Quantum communication exploiting above threshold OPO intensity correlations and polarization encoding

We present a continuous variable quantum communication protocol based on bright continuous-wave twin-beams generated by a type-II OPO. Intensity correlation between the beams is used in conjunction with a binary randomization of polarization to guarantee security and reveal eavesdropping actions. The scheme presented is asymmetric. Bob (the receiver) retains one of the beams and sends the other one to Alice after a random rotation of its polarization. The cryptographic key elements are encoded through amplitude modulation by Alice, who sends back her beam to Bob after a second rotation of the polarization. Eventually, the beams are detected by Bob after a further random polarization rotation. The security of the system and the possibility of revealing the eavesdropping action in the case of an individual attack are demonstrated by evaluating the bit error rates. (C) 2006 Elsevier Ltd. All rights reserved.