  We propose an experiment that permits observation of the de Broglie
two-photon wave packet behavior for a pair of photons, using a Mach-Zehnder
interferometer. It is based on the use of pulsed lasers to generate pairs of
photons via spontaneous parametric down-conversion and the post-selection of
events. It differs from previous realizations by the use of a third
time-correlated photon to engineer the state of the photons. The same technique
can give us which-path information via an ``interaction-free'' experiment and
can be used in other experiments on the foundations of quantum mechanics
related to wave-particle duality and to nonlocality.
