  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.
