Scheme to generate continuous-variable quadripartite entanglement by intracavity down-conversion cascaded with double sum-frequency generations

We theoretically show that continuous-variable genuine quadripartite entanglement can be generated by intracavity cascaded nonlinear optical interactions. The process consists of a parametric down-conversion, a sum-frequency generation of pump and signal and a sum-frequency generation of pump and idler. The characteristics of the quadripartite entanglement among the signal, the idler, and two sum-frequency modes are analyzed by applying a sufficient inseparability criterion for multipartite continuous-variable entanglement proposed by Van Loock and Furusawa. The threshold properties of the system are also discussed.