Experimental observation of three-color optical quantum correlations
  Quantum correlations between bright pump, signal, and idler beams produced by an optical parametric oscillator, all with different frequencies, are experimentally demonstrated. We show that the degree of entanglement between signal and idler fields is improved by using information of pump fluctuations. This is the first observation of three-color optical quantum correlations. 
K. N. Cassemiro, A. S. Villar, P. Valente, M. Martinelli, and P.   Nussenzveig