Faithful entanglement swapping based on sum-frequency generation
  We show that the entanglement resulting from an entanglement swapping operation performed with spontaneous parametric down-conversion can be made faithful without post-selection using sum-frequency generation. This invites us to revisit the sum-frequency process and from a proof-of-principle experiment, we demonstrate that it provides a realistic solution for non-linear optics at the single-photon level. This opens the way to attractive alternatives to six-photon protocols based on linear optics used e.g. for the heralded creation of maximally entangled pairs or for the implementation of device-independent quantum key distribution. 
Nicolas Sangouard, Bruno Sanguinetti, Noe Curtz, Nicolas Gisin, Rob   Thew, Hugo Zbinden
quant-ph