  We report a measurement of the transverse momentum correlation between two
photons by detecting only one of them. Our method uses two identical sources in
an arrangement, in which the phenomenon of induced coherence without induced
emission is observed. In this way, we produce an interference pattern in the
superposition of one beam from each source. We quantify the transverse momentum
correlation by analyzing the visibility of this pattern. Our approach might be
useful for the characterization of correlated photon pair sources and may lead
to an experimental measure of continuous variable entanglement, which relies on
the detection of only one of two entangled particles.
