on an AC-biased SNSPD system with self-differential readout circuit. The system includes a 2-pixel SNSPD consisting of two parallel nanowires, which are biased with 100 MHz sinusoidal current. The output signals of these two nanowires are amplified and combined for the differential readout of the photon response. The resulting response pulse shows a ten times higher signal-to-noise ratio than the one extracted before self-differential readout. In addition, the dark counts are decreased by a factor of 4, and the count rates are increased by a factor of 1.5, comparing to the ones under the DC-bias mode. This work provides a specific method to read out the AC-biased SNSPD.
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