Observation of twin beam correlations and quadrature entanglement by   frequency doubling in a two-port resonator
  We demonstrate production of quantum correlated and entangled beams by second harmonic generation in a nonlinear resonator with two output ports. The output beams at wavelength 428.5 nm exhibit 0.9 dB of nonclassical intensity correlations and 0.3 dB of entanglement. 
O.-K. Lim, B. Boland, and M. Saffman