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象であると考えられる。おわりに光量子制御技術の研究開発について、量子もつれ光源に関する開発を中心に紹介した。高速・高純度、さらに集積化された量子もつれ光源が実現すれば、次世代の量子暗号方式の重要光源となることが期待される。また、より技術的に高度になるが、物質の量子メモリーと光の間で自在に量子情報をやりとりすることができるようになれば、量子中継技術の実現も見えてくる。一方、NICTでは、冒頭で簡単に紹介した光子レベルの微弱なコヒーレント光信号から超高感度で情報を取り出す量子受信技術の研究開発も進めている[13]–[19]。光子レベルの微弱光では、信号の変調に比べて量子雑音の影響が大きく、従来の光通信で用いられる受信技術で信号識別が困難になるが、光子検出技術や信号の量子性を損なわない特殊な位相・振幅変調などにより、信号識別の限界まで感度を上げる技術である。このような量子受信機に、更に量子もつれの制御技術を加えることで、将来的には信号の誤りを量子レベルで訂正する量子復号技術が可能となり、物理学的に許される究極の通信容量限界に迫る光通信が可能となる。【参考文献【1R. Jin, R. Shimizu, K. Wakui, H. Benichi, and M. Sasaki, “Widely tunable single photon source with high purity at telecom wavelength,” Opt. Express 21, 10659, 2013.2R. Jin, K. Wakui, R. Shimizu, H. Benichi, S. Miki, T. Yamashita, H. Terai, Z. Wang, M. Fujiwara, and M. Sasaki, “Nonclassical interference be-tween independent intrinsically pure single photons at telecommunica-tion wavelength,” Phys. Rev. A 87, 063801, 2013.3R. Jin, R. Shimizu, K. Wakui, M. Fujiwara, T. Yamashita, S. Miki, H. Terai, Z. Wang, and M. Sasaki, “Pulsed Sagnac polarization-entangled photon source with a PPKTP crystal at telecom wavelength,” Opt. Express 22, 11498, 2014. 4R. Jin, R. Shimizu, I. Morohashi, K. Wakui, M. Takeoka, S. Izumi, T. Sakamoto, M. Fujiwara, T. Yamashita, S. Miki, H. Terai, Z. Wang, and M. Sasaki, “Efficient generation of twin photons at telecom wavelengths with 2.5 GHz repetition-rate-tunable comb laser,” Sci. Rep. 4:7468, 2014.5R. Jin, M. Takeoka, U. Takagi, R. Shimizu, and M. Sasaki, “Highly effi-cient entanglement swapping and teleportation at telecom wavelength,” Sri. Rep. 5:9333, 2015. 6R. Jin, M. Fujiwara, R. Shimizu, R. J. Collins, G. S. Buller, T. Yamashita, S. Miki, H. Terai, M. Takeoka, and M. Sasaki, “Detection-dependent six-photon Holland-Burnett state interference,” Sci. Rep. 6:36914, 2016. 7R. Jin, T. Gerrits, M. Fujiwara, R. Wakabayashi, T. Yamashita, S. Miki, H. Terai, R. Shimizu, M. Takeoka, and M. Sasaki, “Spectrally resolved Hong-Ou-Mandel interference between independent photon sources,” Opt. Express 22, 28836, 2015.8R. Jin, R. Shimizu, M. Fujiwara, M. Takeoka, R. Wakabayashi, T. Yamashita, S. Miki, H. Terai, T. Gerrits, and M. Sasaki, “Simple meth-od of generating and distributing frequency-entangled qudits," Quantum Sci. Technol. 1, 015004, 2016. 9M. Takeoka, R. Jin, and M. Sasaki, “Full analysis of multi-photon pair effects in spontaneous parametric down conversion based photonic quantum information processing,” New J. Phys. 17, 043030, 2015. 10K. P. Seshadreesan, M. Takeoka, and M. Sasaki, “Progress towards practical device-independent quantum key distribution with spontaneous parametric down-conversion sources, on-off photodetectors, and entan-glement swapping,” Phys. Rev. A 93, 042328, 2016. 11R. Wakabayashi, M. Fujiwara, K. Yoshino, Y. Nambu, M. Sasaki, and T. Aoki, “Time-bin entangled photon pair generation from Si micro-ring resonator,” Opt. Express 23, 1103, 2015. 12M. Fujiwara, R. Wakabayashi, M. Sasaki, and M. Takeoka, “Wavelength division multiplexed and double-port pumped time-bin entangled photon pair generation using Si ring resonator,” Opt. Express 25, 3445, 2017. 13K. Tsujino, D. Fukuda, G. Fujii, S. Inoue, M. Fujiwara, M. Takeoka, and M. Sasaki, “Quantum receiver beyond the standard quantum limit of coherent optical communication,” Phys. Rev. Lett. 106, 250503, 2011. 14S. Guha, J. L. Habif, and M. Takeoka, “Approaching Helstrom limits to optical pulse-position demodulation using single photon detection and optical feedback,” J. Mod. Opt. 58, 257, 2011. 15C. R. Müller, M. A. Usuga, C. Wittmann, M. Takeoka, C. Marquardt, U. L. Andersen, and G. Leuchs, “Quadrature phase shift keying coher-ent state discrimination via a hybrid receiver,” New J. Phys. 14, 083009, 2012. 16M. Fujiwara, S. Izumi, M. Takeoka, and M. Sasaki, “Performance gain of displacement receiver with optimized prior probability," Phys. Lett. 377, 2723, 2013. 17S. Izumi, M. Takeoka, M. Fujiwara, N. Dalla Pozza, A. Assalini, K. Ema, and M. Sasaki, “Displacement receiver for phase-shift-keyed coherent states,” Phys. Rev. A 86, 042328, 2012. 18S. Izumi, M. Takeoka, K. Ema, and M. Sasaki, “Quantum receivers with squeezing and photon-number-resolving detectors for M-ary coherent state discrimination,” Phys. Rev. A 87, 042328, 2013. 19S. Izumi, M. Takeoka, K. Wakui, M. Fujiwara, K. Ema, and M. Sasaki, “Optical phase estimation via the coherent state and displaced-photon counting,” Phys. Rev. A 94, 033842, 2016.武岡正裕 （たけおか　まさひろ）未来ICT研究所量子ICT先端開発センターセンター長博士（工学）量子光学、量子情報理論藤原幹生 （ふじわら　みきお）未来ICT研究所量子ICT先端開発センター研究マネージャー博士（理学）量子鍵配送、光子検出技術、極低温エレクトロニクス和久井健太郎 （わくい　けんたろう）経営企画部企画戦略室プランニングマネージャー博士（工学）量子光学5454-1　光量子制御技術