  Over the past several decades, single photon-based path interference in
double-slit experiment has been well demonstrated for the particle nature of
photons satisfying complementarity theory, where the quantum mechanical
interpretation of the single photon interference fringe is given by Born rule
for complex amplitudes in measurements. Unlike most conventional methods using
entangled photon pairs, here, a classical coherent light source is directly
applied for the proof of the same self-interference phenomenon. Instead of
double slits, we use a Mach-Zehnder interferometer as usual, where the
resulting self-interference fringe of coherent single photons is exactly the
same as that of the coherent photon ensemble of the laser, demonstrating that
the classical coherence based on the wave nature is rooted in the single photon
self-interference. This unexpected result seems to contradict our common
understanding of decoherence phenomena caused by multi-wave interference among
bandwidth distributed photons in coherence optics.
