  Contrary to mechanical waves, the two-slit interference experiment of single
photons shows that the behavior of classical electromagnetic waves corresponds
to the quantum mechanical one of single photons, which is also different from
the quantum-field-theory behavior such as the creations and annihilations of
photons, the vacuum fluctuations, etc. Owing to a purely quantum effect,
quantum tunneling particles including tunneling photons (evanescent modes) can
propagate over a spacelike interval without destroying causality. With this
picture we conclude that the superluminality of evanescent modes is a quantum
mechanical rather than a classical phenomenon.
