  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
$1/\sqrt{2N}$ 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.
