  We investigate the second-order non-linear interaction as a means to generate
entanglement between fields of differing wavelengths. And show that perfect
entanglement can, in principle, be produced between the fundamental and second
harmonic fields in these processes. Neither pure second harmonic generation,
nor parametric oscillation optimally produce entanglement, such optimal
entanglement is rather produced by an intermediate process. An experimental
demonstration of these predictions should be imminently feasible.
