  Detailed investigations of single photon 1st-order interferences of separated
emitters allow for analyzing the role of the mode function in quantum optics.
In an earlier Young's double slit experiment using a higher spatial light mode
some possibly new aspects of complementarity were demonstrated. Here the
chances of using induced coherence in spontaneous down conversion (SPDC) are
experimentally investigated. SPDC in a nonlinear crystal generates two single
photons (signal and idler) with random, but conjugated phases. Thus no
interference between them occurs. But coherence can be induced between the two
separated crystals. Decreasing the pump intensity allows the generation of just
one photon pair from one of the crystals during one measuring interval,
resulting in 1st-order interference of single photons. Varying the phase delay
between the pump beams for the two crystals results in 1st-order single photon
interference but with a fringe period of about half of the photon wavelength.
This phase memory effect causes coherence across the two coupled
interferometers. And even for pump photon delays much larger than the coherence
length of the detected single photons the visibility is above 90 percent.
Therefore this phase memory is a useful tool for investigating the
wave-particle dualism in future experiments.
