Two experiments of four-photon interference are performed with two pairs of photons from parametric down-conversion with the help of asymmetric beam splitters. The first experiment is a generalization of the Hong-Ou-Mandel interference effect to two pairs of photons while the second one utilizes this effect to demonstrate a four-photon de Broglie wavelength of λ/4 by projection measurement.
@article{arxiv.quant-ph/0612006,
title = {Four-Photon Interference with Asymmetric Beam Splitters},
author = {B. H. Liu and F. W. Sun and Y. X. Gong and Y. F. Huang and G. C. Guo and Z. Y. Ou},
journal= {arXiv preprint arXiv:quant-ph/0612006},
year = {2009}
}