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B Entangled photons thought experiment

  1. May 24, 2016 #1
    Following scenario:

    One million entangled photon pairs are created and are sent to Alice's and Bob's laboratory such that Alice receives 1 million photons(or any other large number) of which each is entangled to one of the million photons Bob receives, their laboratories being at a distance to each other.

    Bob is to just have the 1 million photons run through a double slit and check if there is an interference pattern or not.

    Also, Bob's laboratory is further away from the point the photons were entangled compared to Alice's laboratory, giving Alice some time to perform some tasks/experiments on the photons reaching her, before the photons reaching Bob go through the double slit.

    My question is:

    Can Alice affect if Bob will see an interference pattern or not in any way, by doing any kind of experiments on the 1 million photons reaching her, before the photons reaching Bob go through the double slit?

    I leave the way of how the photons have been entangled and what kind of experiments Alice could do to affect the pattern Bob will see open to your imagination/expertise. I am only interested to know if Alice could or could not affect the pattern Bob would see.
  2. jcsd
  3. May 24, 2016 #2


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    It depends on what you mean by "affect". What Alice cannot do is change what Bob sees in a way that would allow Bob to conclude that Alice had done something. Anything that Bob sees is consistent with Alice doing nothing.
  4. May 24, 2016 #3
    I think i was pretty clear about it. Affect as in "manipulate" the photons on her side, such that their entangled counterparts on Bob's side would display an interference pattern necessarily, or no interference pattern necessarily depending on what Alice did or did not to the photons.

    And yes, it seems like if Alice could do that, then she could send information to Bob that way. So your answer is basically "No", Alice cannot affect if Bob will see or will not see an interference pattern, meaning that Bob will always or will never see an interference pattern on his side, no matter what Alice does to the photons on her side.
  5. May 24, 2016 #4


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    Right. Alice can't do anything that Bob would notice. However, I forget the details, but I think it is possible to set things up so that for certain subsets of the photons, there will be an interference pattern, and for other subsets there will not be. But in such a case, it's impossible for Bob to know ahead of time which subset to look at (although Alice will know).
  6. May 24, 2016 #5
    But if Alice could do that, being able to have a subset interfere or not, then to me this looks like she could send information that way. Because she could also decide to not affect the photons at all such that they either all interfere or all do not interfere(not sure which one would be the case).
    So if Alice was to not be able to send any information, at least as far as i can tell, then the photons reaching Bob would either ALL have to interfere or all would have to not interfere. Or to be more precise, they would have to always act the same way, even if it meant that half of them would always interfere while the other half would not.
    Anything else would allow Alice to send information FTL IMO.
  7. May 24, 2016 #6


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    I should try to find the details, but if I'm remember correctly, Alice can't CAUSE one set or another to show interference, she can just figure out which set will show interference, based on her observations.
  8. May 24, 2016 #7
    So it would be correct to state, that even if Alice would run the photons through a double slit herself, prepared in such a way that it would result in no interference pattern on her side, it would not affect the later outcome on Bob's side. The outcome on Bob's side would solely depend on how the entangled photons were prepared at the source.
    So while Alice could end up seeing no interference pattern at all while Bob does, even though the photons were entangled?
  9. May 24, 2016 #8


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    Here's a discussion of how interference can be created and destroyed. I haven't completely absorbed to the extent of being able to answer your questions adequately:


    But note the comment:
    Alice might view her actions as affecting Bob's interference pattern (or destroying his interference pattern), but Bob can't actually see this effect without knowing what Alice knows.
  10. May 24, 2016 #9


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    You can build a quantum eraser:


    I think it's understandable if you know about classical optics and polarization states as well as simple optical elements like quarter-wave plates and employ a very naive photon picture just reducing everything to the polarization states only!

    Of course, all this doesn't imply anything esoteric (no action at a distance, no retrocausation) as long as you stick to the minimal statistical interpretation. The statistical properties of the various ensembles, being defined by choosing partial ensembles from the total ensemble of photon observations according to the once and for all fixed measurement protocols by A and B, and these statistical properties is all that is predicted by QT, and that's what's observed in the experiment. It's fascinating and very "quantum" but it's not esoteric!
  11. May 24, 2016 #10
    The part i am not understanding is as follows.

    Given both Alice and Bob allow the entangled photons to go through the double slits undisturbed, i ASSUME that both would just see an interference pattern as expected to be seen in the usual double split experiments when not messing with the photons. It would be nice if someone could confirm this or deny it.

