Multimode interference allows the use of multimode waveguides as multiport beam splitters for optical communications [1]. An extension of this equivalence to the quantum regime was recently demonstrated using single photons [2]. Here we show how path-entangled quantum states of light evolve through multimode waveguides. We produce two-photon, two-path-entangled states by spontaneous parametric down conversion and study their propagation through a two-mirror planar multimode waveguide of variable width, serving as various types of multiport beam splitters. We measure the intensity correlations between the output ports of the waveguide as a function of the relative phase between the two paths of the input state. We find a rich variety of spatial correlation patterns which strongly depend on the input phase. The measured correlations agree very well with a simple theoretical analysis indicating the creation of various multi-path-entangled states of light.

[1] L. B. Soldano and E. C.M. Pennings, "Optical multi-mode interference devices based on self-imaging: principles and applications", J. Lightwave Technol. 13, 615 (1995).

[2] A. Peruzzo et al., "Multimode quantum interference of photons in multiport integrated devices", Nat. Commun. 2, 224 (2011).