  Four-wave mixing can be stimulated or occur spontaneously. The first process
is intrinsically much stronger, and well understood through classical nonlinear
optics. The latter, also known as parametric fluorescence, can be explained
only in the framework of a quantum theory of light. We experimentally
demonstrate that, in a micro-ring resonator, there exists a simple relation
between the efficiencies of these two processes, which is independent of the
nonlinearity and size of the ring. In particular we show that the average power
generated by parametric fluorescence can be immediately estimated from a
classical FWM experiment. These results suggest that classical nonlinear
characterization of a photonic integrated structure can provide accurate
information on its nonlinear quantum properties.
