We demonstrate a Fock-state filter which is capable of preferentially blocking single photons over photon pairs. The large conditional nonlinearities are based on higher-order quantum interference, using linear optics, an ancilla photon, and measurement. We demonstrate that the filter acts coherently by using it to convert unentangled photon pairs to a path-entangled state. We quantify the degree of entanglement by transforming the path information to polarisation information, applying quantum state tomography we measure a tangle of T=(20+/-9)%.
@article{arxiv.quant-ph/0608052,
title = {Entanglement generation by Fock-state filtration},
author = {K. J. Resch and J. L. O'Brien and T. J. Weinhold and K. Sanaka and A. G. White},
journal= {arXiv preprint arXiv:quant-ph/0608052},
year = {2007}
}