Security of continuous-variable quantum cryptography - art. no. 062306

We discuss a quantum key distribution scheme in which small phase and ampli tude modulations of cw light beams carry the key information The presence o f EPR-type correlations provides the quantum protection. We identify univer sal constraints on the level of shared information between the intended rec eiver (Bob) and any eavesdropper (Eve) and use this to make a general evalu ation of security. We identify teleportation as an optimum eavesdropping te chnique.