In the era of exponentially growing sensitive data, secure key distribution mechanisms are urgently needed to establish reliable encrypted communication links. Channel reciprocity-based key distribution technologies possess significant advantages in compatibility with existing fiber-optic infrastructures, as they can share the same fiber channel with data transmission. However, constrained by the bandwidth of environmental fluctuations, such technologies generally suffer from low key generation rates, typically on the order of kbit/s. Although active channel scrambling schemes can increase the key distribution rate to the Gbit/s level, they require pre-shared pseudo-random algorithms or the introduction of trusted third parties, thus posing potential security vulnerabilities.
In this paper, a novel channel reciprocity scheme based on local true random phase-keying is proposed for high-speed secure key distribution. The scheme establishes a phase-concealed transmission structure using a broadband optical carrier and an asymmetric Mach-Zehnder interferometer. It employs a physical random number generator at the local end to generate random keys and performs phase-keying modulation on the broadband optical carrier. Time-delay compensation technology is adopted to ensure channel reciprocity during bidirectional transmission, enabling both communication parties to obtain highly correlated interference signals. Subsequently, a passive phase demodulation algorithm is used to extract the random phase-keying codes from the interference signals to achieve key sharing. Experimental results over a 25 km standard single-mode fiber link demonstrate that the proposed scheme achieves a secure key distribution rate of 1 Gbit/s at a phase-keying modulation frequency of 500 MHz. The bit error rate is as low as 2.4‰, which is well below the threshold of hard-decision forward error correction (3.8‰) widely adopted in commercial optical communication systems.