  We experimentally show that two-photon path-entangled states can be
coherently manipulated by multi-mode interference in multi-mode waveguides. By
measuring the output two-photon spatial correlation function versus the phase
of the input state, we show that multi-mode waveguides perform as nearly-ideal
multi-port beam splitters at the quantum level, creating a large variety of
entangled and separable multi-path two-photon states.
