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Quantum Physics ?

  1. Apr 17, 2009 #1
    Hi, as my previous thread was closed, I'd like to continue asking some questions here regarding QM (of course if the mentors don't mind that ?).
    So, has entanglement been experimentally proven?.
     
  2. jcsd
  3. Apr 17, 2009 #2

    DrChinese

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    Yes, in literally thousands of experiments covering dozens of ways that particles can be entangled.

    Entanglement can be verified a number of ways. The usually starting point is to generate a stream of particle pairs that exhibit a maximum of "perfect correlations". Only entangled pairs exhibit this, which is the cos^2(theta) function for photons. That reachs its max of 1 at theta=0 or 90 degrees. Unentangled photons follow a different correlation function, with a max of only .75 at those angles. The unentangled function is .25+.5(cos^2(theta)).

    The next step is usually to form a Bell Inequality and demonstrate its violation. Obviously, for both the first step and the second step, the exact functions are dependent of the particle types, the setup, what is being measured, number of particles (entanglement can be 2 or more), etc. So the exact methodlogy varies, but is always described.
     
  4. Apr 17, 2009 #3
    Yes.

    In the early 80s http://en.wikipedia.org/wiki/Alain_Aspect" [Broken] that confirmed entanglement.
     
    Last edited by a moderator: May 4, 2017
  5. Apr 17, 2009 #4
    Last edited by a moderator: Apr 24, 2017
  6. Apr 17, 2009 #5
    I still don't get it, could someone explain to me, from physics point of view, how Bell's experiment proved entanglement?.
     
  7. Apr 17, 2009 #6

    Fredrik

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    You don't prove a thing like entanglement (or gravity). What you do is to compare the predictions of different theories to see if you can dismiss some of those theories.

    A large class of hidden variable theories make predictions that satisfy Bell inequalities. Quantum mechanics makes predictions that violate those inequalities. Experiments were performed, and the results weren't even close to satisfying the Bell inequalities. Therefore, those hidden variable theories have been falsified.
     
  8. Apr 17, 2009 #7

    ZapperZ

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    Note that violation of Bell inequality is only ONE aspect of the test of quantum entanglement. The other is the breaking of the diffraction limit using these entangled particles.

    http://physicsworld.com/cws/article/news/19514

    Such experiments were done 5 years ago!

    Zz.
     
    Last edited by a moderator: Apr 24, 2017
  9. Apr 18, 2009 #8
    OK, but do the loopholes still apply to those new experiments, specifically http://physicsworld.com/cws/article/print/1332" [Broken]?:
     
    Last edited by a moderator: May 4, 2017
  10. Apr 18, 2009 #9

    ZapperZ

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    There are no loopholes in the diffraction limit experiments.

    Zz.
     
    Last edited by a moderator: May 4, 2017
  11. Apr 18, 2009 #10
    I'm sorry, I meant do the diffraction limit experiments (as Bell's experiments) favor the completeness of QM?. If so do the loopholes still apply to them?.
    Thanks.
     
  12. Apr 18, 2009 #11

    ZapperZ

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    You are no longer making any sense.

    At some point, YOU have to do your own homework. We have arrive at that point where you have to read those experiments yourself, understand the physics, and then figure out if your question makes any more sense.

    Zz.
     
  13. Apr 18, 2009 #12
    Ok, Here is why I think so. Electrons interference pattern :
    http://www.hitachi.com/rd/research/em/doubleslit.html" [Broken]

    Photons interference pattern:
    http://ophelia.princeton.edu/~page/single_photon.html"

    But why this happens?.
     
    Last edited by a moderator: May 4, 2017
  14. Apr 18, 2009 #13

    ZapperZ

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    These are not "tiny holes"!!

    Look at the nature of the detector. In many cases, these are CCD detectors. It means that whatever is triggering it has a finite "boundary" of detection.

    If I make the CCD camera pixels bigger, you get bigger "holes".

    Zz.
     
  15. Apr 18, 2009 #14

    Doc Al

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    If you are asking about why the patterns are different, it looks to me that the electrons were diffracted through slits while the photons were diffracted through holes. :wink:
     
  16. Apr 18, 2009 #15
    I know, I didn't mean that. If you compare the two patterns, you will notice that when photons go through the slits, they are more concentrated at one entrance point (tiny hole) along the slits axis unlike electrons.

    No!, if you are sure about that, you would have said this before :smile:.
     
    Last edited: Apr 18, 2009
  17. Apr 18, 2009 #16

    Doc Al

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    Huh? Did you show these pictures before? Do you know what they represent?
     
  18. Apr 18, 2009 #17

    DrChinese

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    There are no loopholes in the proof of entanglement. What you are trying to refer to is the alleged loopholes in tests of Bell's inequality, which is used to rule out local realism. These are not the same things as proof of entanglement. Even folks that assert local realism is not ruled out by these tests must acknowledge that entangled pairs do NOT act like unentangled pairs.
     
    Last edited by a moderator: Apr 24, 2017
  19. Apr 18, 2009 #18
    I meant that you saw pictures of photons/electrons interference pattern before?.
    Yeah, I know, maybe I should have said "entanglement interpretation". But have all the alleged loopholes been argued as not happening in Bell's experiment?.
     
    Last edited: Apr 18, 2009
  20. Apr 18, 2009 #19

    Doc Al

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    I have certainly seen them before. What's your point?
     
  21. Apr 18, 2009 #20
    I'm sorry, it was maybe a misunderstanding.
     
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