  We discuss a quantum key distribution scheme in which small phase and
amplitude modulations of quantum limited, CW light beams carry the key
information. We identify universal constraints on the level of shared
information between the intended receiver (Bob) and any eavesdropper (Eve) and
use this to make a general evaluation of the security and efficiency of the
scheme. We show that a scheme based on coherent light offers competitive secret
key bit transmission rates.
