Hong-Ou-Mandel interference without beam splitters

We propose a new interferometric setup which displays a completely destructive generalized N-photon Hong-Ou-Mandel interference. The key property of this scheme is that is does not require any optical elements like beam splitters or integrated waveguide structures. The interference is intrinsically produced by the evolution of N photons in free space when emitted by N identical statistically independent single photon sources and measured by N detectors in the far field. In this sense the setup is a most simple and natural implementation of the Hong-Ou-Mandel interference effect, i.e., of a completely destructive multi-photon interference produced by statistically independent incoherent sources.