Direct Production of Tripartite Pump-Signal-Idler Entanglement in the   Above-Threshold Optical Parametric Oscillator
  We calculate the quantum correlations existing among the three output fields (pump, signal, and idler) of a triply resonant non-degenerate Optical Parametric Oscillator operating above threshold. By applying the standard criteria [P. van Loock and A. Furusawa, Phys. Rev. A 67, 052315 (2003)], we show that strong tripartite continuous-variable entanglement is present in this well-known and simple system. Furthermore, since the entanglement is generated directly from a nonlinear process, the three entangled fields can have very different frequencies, opening the way for multicolored quantum information networks. 
A. S. Villar, M. Martinelli, C. Fabre, and P. Nussenzveig