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Just wondering, what would happen in this experiment.

In the experiment you would first have two entangled particles.

Then you fire one of the particles, lets say "Particle A", at a double slit towards a detector.

While in transit to the detector, what if the other entangled particle, lets call it "Particle B" was observed / had it's wave function collapsed?

Would "Particle A" still generate a wave-like interference pattern or would the wave function for both be collapsed?

In theory you cannot send classical data by entanglement, so this experiment must somehow fail, but I can't quite figure out why. If this experiment were to succeed, then you could read and send data about wave function states over entangled particles.

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  • $\begingroup$ See this paper for more details. $\endgroup$
    – Trimok
    Commented Jan 13, 2014 at 20:25
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    $\begingroup$ I asked a very similar question here and eventually posted an answer to my own question. $\endgroup$
    – user1247
    Commented Jan 14, 2014 at 1:35
  • $\begingroup$ I see, got it. Yes that answers my question completely, thanks! $\endgroup$
    – Aniseedwolf
    Commented Jan 14, 2014 at 15:53
  • $\begingroup$ It always pays, when you're doing this sort of thought experiment, to write down an explicit example --- specify the state spaces, specify the initial joint state of the particles, specify the measurements you're going to make, and then work out exactly what happens. Usually a single example, fully worked out, will give you all the intuition you need to see what happens in general. $\endgroup$
    – WillO
    Commented Jun 21, 2016 at 17:18

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This experiment has been done, first by Birgit Dopfer in 1998, then later by Dr. John Cramer of the University Of WA. In Dopfer's experiment, there was a "coincidence detector" which is basically an AND gate to filter out only the entangled pairs. By moving the detector in the beam of photons not going to the double-slit, the information about the photon's momentum could be preserved or erased, affecting whether the interference pattern showed up or not.

However, this coincidence detector is a classical information channel. It prevents using entanglement to send information, unfortunately. Cramer tried several different methods to eliminate the coincidence detector and what he found was that there is an "anti-interference" pattern that drowns out the signal. I wonder what the properties of this anti-signal are and whether there's some way to get around it. The answer is almost certainly "no," but it's an interesting thing to study.

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