Reducing of the Uncertainty Product of Coherent Light through Multi-Photon Interference

We demonstrate theoretically and experimentally how the diffraction and interferometric resolution limit for single-mode coherent cw laser light can be overcome by multi-photon interference. By use of a Mach-Zehnder interferometer, operated in the single input and single or double output port geometries, we observe a fringe width reduction of the conventional interference pattern, predicted by the wave or single photon quantum theory, by a factor of up to 1/2N through coincident detection of N=2, 3, 4 photons. Our scheme does not require squeezed or entangled light to overcome the standard quantum limit and greatly facilitates precision interferometry experiments.