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are also underway in NICT, aiming at establishing an ultra-high sensitivity method for extracting information from feeble coherent light signals, as described in the introduc-tion[13]–[19]. Quantum noise overpowers modulation signal in the domain where the handling of extremely weak photo-level light is of concern, almost ruining the validity of conventional technologies used in optical communica-tion. e novel approach NICT is undertaking represents the technologies that pursue the ultimate limits of signal identication, where an important role is played by tech-nologies such as photon detection, specialized schemes of phase, and amplitude modulation that can preserve the quantum nature of the signal. In the future, the develop-ment of quantum entanglement control technologies as applied to such quantum receivers is expected to make quantum decoding, or signal error correction on quantum levels, feasible in the future. is would further pave the way into the realm of optical communication where com-munication capacity comes close to the limits allowed by physical laws.ReferenceR1R. 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 between inde-pendent intrinsically pure single photons at telecommunication 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 efficient 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 method 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 informa-tion 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 entanglement 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 genera-tion 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 feed-back,” 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 coherent state discrimination via a hybrid receiver,” New J. Phys. 14, 083009 (2012). 16M. Fujiwara, S. Izumi, M. Takeoka, and M. Sasaki, “Performance gain of dis-placement 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 squeez-ing and photon-number-resolving detectors for M-ary coherent state discrimi-nation,” 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).Masahiro TAKEOKA, Ph.DDirector, Quantum ICT Advanced Development Center, Advanced ICT Reseach InstituteQuantum optics, Quantum information theoryMikio FUJIWARA, Ph.DResearch Manager, Qunatum ICT Advanced Development Center, Advanced ICT Reseach InstituteQuantum key distribution, Photon detection techology, Cryogenic electronicsKentaro WAKUI, Ph.DPlanning Manager, Strategic Planning Office, Strategic Planning DepartmentQuantum optics, Quantum electronics4 Quantum Node Technology54　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)