Quantum Entanglement, Dark Counts, Coincidence Detection

[mindwalker_i]

48. Type II Collinear?

I have not come across the term yet - my knowledge of nonlinear optics is a bit raw - but as I understand it, type I is the mode that gives off a cone, where photons on opposite sides are entangled. Type II is two sets of three concentric cones, and where the two middle cones intersect, there are entnagled pairs. I've read that type I has a lot more entangled pairs than type II. So what is this type II collinear? A quick google search gave me a bunch of PDFs that I can look through later. And it produces a lot of entangled pairs? That could be very useful, and separating with a polarizing beam splitter is not problem.

What I'm thinking is this: two beams are generated (or one beam, which is split via polarization), and one goes into the MZ interferometer. That forms two interference patterns, one at each output of the second beam splitter/recombiner (BS2), and those should be negative images of each other. In the interference pattern of the orthogonal output of BS2, there will be dark spots, but when the entangled photons in the beam input to the MZ are knocked output of superposition, some of them will land in those dark spots. Any photons that are not entangled will just keep doing what they've always done, forming the same patterns on both outputs of BS2. So the MZ kind of plays a similar filtering role as the coincidence detector in Dopfer's experiment.

So, the percentage of photons that are entangled with photons in the other beam shouldn't be too important. The raw number of entangled photons does matter, because that determined how many end up in the dark spots when they get knocked out of superposition (superposition in terms of momentum). From what I glean from your description of collinear mode above, there would be more entangled photons and they would already be in a beam rather than in a cone that has to be filtered (positionally, with bricks that have holes in them) to get beams. Am I on the right track?

My understanding of nonlinear optics is weak. My searches for information on google have turned up some stuff, but I think I need more depth. For my physics major, I didn't take a QM course but have learned just about everything on my own since school, which resulted in a qualitative understanding of a lot of concepts and ideas, but definitely a lack of understanding for the mathematics. Sadly, the book QWuantum Physics For Dummies kicked my ass on the second chapter and continues to do so occasionally when I get ambitious and pick it up again. There is, however, an absolutely awesome book, "The Quantum Universe" by Brian Cox and Jeff Forshaw that is really explaining some interesting things, mostly qualitatively, but it doesn't relate to entanglement (yet). Hopefully it will provide enough understanding so I can get through the Dummies book