Dark states ultra-long fiber laser for practically secure key distribution

We present and demonstrate a novel ultra-long fiber laser key distribution system (UFL-KDS). The scheme quenches the lasing process when in its secure states, thus forming “dark states” which provide simple detection on one hand and increased difficulty of eavesdropping on the other. We analyze the practical aspects of previously studied UFL-KDS schemes as well as those of the one presented here and demonstrate successful key distribution across a 200 km link with bit-rates that can exceed 0.5 kbps. Spectral and temporal passive attack strategies are analyzed and discussed in details.