Superconducting nanostrip single photon detectors (SNSPDs) are attractive device due to its high detection efficiency, high counting rate, low dark count rate, low timing jitter, and high sensitivity to photons with wide-band wavelengths, and already introduced to various applications such as quantum information systems [1]. While single-pixel SNSPDs are widely used in practical applications, multi-pixel SNSPDs enables higher counting rate with larger detection area, pseudo photon-number resolution and ultra-high-sensitive single-photon imager, allowing to expand its application to a wider area.
We have developed a cryogenic digital signal processor based on a single flux quantum (SFQ) circuit for a multi-pixel SNSPDs system. The SFQ-based signal processing is a powerful tool for the SNSPDs because of its high-speed operation at tens of GHz with small power dissipation and low timing jitters. Since the SFQ circuits based on Nb/AlOx/Nb Josephson junctions (JJs) can operate at the same temperature of around 2.2 K as that of NbTiN SNSPDs, we can make a compact SNSPD system by using a 0.1-W Gifford-McMahon (GM) cryocooler. We have demonstrated a various SFQ signal processor for SNSPD readout such as a 16-channel signal merger for higher counting rate [2], and an event-driven encoder for a single-photon imager [3]. The SFQ event-driven encoder has 64 inputs, which can handle 32×32-pixel SNSPDs by a row-column readout architecture in 0.1-W GM cryocooler, and 4×4-pixel SNSPDs with 2×4-channel SFQ event-driven encoder was already demonstrated [4].
In the above-mentioned systems, an SNSPD chip and SFQ chip is separated, and two devices are connected by microstrip lines on a printed circuit board (PCB) and aluminum bonding wires, in which the occupied area by interconnections will suppress the number of SNSPDs. In order to achieve much larger-pixel SNSPDs system, we developed a single-photon detection system based on multi-pixel SNSPDs monolithically integrated with SFQ signal processor. The 16-pixel SNSPDs monolithically integrated with 16-channel SFQ signal merger was demonstrated by photon illumination, and the obtained timing jitter of 41.4 ps, which is almost same as that of the single-pixel SNSPD [5].
For further scale up of the multi-pixel SNSPDs system, the Joule heat by the DC bias current for the SFQ circuits must be suppressed. Although the power dissipation of the SFQ circuit is quite small, the Joule heat by the large DC bias currents can raise up the temperature of cryocooler easily. We investigated a new package for reduction of Joule heat, and successfully suppressed the increase in the temperature of cryocooler. The temperature of cryocooler increased from 2.2 K to 2.3 K with the DC currents of 400 mA, which can indicate a 50×50-pixel SNSPDs system based on the row-column readout architecture can be achieved in 0.1-W GM cryocooler.
[1] H. Takesue, et al., Nat. Photonics 1(6), 343 (2007)
[2] S. Miki, et al., Opt. Lett., 46(24), 6015 (2021)
[3] S. Miyajima, et al., Opt. Exp., 26(22), 29045 (2018)
[4] M. Yabuno, et al., Opt. Exp., 28(8), 12047 (2020)
[5] S. Miyajima, et al., Appl. Phys. Lett., 122(18), 182602 (2023)
This work was partly supported by JSPS KAKENHI Grant Number 18H05245 and JST Moonshot R&D Program Grant Number JPMJMS2066. The circuits were fabricated in the clean room for analog-digital superconductivity (CRAVITY) with the AIST-STP2.