One and two photon quantum interference in a Mach-Zehnder interferometer

A re-visitation of the well known free space Mach Zehnder interferometer is here reported. Coexistence between one-photon and two-photons interference from collinear color entangled photon pairs is investigated. This is seen to arise from an arbitrarily small unbalance in the arm transmittance. The tuning of such asymmetry is reflected in dramatic changes in the coincidence detection, revealing beatings between one particle and two particle interference patterns. Our configuration explores new physics of the real Mach Zehnder interferometer especially useful for quantum optics on a chip, where the guiding geometry forces photons to travel in the same spatial mode.