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Breaking quantum entanglement

  1. Jan 13, 2008 #1
    Hello !

    I was wondering about conditions which lead to breaking of entanglement other than measuring observables that are binded in such way. Classical example : entangled spins of e-p pair:

    |a> = |u>|d>-|d>|u>

    Will this 'bond' break if I rotate one of the particles by 4*Pi around z-axis ( or any other axis for that matter=) ? If I rotate them both in the same way( I think no on this one) ? So, let say I separate *one* of particles and put in in homogeneous magnetic field. I guess correlation will somehow 'decay' in time or angle, so I was wondering are there any measurements or mathematical models that describe such 'decay'? My assumption is that break of bond is not instantaneous as a state collapse, but has some time(angle) dependence. Any thoughts on this?


    sorry for not-so-good english =)
    Last edited: Jan 13, 2008
  2. jcsd
  3. Jan 13, 2008 #2
    experimentaly manipulations with entanglement are unsuccessfull so it's hard to say does entanglement realy exist.
  4. Jan 13, 2008 #3


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    How are you going to do that without knowing where the particle is- which would require measuring observables?

    Your English is excellent. You should see my (insert language of your choice here)!
  5. Jan 13, 2008 #4
    Since x and Sz comute, feel free to kick my particle around at will =) My knowledge of experimental setups is, sadly, poor, but here it goes : Let say I have neutral pseudomeson of mass over double muon mass. It will have muon+muon- decay channel. If decay should happen in homogeneous electric field, they will separate - each one heading towards volume where we have SG apparatus that will make Sz measurement. I belive that TPC is constructed similar : "just" (I write "", because I don't want to insult experimentalists that actually can make such device - I know it's not easy=) evacuate box and set SG instead of wires at its end. Like I said, I don't have much of experimental knowledge : can you give me your opinion on my original question considering it as gedankeexperiment? That is, if there is no principle objection to the situation.

    Thank you! =)
  6. Jan 13, 2008 #5
    I think Aspect and coworkers proved beyond doubt that q.e. exists. Also I believe that some college in Swiss proved violation of Bell's equations by more than 36 std. deviations. q.e. is implied in such experiment, if I'm wrong, please correct me.
  7. Jan 13, 2008 #6
    I think what fermio means is whether there is a physical connection between the particles that can be "broken" or if it's just a statistical correlation with no physical meaning. Jury's still out.
  8. Jan 13, 2008 #7
    The correlation depends on the relative measurement angles in such a way that one can say the result at B also (but not only) depends on the angle at A. So a connection is proven, with the violation of Bell's theorem, save a few very tiny loopholes. A theoretical possibility seems to be 'non-realist' rather than 'non-local', but I haven't yet heard of an actual theory that would explain what happens in a 'non-realist' way without also being 'non-local'.
  9. Jan 13, 2008 #8
    Regarding the question: I think, but am not sure, that some operations, such as rotation, can be performed. In general anything that allows the entangled property to remain in superposition, would allow the entanglement to continue. AFAIK.
  10. Jan 15, 2008 #9
    I can not agree on this one. If we choose to believe that QM accurately describes spin entanglement, than Q.E. obviously is not without physical meaning. Take for example entangled spins and one is up, other is down (don't ask how I know=) . If there is no influence between them and we select Sx to measure, there is same probability that results are going to agree and to disagree thus making angular moment conservation law untestable for such system. This is not so. Experiments say that there is correlation that measurements on the *same* axis will always disagree thus actively enforcing angular moment conservation law by collapsing entangled spin state of *other* particle. I know that this is well known and you are aware of this, but I state it since I don't know how can this be explained without spooky action on distance e.g. physical meaning. OTOH I'm quite new to this QM stuff so feel free to correct me if didn't understand you and/or there are alternative theories.
  11. Jan 15, 2008 #10
    None of this proves that there is a "connection" that can be "broken." FTL signaling is not the only problem with such an interpretation. Other problems include:
    - Inability to say, in a Lorentz-invariant way, which measurement occurs first, therefore impossible to say one measurement causes the outcome of another
    - If there actually was an exchange of information between the particles, there'd have to be a boson mediating it. Where would the energy come from to create such a boson? It also, incidentally, would have to be a tachyon.

