  Presently, classical optical communication systems employing strong laser
pulses and quantum key distribution (QKD) systems working at single-photon
levels are very different communication modalities. Dedicated devices are
commonly required to implement QKD. In this paper, we propose a scheme which
allows classical communication and QKD to be implemented simultaneously using
the same communication infrastructure. More specially, we propose a coherent
communication scheme where both the bits for classical communication and the
Gaussian distributed random numbers for QKD are encoded on the same weak
coherent pulse, and decoded by the same coherent receiver. Simulation results
based on practical system parameters show that both deterministic classical
communication with a bit error rate of 10^{-9} and secure key distribution
could be achieved over tens of kilometers single mode fibers. It is conceivable
that in the future coherent optical communication network, QKD is operated in
the background of classical communication at a minimal cost.
