Experimental realization of optical key distribution under strong wiretap eavesdropping.

We report the experimental distribution of a cryptographic key based on binary intensity-modulation/direct-detection (IM/DD) transmission of an optical signal. The implemented secret key agreement protocol ensures security against passive wiretap eavesdropping, notably in the regime when the eavesdropper has an advantage in the intercepted signal strength over the legitimate recipient. Operating at a 1 GBaud symbol rate, the experimental fiber optic testbed yielded secret key rates between 43.5 Mbps at 61.1 km and 640 kbps at 81.6 km of the effective link length, with a 9% wiretap installed at the transmitter. The protocol performance was analyzed using a theoretical model incorporating transmitter noise and non-ideal key reconciliation efficiency. The presented results validate IM/DD optical key distribution as an efficient low-complexity solution for protecting optical communication systems against the predominant physical-layer threat of wiretap eavesdropping.