For quantum entanglement experiments in the mm-wave band it is necessary to have a source of entangled photons. Generating these from a pump of radiation by spontaneous parametric down conversion (aka parametric fluorescence) requires a highly nonlinear material or device. A material is sought which has an extremely high second order non-linear (electric or magnetic) susceptibility. Ferrites or non-centrosymmetric (BBO & LiNbO3) crystals don't seem to have a sufficiently high nonlinear susceptibility. What other materials or devices may be used that have the extremely high non-linear characteristics?