Quantum Entanglement, Dark Counts, Coincidence Detection

[mindwalker_i]

50. Ok, I see basically what this is

Type II does have the two sets of concentric cones, and where the two middle cones intersect, there are entangled photons. For collinear type II, the angle between the two sets of cones is set so that there is only one point where the middle concentric cones intersect. Since in normla type II, the entangled photons in the two intersecting points (or lines, sort of) have opposite polarization, in the collinear type II, entangled photons are in the same beam but have opposite polarizations - or different, or 90 degrees off, or whatever.

The entangled photons will be in the same line as the pump beam that remains after downconversion, which is the overwhelming majority of the pump. Therefore that needs to be broken off with a dichroic mirror, prism, or something like that. The remaining IR photons can go through a polarizing beam splitter and sent in different direction to be messed with however necessary.

I see one big advantage would be that the entangled photons don't get spread out due to the location where downconversion happens being anywhere within the depth of the crystal - the violet photons downconvert into IR, and the IR photons continue going in exactly the same direction rather than deflecting at 3 degrees or so. That means the BBO can be arbitrarily deep without smearing out the downconverted photons. And therefore, lots of downconversion can be made to happen by extending the depth of the crystal.

This sounds like a much better way to go than the paired BBO crystals. Is there any reason not to go with this method? What should I look for when picking a crystal to buy (obviously type 2 collinear)?