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.
@article{arxiv.1101.1009,
title = {Faithful entanglement swapping based on sum-frequency generation},
author = {Nicolas Sangouard and Bruno Sanguinetti and Noe Curtz and Nicolas Gisin and Rob Thew and Hugo Zbinden},
journal= {arXiv preprint arXiv:1101.1009},
year = {2015}
}