  It is shown that the visibility of multi-photon interference for classical
sources grows rapidly with the order of interference. For three-photon and
four-photon interference of two coherent sources, the visibility can be as high
as 81.9% and 94.4%, respectively, - much higher than the 'classical limit' of
two-photon interference (50%). High-visibility three-photon and four-photon
interference has been observed in experiment, for both coherent and
pseudo-thermal light.
