  The photonic de Broglie wave, in which an ensemble of $N$ identical photons
with wavelength $\lambda$ reveals $\lambda/N$ interference fringes, has been
known to be a unique feature exhibited by the photon number-path entangled
state or the NOON state. Here, we report the observation of the photonic de
Broglie wave for a pair of photons, generated by spontaneous parametric
down-conversion, that are not photon number-path entangled. We also show that
the photonic de Broglie wave can even be observed for a pair of photons that
are completely separable (i.e., no entanglement in all degrees of freedom) and
distinguishable. The experimental and theoretical results suggest that the
photonic de Broglie wave is, in fact, not related to the entanglement of the
photons, rather it is related to the indistinguishable pathways established by
the measurement scheme.