    So this leaves the cases in which Alice runs the photons through polarizes/double slits or whatever else one could come up with.
    If however above description for the case of not disturbing the photons at all is correct, THEN,as far as i can think, no matter what Alice would do to the photons on her side, Bob would have to see an interference pattern still, because if he didn't, simply by Alice deciding to either mess with the photons or not, she could pass yes/no messages to Bob.
  12. May 24, 2016 #11


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    I'm not sure about the details, but as a rule of thumb, entangled photons do not produce interference patterns. In "quantum eraser" experiments, it's a little more complicated than that...
  13. May 24, 2016 #12
  14. May 24, 2016 #13
    I found this paper but i cannot evaluate it's validity.

    It's a similar scenario to the one i am trying to understand

    http://exvacuo.free.fr/div/Sciences/Dossiers/FTL/R Jensen - Is faster-than-light communication possible using entangled photons.pdf

    part of the paper


  15. May 25, 2016 #14
    It relies on comparing the two signals, both of which are limited to the speed of light!
  16. May 25, 2016 #15


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    The explanation is that entangled photons do not produce an interference pattern as you might expect. The easiest way to understand this is that they are not coherent.
  17. May 25, 2016 #16


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    Regardless of what Alice does on her end, Bob sees a bland lack-of-interference pattern.

    The only way to find an interference pattern is for Alice to measure her copy in a particular way, have her tell Bob the result, then have Bob combine the results together to find that Alice's results split Bob's results into two complementary interference patterns.


    An equivalent way to think about this is that Alice isn't controlling Bob's outcome, she's getting a noisy prediction of what it was or will be. When Bob's photon hits a point associated with a peak in 'Erased Case 1' and a trough in 'Erased Case 2', Alice is much more likely to measure the result corresponding to 'Erased Case 1'.
  18. May 26, 2016 #17
    Even if they did, you still couldn't send information that way, because Bob has no way of knowing whether or not the pattern ( or its form ) is due to anything Alice did on her side. In a sense, the very meaning of "entanglement" already precludes the instantaneous exchange of information, in the absence of any other channels.
  19. May 26, 2016 #18
    Yes, that is probably the case, but it is not unchallenged it seems.

    Someone on stackexchange asked a similar question which was surprisingly answered by R.Jensen, the original author of the paper i linked above and seemingly wasn't challanged by anyone even though it had almost 400 views.

    see more here http://physics.stackexchange.com/qu...ment-require-coincidence-counting/81567#81567

    he then links to another paper which describes an experiment with entangled photons and a mach zender inferometer, with all the mathematics included, claiming that FTL communication is possible once again. The paper can be found here http://vixra.org/pdf/1103.0095v1.pdf

    quoting the introduction

    I am looking if such an experiment has actually been performed still.

    He is not the first one to claim such a thing. There exists another physicist, John G Cramer, who was funded by the public to perform a similar experiment claiming FTL communication pictured here http://www.physics.ohio-state.edu/~lisa/CramerSymposium/talks/Cramer.pdf (also included the Birgit Dopfer experimental setup in a more detailed version). Unfortunately, i cannot find any information on if he ever actually performed the experiment or any conclusive results on it. Which is strange in itself. It almost looks like he ran with the bag.

    edit: Looks like i was too quick on Cramer. He actually admitted that it did not work.

    A more detailed explanation can be found in this paper http://arxiv.org/pdf/1409.5098.pdf

    The Birgit Dopfer paper (1988) which seems to have triggered most of this, can be found here, https://users.isy.liu.se/jalar/kurser/QF/assignments/Dopfer1998.pdf but it is available in German only it seems.
    Last edited: May 26, 2016
  20. May 26, 2016 #19


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    The fact that the answer has a positive vote count is more significant than the fact that it has views. I have no idea why it has a positive upvote count.

    The problem with the paper is really obvious: the author thinks that the quantum eraser leaves the system in the state ##|+\rangle + |-\rangle## with certainty, when actually it leaves the system in the state ##|+\rangle + |-\rangle## 50% of the time and the state ##|+\rangle - |-\rangle## 50% of the time. So the author thinks they have a pure state ##|+\rangle + |-\rangle## in one case and a maximally mixed state 50% ##|+\rangle## + 50% ##|-\rangle## in the other case (though they don't seem familiar with that terminology). But actually it's a maximally mixed state in both cases.

    The rest of the paper is about distinguishing a pure state from the maximally mixed state. Which is trivial, and doesn't need to be explained in a paper.

    That's why the paper is on vixra (it couldn't even make it to arxiv): it's junk with trivial flaws.
  21. May 26, 2016 #20


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    An unfiltered entangled source won't produce an interference pattern, that is something that is experimentally verifiable and hopefully not in question. The coincidence filtering (a la quantum erasers) is a whole 'other story and involves a lot of complexity. You can go 'round and 'round on that. :)
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