Four-Photon Interference with Asymmetric Beam Splitters
  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 $\lambda/4$ by projection measurement. 
B. H. Liu, F. W. Sun, Y. X. Gong, Y. F. Huang, and G. C. Guo, Z. Y. Ou