  Continuous-variable encoding of quantum information in the optical domain has
recently yielded large temporal and spectral entangled states instrumental for
quantum computing and quantum communication. We introduce a protocol for the
generation of spatial multipartite entanglement based on phase-matching of a
propagation eigenmode in a monolithic photonic device: the array of quadratic
nonlinear waveguides. We theoretically demonstrate in the spontaneous
parametric downconversion regime the generation of large multipartite entangled
states useful for multimode quantum networks. Our protocol is remarkably simple
and robust as it does not rely on specific values of coupling, nonlinearity or
length of the sample.
