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 the total intensity of 22mW and the frequency difference of 1nm, 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 semi-classical analysis of the noise characteristics given by C. Fabre et al. 
Xiaolong Su, Aihong Tan, Xiaojun Jia*, Changde Xie, Kunchi Peng