  An experiment is proposed in which the overall path taken by a photon is
indicated by the timing of a twin herald photon, while a particular segment of
that path is determined by interference. The experiment is to be carried out in
two parts. In the first, coincident pairs of photons generated by type I
spontaneous- parametric-down-conversion are diffracted by a large-width grating
to increase the coherence length of their associated waves, and to produce two
sources of coincident photons. Upon experimental confirmation that coincidences
have been maintained, one of the sources is used to furnish timing heralds, and
the other to send photons to an unequal path interferometer. A photon path
through the interferometer via the short arm is indicated by count
synchronization with the herald. The exit output port used and final detection
location are determined by the phase in the long arm. If output port usage can
thus be controlled by the phase in the photon free arm, the path of the photon
as particle will have been controlled by interference with an accompanying
photon-empty wave.
