  The complementarity of single-photon's particle-like and wave-like behaviors
can be described by the inequality $D^2+V^2 \leq 1$, with $D$ being the path
distinguishability and $V$ being the fringe visibility. In this paper, we
generalize this duality relation to multi-photon case, where two new concepts,
higher order distinguishability and higher order fringe visibility, are
introduced to quantify the higher order particle-like and wave-like behaviors
of multi-photons.
