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I What exactly does quantum entanglement imply?

  1. Dec 1, 2015 #1
    I've been having issues understanding quantum entanglement and non-locality recently, and certain explanations that I have been told has just made the matter more confusing. The first portion of this thread will be explaining what I know and the second part will contain the questions.

    First Part: What I understand from entanglement is that a particle becomes connected with another particle, and this generates a phenomenon in which neither particle can any longer be considered independent, but rather intrinsically intertwined to the corresponding entangled particle. Any affects that one particle experiences will affect the other particle instantaneously.

    To explain quantum entanglement, physicists postulated the idea of non-locality. Non-locality states that an object can affect other objects that are not in its immediate surroundings.


    Second Part: My first question lies within non-locality. It seems to me that non-locality was postulated because physicists could just not explain why can object is affected by another object at a distance where faster than light speeds would have to be necessary. Is this true?

    Second question lies within a certain entanglement explanation that I've been told. This explanation goes as follows: "Quantum entanglement does not imply faster than light speeds because nothing is being communicated, rather it's just merely correlation between the particles. If one particle is measured to contain spin up, then the other entangled particle can be sure to contain spin down. Nothing is being communicated; we just know what spin the other particle is going to be." This explanation would make sense to me, although the particles do seem to communicate something. Communication can be seen observed by the other particle always containing an opposite spin. The only to explain this using this explanation is to say that particles always have to be opposite spin to entangle together, although isn't this just the hidden variable theory?

    Any feedback would be appreciated!
     
    Last edited: Dec 2, 2015
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  3. Dec 2, 2015 #2

    Simon Bridge

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    1. Yes. It's the name for the effect; see:
    https://en.wikipedia.org/wiki/Bell's_theorem

    2. I've been having trouble finding a decent lay description ... when I had the same issue you did I basically had to do the math. The statistics are different.
    You can imagine a situation where there are two boxes, a blue ball in one box and a red one in the other - you dunno which is where but looking in one box tells you right away what is in the other one. That is hidden variables - where it does not match up with the entangled case is in the statistics: in a nutshell, the example is too simple.
    This looks promising: https://cs.uwaterloo.ca/~watrous/CPSC519/LectureNotes/20.pdf
     
  4. Dec 2, 2015 #3
    also my 2 cents if I may, just because once you see one particle reveals the other doesn't mean communication , because for communication the other end (whoever may be there) needs to also know what you saw at your end of the line.

    you could say , well okay make a code sheet and then give it to each of the observers at each end so they dont have to communicate back to each other to confirm but as they see their result they can compare it wih what has been writen for that exact result but this would work if one could determine the outcomes of the entangled particles spin states etc, but we can't determine those , we only know the other particles state once the first one is revealed, so it's more of a lottery.
     
  5. Dec 2, 2015 #4

    DrChinese

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    Just to be clear: there are no such code sheets possible which would agree with the predictions of QM.
     
  6. Dec 2, 2015 #5
    My two cents: the measured value by Bob depends on the measured value by Alice, or vice-versa, which is equivalent. So, Alice's measurement basis of her free choice influences the correlation (between Alice and Bob), and similarly for Bob. This is non-locality. The problem is that you can't tell who influences who, the correlation just is! So in this case, local hidden variables are ruled out, though non-local hidden variables are not.
     
    Last edited: Dec 2, 2015
  7. Dec 2, 2015 #6
    A good lay description I've heard is: imagine walking into a shoe store taking a box off the shelf and inside you have a LEFT white size 9 nike with blue trim, immediately you know that somewhere regardless of distance space or time, that there is a RIGHT white size 9 nike with blue trim, we know this simply because they are a correlated pair. As for this spooky "action" at a distance that's a bit harder to explain, entangled pairs act as one.
     
  8. Dec 2, 2015 #7

    Nugatory

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    Unfortunately, this is the classic example of local variables at work - we know that a shoe is created with a size, a color, and a handedness footedness and it has these properties even if we don't measure them. Entangled quantum particles don't behave like that..

