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Timing in entanglement investigations

  1. Dec 21, 2013 #1
    Hello all,
    I'm trying to get clarification about the experimental investigation of quantum entanglement and I am stuck on one main thing. How is it shown that the correlated properties between particle properties can be revealed instantaneously(or extremely rapidly) upon making the appropriate measurement on one of the particles?
    Amongst other things this requires extremely precise timing methods.There are other problems such as how do we specify the exact time that Alice makes her measurements and Bob makes his?
    I have spent a fair bit of time searching this but to no avail.
    Thank you.
  2. jcsd
  3. Dec 21, 2013 #2
    Last edited: Dec 21, 2013
  4. Dec 21, 2013 #3


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    Gold Member

    Either I'm misunderstanding your question or you have wrong ideas about quantum entanglement.
    Nothing is shown instantaneously(or extremely rapidly) in quantum entanglement experiments. And certainly nothing is revealed after measurement of only one of the particles.

    To find coincidences you use so called coincidence window. And speaking about timing we have very accurate clocks - where is the problem?
    I would say that bigger problem for timing is detector jitter.
  5. Dec 21, 2013 #4


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    Time transfer for a precise reference time is not extremely difficult these days, and in the linked paper they utilized laser synchronization with a timing accuracy better than 1 ns, while the coincidence window was 3 ns = nema problema.

    Of course there are other relativistic situations (moving detectors) where it is theoretically impossible to define simultaneity because of RoS. However, entanglement works as smooth as any other day in the week... don’t ask me how/why because I have absolutely no idea... :rolleyes:
    Last edited by a moderator: Dec 22, 2013
  6. Dec 21, 2013 #5
    Thank you Jilang and DevilsAvocado. That's clarified things for me. One ns and three ns is pretty impressive.

    Thank you also zonde but your first paragraph seems to contradict what I have read so far. What I was interested in is details of the experimental techniques used and the results obtained.For the time being I don't think I have any further problems on this.
    Last edited: Dec 21, 2013
  7. Dec 21, 2013 #6


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    In general, we can't pin down the exact moment of detection without pinning down the exact point of detection, and that's limited by the size of the detector ("The detection happened somewhere in this volume of space, so happened somewhere between these two points in time"). That's why we spea in terms of detection "windows". When we separate the detectors by a distance that is large compared with the size of the detectors then the travel time is large compared with the window size, so we don't have to worry about it.
  8. Dec 21, 2013 #7


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    Maybe. But surely nothing like that can be directly revealed in entanglement experiments. And your question was about experiments, right?

    I would propose to speak about entanglement of photon polarization as experiments with this type of entanglement are most advanced.
    Now if we would speak about polarization of photon you learn about polarization when photon is detected in detector. What happens before detection is subject to interpretation.
  9. Dec 21, 2013 #8


    Staff: Mentor

    Closed for moderation. EDIT: and reopened.
    Last edited: Dec 22, 2013
  10. Dec 22, 2013 #9
    Thank you. It seems that increasingly longer distances have been tested and are planned so the size of the detectors becomes less significant.Interesting stuff.
  11. Dec 22, 2013 #10
    1.I'm still reading up on the experiments that have been carried out so far and the results seem to support the idea that a measurement on one particle instantly collapses the state of the other particle. I haven't seen anything to the contrary yet.

    2.The remarks I made previously apply to polarisation as well as to spin (and other correlated properties....if any)
  12. Dec 22, 2013 #11
    This is a really fascinating area of experimentation. Please keep us posted with any new developments you come across!
  13. Dec 22, 2013 #12

    time ordering or time detection ?

  14. Dec 22, 2013 #13
    Now that's a completely different question! The time ordering depends on your frame of reference... Do you really want to go there?
  15. Dec 22, 2013 #14

  16. Dec 22, 2013 #15
    Thought not.
  17. Dec 22, 2013 #16
    Class. Quantum Grav. 29 224011

    "So the motivation of the fast moving observers experiment is that each observer would have a different notion of what that moment in time is, according to special relativity. If the two satellites that are making the measurements are approaching each other at relativistic speeds, then an observer on each satellite would have the opinion that their measurement took place before the measurement of the other observer. If we wanted to take quantum mechanics literally then there is an open question—a paradox of sorts—as to what would happen in this situation. Future experiments could test this paradox and see how nature behaves in such a scenario."
  18. Dec 22, 2013 #17
    Thanks. I wanted to know about the time interval,if any, between observing one of the particles and the other particle. I think I have the answers I need.
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