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I don't know about all the details of Bell tests using methods like parametric down conversion, but at least in some versions of the EPR paradox you get two photons moving apart in opposite directions. I wonder if you can look for detection coincidences by using photographic plates instead of coincidence counters? The idea would be that if you had two photographic plates on opposite sides of the source, you would get some instances where you would have a perfect matchup of reduced silver crystals, or dots. And that if you inserted crossed polarizers in front of the plates, the coincidences would disappear. I wonder if this analysis is correct, and if so, whether such experiments have been done or proposed?

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OK, Marty, I've voted to delete my Answer. I agree that I didn't "get" your Question, which I can see may have been because of my prejudices, but I suggest you should also look at your wording. Perhaps you need a more detailed account of spatial coincidence instead of temporal coincidence? What quantum mechanical model would there be for such an experiment? What would the measurement operators and the state be? I'm sorry to say that I'm a bit at a loss. – Peter Morgan Apr 28 '11 at 2:14
@Peter Morgan Thanks, Peter. I'm thinking of something like the decay of positronium as described in the Feynmann Lectures. The entangled photons fly off in opposite directions. Shouldn't two photographic plates at opposite ends of the room record hits diametrically opposed? So you could count coincidences just by comparing the two images? There is an unfortunate complication in that we do not have good polarizers for gamma rays; but if we did, doesn't the theory say the coincidences should disappear when the polarizers are crossed wrt each other? – Marty Green Apr 28 '11 at 2:42
There would have to be a subset of almost perfectly spatially matched pairs that is clearly distinguishable from noise (perhaps my Answer does help a little here, one can see very clearly that there is a distinguishable subset of time-coincident events). I suppose such an experiment would have nothing to say about causality/locality, but is there another reason for this to be of foundational or other interest? – Peter Morgan Apr 28 '11 at 12:18
I don't see how the Poisson-type statistics that distinguish time-coincident events from background noise would be any different in the spatial case? – Marty Green Apr 28 '11 at 23:39
The disappearance of coincidence counts with crossed polarizers is equally spectacular whether it is seen in spatial or time-like coincidences, because it is not what one would expect from particles whose polarization was determined at the moment of its creation. – Marty Green Apr 28 '11 at 23:41

Yes, something like that is possible, and they've done it in Anton Zeilinger's lab. It's not a Bell's inequality test, and to get it to work it's more complicated than what you're describing, but: they do see spatial (i.e. position) correlations/interference disappear or not based on measurement of one of the entangled particles.

It's described in a nice review paper (which has a bunch of other nice EPR/Bell experiments): Rev. Mod. Phys. 71 S288 (1999)

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I'm grateful to the "community" for poking this question; I'm presuming on the grounds that it has gone unanswered. I have to say I was never satisfied with AC's answer: a) because I think I am asking for a demonstration of one of the very simplest cases of entanglement; b), because I don't believe PDC photons have the spatial "anti-coherence" necessary for my experiment as would, say, photons from the decay of positronium; c), because experiments like I describe, even WITHOUT the polarizers, should logically have predated the whole fuss generated by Bell in the sixties. – Marty Green May 28 '11 at 23:13

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