    Suppose you and I somewhere else in the universe are looking at the pairs of shoes as they come by, but we're each only allowed to measure at random one of the three properties: color (white or red), size (9 or 10), left-foot or right-foot. If, for a given pair, you measure the size and get nine and I measure the color and get red, we might conclude when we compare notes that you had a white size nine shoe of unknown footedness while I had a red size nine shoe of unknown footedness; similar logic works for all the other possible pairs of measurements. (If we both randomly measure the same property, the other two will be unknown even after we compare notes).

    However, suppose that when we compare notes we discovered that the number of white size nine shoes (using the logic above, where we combine your measurement and mine to infer something about my shoe) that I saw is more than the sum of the number of white left shoes that passed me plus the the number of size nine right shoes? That's the equivalent quantum mechanical prediction; it has been confirmed experimentally and it tells us pretty clearly that the two particles in the entangled pair were not created with definite values of all three attributes.
     
    Last edited: Dec 2, 2015
  9. Dec 2, 2015 #8
    And so something is being communicated between the entangled particles, right? From my research I know that we cannot tell what is being communicated or who's doing the communication, but just to clarify, we know that something is being communicated?
     
  10. Dec 2, 2015 #9

    Simon Bridge

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    That would be the classical interpretation of the results... otherwise, how does the distant particle know which set of statistics to follow?

    Technically, what we are saying is that any "hidden variables" style communication would have to be ftl... this potentially allows for causality violations[1] so we don't think this is what happens. Instead we have to face the idea that reality can be non-local. ie action at a distance is real. In this case, two objects can be part of the same quantum system even at very large distances.

    [1] Summary of (2012) Nature paper, link to paper at bottom.
    http://arstechnica.com/science/2012/10/quantum-entanglement-shows-that-reality-cant-be-local/
     
  11. Dec 2, 2015 #10

    bhobba

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    Before discussing entanglement you need to understand what it is.

    Simply its an extension of the principle of superposition to different systems. Suppose two systems can be in state |a> and |b>. If system 1 is in state |a> and system 2 is in state |b> that is written as |a>|b>. If system 1 is in state |b> and system 2 is in state |a> that is written as |b>|a>. But we now apply the principle of superposition so that c1*|a>|b> + c2*|b>|a> is a possible state, The systems are entangled - neither system 1 or system 2 are in a definite state - its in a peculiar non-classical state the combined systems are in.

    If you observe system 1 and get state |a> then you know system 2 is in state |b>, and similarly if you observe system 1 and get |b> you know system 2 is in state |a>. That's all entanglement is - a correlation. That's it, that's all. Let that sink in devoid of the stuff you read about it in the pop sci literature.

    Imagine you have two slips of paper a red and a green one and put them in envelopes. Send one to the other side of universe and keep the other. Open the envelope and you see red - you immediately know the other is green, and conversely. Nothing weird or mysterious here. That's all that's going on with entanglement with a twist I will explain.

    Now for the QM twist. It turns out the paper analogy is not quite the same as QM. The correlation is a bit different - its still just a correlation - but has statistical properties different to the paper example. Why the difference? The difference is in QM things do not have properties until observed to have them, whereas the slips of paper remain red or green at all times. But what if we insist it's like the slips of paper - then it turns out you need some kind of non local superluminal communication. That's really weird. But there is nothing compelling anyone to insist its like the slips of paper - simply accept QM allows a different kind of correlation and things are no longer mysterious.

    Without going into the details there is good reason to not apply the concept of locality to correlated systems (its to do with QFT an the so called cluster decompostion property). You can define it to be applicable but it doesn't sit well with other things. If you do that then you don't run into issues in the first place. Only by allowing it to apply can you say its like the slips of paper. If you say its not an applicable concept then the answer is simple - it can never be like the slips of paper. QM correlations are different from classical ones - big deal.

