  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.
