  A measurement process is constructed to project an arbitrary two-mode
$N$-photon state to a maximally entangled $N$-photon state (the {\it
NOON}-state). The result of this projection measurement shows a typical
interference fringe with an $N$-photon de Broglie wavelength. For an
experimental demonstration, this measurement process is applied to a
four-photon superposition state from two perpendicularly oriented type-I
parametric down-conversion processes. Generalization to arbitrary $N$-photon
states projection measurement can be easily made and may have wide applications
in quantum information. As an example, we formulate it for precision phase
measurement.
