Testing the entanglement of intense beams produced by a non-degenerate optical parametric oscillator

We propose a direct measurement of the quadrature correlations of signal and idler beams in a non-degenerate optical parametric oscillator operating above threshold. We investigate the experimental limits where quantum correlations can be observed, fulfilling an inseparability criterion for defining them as intense entangled beams. The use of optical cavities to access quadrature noise in this situation is studied, and its advantages over homodyne detection are discussed. We also consider the application of this entanglement and the quadrature noise measurement technique to quantum cryptography. (C) 2004 Elsevier B.V. All rights reserved.