How do you measure entanglement for type-II downconversion? I am going to do an experiment for a class, which will be using type-II downconversion to entangle pairs of photons pumped from a laser diode. We're planning on using a camera to take pics of the rings that'd be formed, but we need to also measure/quantify our results somehow. The only possibility I've been able to find from research online is measuring coincidence counts for the photon pairs, but I don't see how that really provides any interesting/useful information, and I'd have to find someone to loan us their single photon detectors or whatever equipment would need to be used. 
So my question is, how can the entanglement be measured? Can the quality of entanglement be quantified, and how? Or is there just anything measureable/testable that wouldn't be too hard or too inane? 
Please help me, really any info at all helps! Thank you!!
 A: So from what I know, proving entanglement in formal terms can be tricky since you need to do a succession of experiments to show that your system breaks Bell's inequalities. I never did it myself but it is documented in literature and even in Wikipedia. 
In your case, if you only want to give a flavour of entanglement, you could simply show that detecting a horizontally polarised photon on one detector is always coincidental with detecting a vertically polarised photon on the other detector. At least to show the polarisation entanglement in type II downconversion it is the simplest way I can think of – but I would be happy to be proven wrong!
A: Coincidence counting alone only establishes that the parametric down conversion process is working.  Entanglement is usually demonstrated by violation of the Bell inequalities, which requires a series of coincidence counts while manipulating the polarization of the two down-converted beams; see Bell Test Experiments.
A number of papers have been published which describe experimental setups for undergraduate physics labs which carry out these tests using a laser, a pair of matched BBO crystals, etc. For example, A Test of Bell’s Inequality for the Undergraduate Laboratory.