    I do not disagree in the least that the correlations are real and that the probability of getting the same result of a photon spin measurement is equal (or very close to) the square of the cosine of the difference between the measurement angles. That does not prove a physical bond between the particles. There might be one but that doesn't prove it. Even Bell did not go so far - he wrote many papers exploring the possibility of hidden variables that were not specific to the particles themselves, and on the nature of causality itself.
  12. Jan 15, 2008 #11
    We I reread my last post, i saw that i made much stronger statements that I intended to make or that I care to defend. Also, I came to realize how deep is my ignorance on the subject so I got myself a book 'Consistent Quantum Mechanics' by R.B. Griffits hoping it will fill in some gaps - looks that the book covers all subjects I'm interested in and I appreciate it's author very much. On the subject : well I find that these problems are much more serious now, since things are now much fuzzier then when I made my original post=)

    I understand why FTL raises so many question,especially on the subject of causality.

    Can't resist ... Goldstone bosons take no energy to be created. Just teasing, I know that they are non-physical=) btw ... how come only boson carry interaction? Only thing I can come up with is that fermion interaction would have somehow ill-defined parity due to change of wave function sign on particle exchange or something?!?!?!
    To conclude,I am grateful for yours (plural=) replies : I got myself lot of interesting physics to think of now.

    Stipe Galić
  13. Jan 15, 2008 #12
    Let's go through your 3 points:
    1. FTL signaling
    2. "which measurement comes first"
    3. There would have to be a Boson (or Tachyon).

    (1.) and (2.) are actually related. The effects of entanglement are symmetrical, in it's currently known form. This means there is no actual contradiction with relativity, since an observer that sees the events in a different order, will perceive the other measurement as being first, but since the effect is symmetrical, that doesn't make a difference. One might think it would contradict relativity, but since causation is symmetrical, it doesn't. It may contradict some theories of causation, but not relativity. AFAIK.

    3. I haven't heard this before. Who says there has to be a Boson, or some other particle? That seems to be someone's personal opinion. One of the more usual explanations is that separation by space isn't what it seems to be, that spatial separation may be less fundamental than it appears. You have even mentioned thoughts in that direction yourself.
  14. Jan 15, 2008 #13
    I'd argue that non-locality contradicts relativity. Now, apparently there's been research to suggest that even a non-local hidden variable theory can't account for the experimental data. Then you'd have to abandon not just locality but REALITY too. Reality I'd associate with _causality_ (i.e. definite cause leads to definite effect in the future irrespective of what's actually measured). If this turns out to be wrong, then all bets are off!

    My opinion, yes, but I think this is very likely. I do not see how any signal can be exchanged if it cannot be measured, and if it can be measured then there must be either a particle or field associate with it. All the forces have bosons except for gravity, and most people think gravity has one too. Even Cramer's TI has something like this; he uses waves of course but, same thing.

    Two points here. One, yes, the spatial separation certainly might be different from what it seems. But even if the spatial separation is extremely small, _something_ must be exchanged between the particles if there is truly a connection that could be broken. Where would the energy for such a somethng come from? I haven't seen _anyone_ try to address this.

    Second, I indeed have expressed these thoughts as conjecture, but I'm just trying to point out that there are arguments on both sides. I certainly don't hold a firm belief either way...
  15. Jan 16, 2008 #14
    How would you argue that? In some other way than I have indicated?

    I think there were (at least) two experiments in that direction, one of them by A.Zeilinger. But they ruled out only certain classes of non-local realist models, Bohmian Mechanics among the exceptions. So you might hold on to Bohmian Mechanics. It is realist, yet non-local. Besides, I think BM can be developed further, so it might appreciate some physicists to take care of this... :)

    As far as I understand, the wavefunction is (for the entangled part) the same, so no need to exchange anything. It is more as if the same wavefunction appears in two places. In any case, I would go by what the wavefunction says, and I don't think the wavefunction requires some kind of exchange. It is just automatically the same, so to speak.
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