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Based on my understanding of the “Double-Slit Quantum Eraser Experiment”, documented here: http://grad.physics.sunysb.edu/~amarch/, it seems that Faster-Than-Light communication is possible. Of course, this violates the “no-communication theorem”, so I assume it’s due to a misunderstanding on my part.

In the experiment, changing whether the polarization of the ‘p’ photon is erased or not changes whether or not interference patterns appear for the ‘s’ photon. Suppose Alice and Bob generate many entangled photons and then travel to opposite sides of the galaxy, Alice taking the ‘p’ photons and Bob taking the ‘s’ photons. At a predetermined time Bob releases his photons one by one into a double slit setup as in the experiment. If he sees an interference pattern, he knows that Alice must have “erased” the polarization information on her corresponding ‘p’ photons, and if not that she has instead measured it.

Of course, even stranger situations are possible. The fact that the ‘s’ photon in the experiment showed interference patterns when the ‘p’ photon was going to be erased, but hadn’t been yet, makes me think that photons can retrieve information from the future. For instance, measuring the ‘s’ photon and detecting interference patterns or not, while the ‘p’ photon is bouncing around in some system, waiting to have its polarization be either erased or measured depending on the result of some future event, like an election result. If the ‘s’ photon shows an interference pattern, then you would know the result of the electron (or know that forces beyond your control erase the information for some other reason).

I’d be very appreciative if you could clear this up for me. My search online has not been fruitful, since the only similar questions I’ve seen just say something like “When you have entangled photons, measurements on one member of the pair cannot tell you anything about whether the other member has been measured or not.”, which seems to be contradicted by the above experiment.

Thanks!

Rookman64
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    Did you read any of the question that came up in the auto-suggester when you types that title? How about some of the ones in the "Related" sidebar? This question has come up many times already. – dmckee --- ex-moderator kitten Aug 12 '15 at 04:00
  • None of them seemed to answer my question well. See my last paragraph. – Rookman64 Aug 12 '15 at 04:06
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    That answer is correct, and your intuition that is disagrees with the other things you have read isn't actually so. It helps to sit down and work out the probabilities by hand. – dmckee --- ex-moderator kitten Aug 12 '15 at 04:14
  • Based on the linked experiment, erasing information on one photon causes a change in the behavior of the other photon. How is this experiment possible if measurements on one member of the pair cannot change anything about the interference patterns of the other member? – Rookman64 Aug 12 '15 at 04:29
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    You really, really have to sit down and work out the probabilities for yourself. If your quantum mechanics isn't up to that you either have to bone up or take someones word for it. – dmckee --- ex-moderator kitten Aug 12 '15 at 04:38
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    I really don't see the difference between this question and http://physics.stackexchange.com/questions/100864/the-choice-of-measurement-basis-on-one-half-of-an-entangled-state-affects-the-ot – Emilio Pisanty Aug 12 '15 at 07:03
  • Hint: In the experiment you link one crucial element is a coincidence counter. It uses classical communication between the two parties and without it you can't observe the effect (and this is not just a technical limitation). – Emil Aug 12 '15 at 08:13
  • see also https://physics.stackexchange.com/questions/512504/in-layman-terms-why-cant-quantum-entanglement-be-used-to-achieve-ftl-communica/512547#512547 – Andrew Steane Dec 11 '19 at 12:33

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Something that confuse many people on this experiments is that you need to understand that just measuring the s photons will not show an interference pattern. The interference pattern appears only after you have selected the specific photons in s that correspond to the photons in p whose polarization has been erased. So you need this after-measurement in p information in order to see an interference pattern in s. And this information is classical, and cannot be sent faster than light.

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The diagram of the experiment has a large box bearing the legend "coincidence counter". The experiment is measuring fringes in the probability of a match in the photons' location. The probability for each individual photon to arrive at a specific place is not changed by different measurements. The probability of a match in location when the results are compared does change depending on what measurement you do.

Also, quantum systems are governed by local equations of motion, and there is an explanation for how correlations between entangled particles are brought about by local processes:

http://arxiv.org/abs/quant-ph/9906007

http://arxiv.org/abs/1109.6223.

alanf
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The entanglement is always based on pair production of the involved particles. And all thoughts about faster-then-light evaporates to dust, if one understand that the entangled parameters are given at the moment of the pair production.

At the moment, we measure the entangled parameter from one of the particles our unknowledge about the parameter collapses. It is more the artful engineering to find materials for pair production and to prevent destruction of the entangled parameters from the environment what make it possible to send encrypted information over distance then some not existing spukhafte Fernwirkung (spooky influence over distance).

HolgerFiedler
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