Secret key distribution (SKD) is used to generate a common secret key between two legitimate users, which enables the users to communicate securely. We proposed a SKD scheme which can achieve information-theoretic security and long-distance key distribution. Our scheme is based on a property called bounded observability, which occurs due to the practical difficulty of completely measuring broadband random light. An implementation based on this concept has been proposed using the synchronized responses of laser systems injected with common random light with broad bandwidth, and it has been experimentally demonstrated to be possible and practicable.
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