Photon counting is an indispensable technology in frontier optical science such as an optical quantum technology. A superconducting nanowire single-photon detector (SSPD) that has high detection efficiency, low dark count rate, low timing jitter and wide detection wavelength band is one of the promising single-photon detectors. It has been applied in a wide range of fields such as remote sensing, biofluorescence measurements and quantum communication. To further improve the detection capability, we develop a multi-pixel SSPD array system. The multi-pixel SSPD enables photon counting imaging, pseudo photon number resolving and dead-time-free photon counting that cannot be achieved by single-pixel SSPD. One of the significant issues to develop the SSPD array system is its signal readout, because the increase in the number of pixels requires many readout cables which causes a serious heat inflow into the cryocooler. To overcome this issue, we proposed signal multiplexing using single-flux-quantum (SFQ) circuit. The SFQ circuit can operate under cryogenic environment with low internal timing jitter, which enables to handle many pixels with a small number of readout cables maintaining the jitter performance of the SSPD. We have demonstrated a 64-pixel SSPD array system with a 64-input event-driven SFQ encoder that converts the pixel address to a serial bit code and output it via a single readout cable [1]. We have recently upgraded our system with a row-column readout architecture that enables to handle an NxN-pixel SSPD array with a 2xN-input SFQ encoder and have successfully demonstrated a 4x4=16-pixel SSPD array system [2]. This allows more pixels to be handled without increasing the power consumption of SFQ circuit. We are developing a 1024-pixel SSPD array system, and the monolithic integration of these systems is also underway. In the presentation, we report the progress on these SSPD array systems.

 [1] S. Miyajima et al., Opt. Express 26(22), pp.29045-29054 (2018).
[2] M. Yabuno et al., Opt. Express 28(8), pp.12047-12057 (2020).

Acknowledgements
This work was supported by JSPS KAKENHI (JP18H05245, JP19H02206, JP19K15472) and JST CREST (JPMJCR1671). The SFQ circuits were fabricated in the clean room for analog–digital superconductivity (CRAVITY) of AIST.

Keywords: Nanowire single-photon detector, Multi-pixel array, Single-flux-quantum readout circuit