I do have a BBO (bet-barium-borate) crystal. It's cut for type II, 405nm, colinear spontaneous parametric down conversion (SPDC), which means that it will produce two cones of 810nm light from a 405nm source, and those cones overlap along a line. In that overlap will be pairs of photons that are momentum and polarization entangled. They can be separated with a polarizing beam splitter (PBS) which will break the polarization entanglement but should leave the momentum entanglement intact. That will be great for sending one beam through the double slit while the other goes to the lens and screen/detector.

I have a dichroic mirror to reflect off the non-downconverted light, leaving just the IR light which includes the entangled pairs.

I haven't messed with this yet and am moving >:[ What I want to do is get stuff set up to send a beam from a cheap, ebay laser diode through the BBO and PBS, then detect photons from each and count both raw detections and coincidence detections. That should give me an idea about how many entangles pairs and what percentage of the total I get.

Next is the critical experiment: send one beam through a double-slit. Dr. Cramer said the "anti-signal" will cause photons (that would be filtered out by the coincidence detector - thus, not entangled) to fill in the otherwise dark parts of the interference pattern. So putting one of these beams through a double-slit will NOT produce an interference pattern, if I don't filter with a coincidence detector? That's really hard to believe. Essentially, each photon's wave function goes through each slit, diffracts into concentric arcs from each slit where the arcs are maxima of the wave function. Those arcs overlap and interfere constructively at various angles. It seems to me that in order to get an inverse interference pattern, which is what this "anti-signal" light would do, the phases of the waves at each slit would have to be different by pi, and I can't see how/why that would happen.

There's no help but to build it. Moving. And I need optical mount that I can adjust, which I can't work on because I'm moving! However, I can post the designs for those, possibly.