An Optimal Polarization-Entanglement Source: Frequency-Converted SPDC with Degeneracy, Indistinguishability, and Ultra-High Purity Configurable Over a Large Spectral Range

Modeling and simulations of entanglement-based quantum-networking protocols commonly assume perfect entangled states. Some investigations have been performed which show how imperfections cause the efficiency of the protocols to rapidly deteriorate. For polarization-entangled states created by SPDC, the fundamentals of phase matching lead to a trade-off problem for the optimal properties of the state. We present a new SPDC method which circumvents the trade-off problem and allows one to obtain degeneracy, indistinguishability, and heralded-single-photon spectral purity greater than 99% for any target SPDC wavelength in the visible and NIR spectrum. Therefore, our method can in principle generate optimal polarization-entangled states for polarization-entanglement-based quantum-networking protocols.