We report the first on-line high-speed and large-capacity secure optical communication system. This was realized by combining a quadrature amplitude modulation/quantum noise stream cipher technology where a coherent multi-level optical signal is hidden in quantum noise and a quantum key distribution technology where secure key delivery is realized with an extremely weak laser light comparable to a single photon. In our security analysis, we adopt a realistic assumption, namely, that an adversary does not have a lossless fiber. To show the advantage of this scheme, we performed a 128 QAM, 70-Gbit/s single-channel transmission over 100 km with a spectral efficiency of as high as 10.3 bits/s/Hz. This is the fastest data rate and highest spectral efficiency yet achieved in quantum cryptography with a realistic assumption.
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