Experimental demonstration of quantum entanglement between frequency-nondegenerate optical twin beams

The quantum entanglement of amplitude and phase quadratures between two intense optical beams with a total intensity of $22\mathrm{mW}$ and a frequency difference of $1\mathrm{nm}$, which are produced from an optical parametric oscillator operating above threshold, is experimentally demonstrated with two sets of unbalanced Mach–Zehnder interferometers. The measured quantum correlations of intensity and phase are in reasonable agreement with the results calculated based on a semiclassical analysis of the noise characteristics given by Fabre et al. [J. Phys. (France) 50, 1209 (1989)] .