Demonstration of the complementarity of one- and two-photon interference

The visibilities of second-order (single-photon) and fourth-order (two-photon) interference have been observed in a Young’s double-slit experiment using light generated by spontaneous parametric downconversion. As the effective source size is increased, coherence is diminished while entanglement is enhanced. This is the first experimental demonstration of this complementarity in the spatial domain.