  We propose the concept of pair-wise coherence to study the relation between
the l1 norm of coherence and the quantum interference in Young's multi-photon
multi-path experiments, where the input photons may be entangled each other. We
find that only the local coherence of each single photon can make quantum
interference and the collective coherence between photons has no contribution
to quantum interference. The visibility of interference fringe is commonly less
than the l1norm of coherence of the corresponding input state, suggesting that
the l1 norm of coherence is only the necessary but not sufficient condition for
quantum interference. We also find that the maximal fringe visibility can reach
one. The optimal input states for producing the maximal visibility are
presented.
