Multipartite continuous-variable entanglement from concurrent   nonlinearities
  We show theoretically that concurrent interactions in a second-order nonlinear medium placed inside an optical resonator can generate multipartite entanglement between the resonator modes. We show that there is a mathematical connexion between this system and van Loock and Braunstein's proposal for entangling N continuous quantum optical variables by interfering the outputs of N degenerate optical parametric amplifiers (OPA) at a N-port beam splitter. Our configuration, however, requires only one nondegenerate OPA and no interferometer. In a preliminary experimental study, we observe the concurrence of the appropriate interactions in periodically poled RbTiOAsO4. 
Olivier Pfister, Sheng Feng, Gregory Jennings, Raphael Pooser, and   Daruo Xie