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Uncertainty on entangled electrons

  1. Oct 3, 2013 #1
    We know that cant measure both position and momentum simultaneously whether both are orthogonal.

    If i measure position one entangled electron and momentum on other electron for single state then how heisenberg's uncertainity princple works?? so We may know the both parameter.
  2. jcsd
  3. Oct 3, 2013 #2


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    If you have a strongly entangled pair of electrons in position-momentum, measuring the position of one electron makes the state of the other electron one of well defined position.
    If you instead measure the momentum of the one electron, the state of the other will be one of well-defined momentum.

    If you've already measured the position of the one electron, you will see the position of the other electron to be well-defined if that's what you choose to measure. If you instead try to measure the momentum of the second electron, you will see it is very uncertain just as the uncertainty principle would say.
  4. Oct 3, 2013 #3


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    It works in the same way it works for a single electron. You get some measurement value, but it is not meaningful to say "this is the value the electron has now" for both values. Measure the position of electron A, and you know the position of both, the entanglement is now broken. Measure the velocity of electron B, and you know the velocity (but not the position) of B, and still the position (but not the velocity) of A.

    This is a simplified description, in reality there are no ideal position or momentum measurements, but that does not change the main point.

    Edit: Again too slow :(.
  5. Oct 3, 2013 #4


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    Heisenberg allows to know the position of A and the momentum of B!
    Do not think you have measured something else.
  6. Oct 3, 2013 #5
    if i make two device which one going to measure position of A and 2nd device measure momentum of B in simultaneously??
  7. Oct 3, 2013 #6
    how do u say??need explanation..
  8. Oct 3, 2013 #7


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    It is true that you can measure momentum of A and position of B. When A and B are entangled (as A+B), that does NOT tell you the simultaneous values for either A or B. Your idea was originally considered in a famous paper called EPR from 1935. (Einstein is the E in EPR.) They thought they had found a way around the Heisenberg Uncertainty Principle (HUP), demonstrating that QM was incomplete.

    Later, a gentleman named Bell (1964) figured out a proof that this was not the case. Basically, a measurement on the system of A+B leaves A and B in a similar quantum states. Further measurements are then just like additional measurements on any individual quantum particle. And for that, the HUP still applies.
  9. Oct 3, 2013 #8


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    Then you'll know the position of A and the momentum of B.

    You will also know what result you would have gotten if you had measured the position of B instead of the momentum of B - but that tells us absolutely nothing about B after you measured its momentum instead.

    (Also there is a relativistic pitfall hiding behind that word "simultaneously", but we don't need to worry about that now).
  10. Oct 3, 2013 #9
    why values for diffrnt like A and B whether they are entangled?? and measuring simultaneously(i heard about they correlate faster than speed of light..and its fuzzy to me..)
  11. Oct 3, 2013 #10
    yes i got your views.. i know that we can do any additional measurement for one particle but not simultaneously...but i want to say what happened if doing measurement simultaneously for different particle in entangled condition???
  12. Oct 3, 2013 #11
    mayb i doing stupid questions contino
    usly but i dont know...
  13. Oct 3, 2013 #12


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    By the way, welcome to PhysicsForums!

    It does not matter whether you measure A first, B first, or even if you "think" you measured them simultaneously. The results are exactly the same as if you made two measurements on A. Or two measurements on B. In other words, the HUP is still followed. You do not actually know two values of one particle at the same time.

    If you DID actually know 2 values of A simultaneously, then you would be able to measure it and confirm. But you will not get the confirmation you seek, just some random value. In other words, experiments do not back up your idea.
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