Science
In reply to the discussion: Quantum Entanglement, Dark Counts, Coincidence Detection [View all]caraher
(6,278 posts)Since it's clear that this measurement is all about momentum and not polarization, and since I know you can get ginormous count rates using Type II collinear downconversion (note that this is what Cramer is doing, only in PPKTP rather than BBO!), you might be better off with Type II. You can separate the photons with polarizing beamsplitters, and they momenta carry the same correlations. And if you want to play with polarization entanglement at some later date, all you do is tilt the crystal a little bit (I'm not sure how many degrees that is at 405 nm to get, say, 3 degree separation of the entangled beams from the pump path, but it's reasonable, and you don't need that phase shifter, though you probably need to compensate for temporal walkoff inside the BBO, which is something you do with, say, quartz, after the crystal).
One handy feature of the collinear geometry is that you can do a lot of alignment using your pump beam (which you can see!). It's only when it comes to any focusing optics or other dispersing elements like prisms that the wavelength difference gives any trouble.