  Dirac has written "Each photon then interferes only with itself. Interference
between two different photons never occurs." Indeed, a practical definition is
that "classical" optics consists of phenomena due to the interference of
photons only with themselves. However, photons obey Bose statistics which
implies a ``nonclassical'' tendency for them to ``bunch''. For a simple example
of nonclassical optical behavior, we consider two pulses of photons of a single
frequency that are simultaneously incident on two sides of a lossless, 50:50
beam splitter.
