Phase memory across two single photon interferometers including   wavelength conversion
  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. 
Axel Heuer, Sebastian Raabe and Ralf Menzel
quant-ph