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また、NbN集積回路に関してもNICTではNbNを用いた磁性接合による量子ビットの実証に成功しており、20 mKで動作するNbN集積回路と組み合わせることで従来の手法では到達できない規模の超伝導量子ビット数の実現が期待される。SFQ回路による極低温信号処理は、既存の超伝導デバイスのシステムに様々な機能を付加することが可能であり、本稿で述べたシステム以外にも多岐にわたる用途が考えられる。特にSSPDに関しては、様々な研究機関からデバイスの提供が求められており、ユーザーは非常に多い。SFQ回路を組み込んだシステム実用化にはこれらのユーザーからの要求に応じて的確な信号処理回路の開発を行いシステムの提供を行うことが重要であり、応用分野の拡大につながると考えている。謝辞本稿の執筆にあたり、日頃から議論していただいているフロンティア創造総合研究室・超伝導デバイスプロジェクトの寺井弘高上席研究員、三木茂人主任研究員、丘偉主任研究員、藪野正裕研究員、知名史博研究員、川上彰主任研究員、今村三郎研究技術員、菱田有二研究技術員に深く感謝する。また、SFQ回路の作製でご協力いただいた産業技術総合研究所の永沢秀一氏、日高睦夫氏、2光子同時計数システムに関して有益なご議論を頂いた大阪大学大学院基礎工学研究科の山本俊教授、生田力三助教に感謝する。本研究の一部はJSPS科研費（JP18H05245, JP19H05615, JP19H02206, JP19K15057）、JST CREST （JPMJCR1671）の助成を受けたものである。【参考文献【1Y. Kameda, Y. Hashimoto, and S. Yorozu, “Design and Demonstration of a 4 x 4 SFQ Network Switch Prototype System and 10-Gbps Bit-Error-Rate Measurement,” IEICE Trans. Electron., vol.E91-C, no.3, pp.333–341, March 2008. 2Y. Yamanashi, M. Tanaka, A. Akimoto, H. Park, Y. Kamiya, N. Irie, N. Yoshikawa, A. Fujimaki, H. Terai, and Y. Hashimoto, “Design and Implementation of a Pipelined Bit-Serial SFQ Microprocessor, CORE1β,” IEEE Trans. Appl. Supercond., vol.17, no.2, pp.474–477, June 2007. 3N. Takagi, K. Murakami, A. Fujimaki, N. Yoshikawa, K. Inoue, and H. Honda, “Proposal of a desk-side supercomputer with reconfigurable data-paths using rapid single-flux-quantum circuits,” IEICE Trans. Elec-tron., vol.E91-C, no.3, pp.350–355, March 2008.4R. Sato, Y. Hatanaka, Y. Ando, M. Tanaka, A. Fujimaki, K. Takagi, and N. Takagi, “High-speed operation of random-access-memory-embed-ded microprocessor with minimal instruction set architecture based on rapid single-flux-quantum logic,” IEEE Trans. Appl. Supercond., vol.27, no.4, 1300505, June 2017.5A. Fujimaki, K. Nakazono, H. Hasegawa, T. Sato, A. Akahori, N. Takeuchi, F. Furuta, M. Katayama, and H. Hayakawa, “Broad Band Software-Defined Radio Receivers Based on Superconductive Devices,” IEEE Trans. Appl. Supercond., vol.11, no.1, pp.318–321, March 2001.6D. Gupta, T. V. Filippov, A. F. Kirichenko, D. E. Kirichenko, I. V. Vernik, A. Sahu, S. Sarwana, P. Shevchenko, A. Talalaevskii, and O. A. Mukhanov, “Digital Channelizing Radio Frequency Receiver,” IEEE Trans. Appl. Supercond., vol.17, no.2, June 2007.7A. Bozbey, S. Miyajima, H. Akaike, and A. Fujimaki, “Single-Flux-Quantum Circuit Based Readout System for Detector Arrays by Using Time to Digital Conversion,” IEEE Trans. Appl. Supercond., vol.19, no.3, pp.509–513, June 1999.8S. Miyajima, T. Kusumoto, K. Ito, Y. Akita, I. Yagi, N. Yoshioka, T. Ishida, S. Miki, Z. Wang, and A. Fujimaki, “High-Throughput RSFQ Signal Processor for a Neutron Diffraction System with Multiple MgB2 Detec-tors,” IEEE Trans. Appl. Supercond., vol.23, no.3, 1800505, Jan. 2013.9K. Sano, Y. Yamanashi, and N. Yoshikawa, “Design and Demonstration of a Single-Flux-Quantum Multi-Stop Time-to-Digital Converter for Time-of-Flight Mass Spectrometry,” IEICE Trans. Electron., vol.E97.C, no.3, pp.182–187, March 2014.10G. N. 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Yamada, A. Fujimaki, S. Nagasawa, K. Hinode, T. Satoh, Y. Kitagawa, and M Hidaka “Demonstration of a 120 GHz single-flux-quantum shift register circuit based on a 10 kA cm−2 Nb process,” Supercond. Sci. Technol., vol.19, no.5, S320, March 2006.19X. Peng, Q. Xu, T. Kato, Y. Yamanashi, N. Yoshikawa, A. Fujimaki, N. Takagi, K. Takagi, and M. Hidaka, “High-speed demonstration of bit-serial floating-point adders and multipliers using single-flux-quantum circuits,” IEEE Trans. Appl. Supercond., vol.25, no.3, 1301106, June 2015.20H. Terai, T. Yamashita, S. Miki, K. Makise, and Z. Wang, “Low-jitter single flux quantum signal readout from superconducting single photon detector,” Opt. Exp., vol.20, no.18, pp.20115–20123, Aug. 2012.21S. Miyajima, S. Miki, M. Yabuno, T. Yamashita, and H. Terai, “Timing discriminator based on single-flux-quantum circuit toward high time-resolved photon detection,” Supercond. Sci. Technol., vol.30, no.12, 12LT01, Oct. 2017.22S. Miki, S. Miyajima, M. Yabuno, T. 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