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There are two experiments that are often used to explain Quantum Mechanics: the two-slit experiment and the EPR paradox. I am curious what would happen if you combined them.

Imagine an experiment where you fire pairs of entangled particles at two simultaneous two-slit setups. If you used detectors, you could find out how the entangled particles' paths correlate. Perhaps you'd be able to deduce, based on the result of one detector, which slit the other particle went through. Now, if you were to run the experiments with a detector on one side and no detector on the other side, would the unobserved particles still form an interference pattern, even though you know which slit they would have went through?

My intuition is that the answer is yes. Despite being entangled, the particles should not have correlated actions, otherwise we would have invented faster-that-light communication. You could create a device that constantly fired entangled particles toward two far-apart worlds, and if one world suddenly started observing the particles on their side, the particles arriving at the other world would instantly cease to create an interference pattern.

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Related: physics.stackexchange.com/q/4353 and physics.stackexchange.com/q/4345. But I don't think either of those is really a duplicate. I like this question, it's fairly precisely stated. –  David Z Apr 15 '11 at 21:20

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An experiment very close to the proposed experiment has been done by Zeilingers grad-student Dopfler in 1998. She used a down-conversion crystal to produce pairs of entangled photons in a quantum-eraser type of experiment involving a dual-slit.

One of the members went through the dual-slit and was detected by a detector A which is scanning the space behind the slits to see if there is an interference pattern or not. The other member passes through a lens to another detector B whose distance to the lens can be varied (angle is fixed). By moving the detector B into or out of focus with the lens, the other pair member can be detected as passing through one of the slits (in focus, it "sees" the slit holes) or the which-way information is erased (out of focus, information from both slits are merged).

A useful way of looking at these experimental setups is to pretend that the photon is emitted by one of the detectors, passing backwards through the experiment, through the down-conversion crystal with momentum intact, and finally being absorbed by the other detector.

This experiment is thus described simply as the lens + detector B either watching the which-slit information or not at the other section of the experiment.

According to Dopfler and Zeilinger the experiment worked, but I have not read anything about it since and the original dissertation has been pulled from the web, but a copy can be found at the internet archive. Since it used a coincidence counter to increase the signal-to-noise between both detectors, they had not really demonstrated any FTL-signalling, however the speculation is that the experiment could run without the counter. Zeilinger calls the conceptual alternative to spacelike signalling "retrocausality" I think, where the cause is timelike in both paths after the down-conversion, but runs backwards in one of them (like in the "pretend" tool mentioned above).

J. Cramer apparently currently works on refining this experiment.

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If this is really is on the right track to transmitting information faster than light, I'm surprised it's not more actively studied. Perhaps because it's impractical? I suppose if light can travel around the world in a matter of milliseconds, sending information any faster is not that important until we get some space colonies. But still, the implications seem important. –  Nick Retallack Apr 18 '11 at 5:33

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