  We show theoretically and experimentally that scattered light by thermal
phonons inside a second-order nonlinear crystal is the source of additional
phase noise observed in Optical Parametric Oscillators. This additional phase
noise reduces the quantum correlations and has hitherto hindered the direct
production of multipartite entanglement in a single nonlinear optical system.
We cooled the nonlinear crystal and observed a reduction of the extra noise.
Our treatment of this noise can be successfully applied to different systems in
the literature.
