Quantum communication with correlated nonclassical states - art. no. 042311

Nonclassical correlations between the quadrature-phase amplitudes of two sp atially separated optical beams are exploited to realize a two-channel quan tum communication experiment with a high degree of immunity to interception . For this scheme, either channel alone can have an arbitrarily small signa l-to-noise ratio (SNR) for transmission of a coherent "message." However, w hen the transmitted beams are combined properly upon authorized detection, the encoded message can in principle be recovered with the original SNR of the source. An experimental demonstration has achieved a 3.2 dB improvement in SNR over that possible with correlated classical sources. Extensions of the protocol to improve its security against eavesdropping are discussed.