Quantum Key Distribution with Continuous Variables in Optics

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.