    Thanks
    Bill
     
    Last edited: Dec 6, 2015
  12. Dec 2, 2015 #11

    zonde

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    Right, as long as you remember that all knowledge in science is tentative.
    Or the longer answer would be that we know there can't be any light speed limit compatible scientific explanation of our loophole free Bell test experiments as far as we can trust that experiments are loophole free.
     
  13. Dec 3, 2015 #12
    That is the explanation in which I am having difficulty understanding. How would system 1 know that system 2 is measured if there is no communication but merely correlation? You said yourself that "QM things do not have properties until you have observed them.." and so how would a certain system know that it has to be opposite to the opposing system, if it has the superposition of both systems prior to measurement?
     
  14. Dec 3, 2015 #13
    I would rephrase that: suppose you have two envelopes and in envelope A you put a green/red striped sheet of paper, and in envelope B a red/green striped sheet of paper. If you send one to Alice and the other to Bob, and Alice finds out that, after putting the envelope through some process, she has a green sheet, then she know Bob has a red one. :wink: But you can view this the other way round: suppose Bob puts his envelope to some process and after opening it finds it contains a green sheet! The he knows Alice must have the red one! Since who opens the envelope first depends on the (relativistic) frame of reference, you can't tell who should be the communicator. You can only establish that there is a correlation. Am I making sense? :smile:
     
  15. Dec 3, 2015 #14

    bhobba

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    Sure. Its just a correlation.

    Thanks
    Bill
     
  16. Dec 3, 2015 #15
    Yes, it's correlation. Through the correlation one can infer that there is some kind of communication being transferred between the two entangled states. You stated yourself that within the confines of quantum mechanics two entangled states are in a superposition of being both state A and B when not measured. So my question is simple: if it is merely correlation and there is no communication, then how does an entangled particle know to be opposite to the other particle?

    Let me try to be my clear and give an example in what I am asking.


    Let's say we have an entangled pair of particles that are separated by an arbitrarily long distance. We'll call the particles system 1 and system 2.
    Both systems are in a superposition of being in state A and B.
    If then system 1 is measured to contain state A, then we can infer that system 2 contains state B. However how does system 2 itself know that it has to be in state B? Prior to measurement, it should be in a superposition of both A and B. And so then how can a measurement that is occurring in some far off galaxy affect the of state of it?
     
    Last edited: Dec 3, 2015
  17. Dec 3, 2015 #16

    zonde

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    Not quite. It does not make sense to incorporate into single model symmetries of relativity and FTL communication. Of course you get contradiction because symmetries of relativity are simply incompatible with any FTL communication or causality.
     
  18. Dec 3, 2015 #17
    In my view, it isn't in state B. The correlation measured leaves room for the interpretation it may have been in state B, but the measurements and their correlations is all we have!
     
  19. Dec 3, 2015 #18

    bhobba

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    That's incorrect. It knows it the same way the colored papers knows it ie you have arranged it so by the entanlement. The only difference is in QM properties do not exist until measured.

    To be specific because |a>|b> and |b>|a> are in superposition you have arranged things so that the only outcomes of an observation are |a>|b> or |b>|a>. That is the exact analogue of the coloured papers where the only outcomes are red and green or green and red.

    If you still dont see it you need to explain exactly why there is no communication between the slips of paper but there must be communication between entangled particles.

    Thanks
    Bill
     
    Last edited: Dec 3, 2015
  20. Dec 3, 2015 #19
    But they both apply to the observable world, don't they? I am just combining the two in this context! :wink:

    By the way, I am on the contrary not talking about FTL communication. I think that decoherence is an important contributor in this case. Hoewever, I have little knowledge of it right now.
     
    Last edited: Dec 3, 2015
  21. Dec 3, 2015 #20

    zonde

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    For deBroglie-Bohm model you have to use preferred frame and incorporate relativity using so called Lorentz relativity interpretation.
     
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