Quantum cryptography using continuous variable Einstein-Podolsky-Rosen correlations and quadrature phase amplitude measurements

Correlations of the type discussed by EPR in their original 1935 paradox for continuous variables exist for the quadrature phase amplitudes of two spatially separated fields. These correlations were experimentally reported in 1992. We propose to use such EPR beams in quantum cryptography, to transmit predetermined messages in such a way that the receiver and sender may later determine whether eavesdropping has occurred. Intriguing is the possibility of using quantum mechanics to transmit signals in a way that any eavesdropping can be detected by the receiver and sender. This new field of quantum cryptography [1,2] has attracted much attention. In the pioneering proposal of Bennett and Brassard [1] the sender (Alice) transmits to the receiver (Bob) photon pulses in one of two orthogonal polarisations (labeled 0 and 1), where the orientation (basis) of polarisation randomly shifts between 0 and 45. The 0, 1 choice of polarisation represents the bit value. Bob randomly selects a basis (0 or 45) for a polarisation measurement, and records the resulting bit value. Alice and Bob later compare notes, through a public channel, on the sequence of orientations (0 or 45) chosen. The bit sequence where Bob selected the same orientation as Alice forms a key, to be used later to