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Do you believe what this physics teacher says about entanglement

  1. Feb 21, 2016 #1
    Deborah Hearn taught physics for many years at the University of Calgary before coming to Nanaimo.
    She is very interested in research about physics teaching and how to make it more effective. She has had a lifelong interest in creativity in science, and its relationship to scientific discovery and problem solving. She is in the process of designing and coding a computer program that solves qualitative physics problems in a way that demonstrates understanding of physics, much as an expert, as opposed to a novice, might do. The program is based on the premise that successful understanding of physics is related to how we structure our knowledge about it.
    Debbie is also very interested in quantum mechanics, and in studying some alternative interpretations of the theory. As well, she continues to be interested in space physics, specifically modeling the Earth's magnetospheric magnetic field in order to better understand the aurora.

    She has a Ph.D. and in her courses, she teaches that if two photons of unknown polarization arrive at the beam splitter at the same time, they become entangled - their polarizations are no longer independent of each other.

    Check it: https://web.viu.ca/hearnd/Courses/Phys212/Quantum%20Teleportation%20P%20212.pdf [Broken]

    Can you believe this? She basically says the beam splitter can correlate polarizations.
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Feb 21, 2016 #2
  4. Feb 21, 2016 #3
    I mean I may believe or not what they say, but I need to be sure before publishing an article with a setup in which a beam splitter correlates polarizations.
    If the beam splitter happens not to correlate them in reality, my setup would be useless.
     
  5. Feb 21, 2016 #4
    Depending on the input to the beam splitter, and the detected output, yes, they can become entangled in one of the four bell-states.
     
    Last edited by a moderator: May 7, 2017
  6. Feb 21, 2016 #5

    Nugatory

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    As been pointed out in some of your other threads, there's a lot more to producing the entangled pair than just having a beam splitter. The presentation you linked is about something that can be done with entangled photons not how to produce them, and slide seven is just there to give you a sense of what sort of interaction is required to produce the entanglement.

    Whether your setup is useless or not will depend on everything else in your setup, not whether a beam splitter will correlate polarizations.
     
  7. Feb 21, 2016 #6

    DrChinese

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    We have already discussed this experiment with you several times. Nothing is said at this powerpoint different than what we have discussed previously. There is no point in posting something and then making an incorrect statement about it, and expecting it to go unnoticed by Nugatory, myself, etc.

    So for the record: she did not say a beamspliter can correlate polarizations.
     
    Last edited by a moderator: May 7, 2017
  8. Feb 22, 2016 #7
    I got the opinion that the beam splitter could be used alone not from these sources describing entanglement experiments, but rather from something called the Michelson interferometer. Unlike you, I do not understand the difference between interference and entanglement. What could it be? Can you help me understand what other elements are needed in this interferometer to entangle the input photons?

    https://en.wikipedia.org/wiki/Michelson_interferometer

    It uses mirrors, a beam splitter and a detector to create interference patterns.
    655px-Michelson_interferometer_fringe_formation.svg.png
     
    Last edited: Feb 22, 2016
  9. Feb 22, 2016 #8
    Entanglement happens when the particles physically interact (exchange information) with each other which correlates their quantum states to a single state; quantum interference is about the superposition of the wave functions which give the probabilities of events.

    Entanglement is pretty much demystified if you think that after their interaction both entangled particles carry information about both particle's state but this information is in superposition prior to measurement as is everything in quantum mechanics.

    Edit: if you think the entanglement of polarizations is weird check this out
    https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser
     
    Last edited: Feb 22, 2016
  10. Feb 22, 2016 #9
    I'm sorry but that page mentions BBO crystals which create entangled particles rather than entangling independent particles. We were asking each other about setups that entangle previously independent photons.
     
  11. Feb 22, 2016 #10
    If the photons interact with eachother why wouldn't they be entangled afterwards? Every time a particle affects the behaviour of another particle (when they exchange information) they become entangled.
     
  12. Feb 22, 2016 #11

    DrChinese

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    The reference kvantti provided you does entangle photons. I don't know if I would use the word "independent" to describe them though. I think you are asking whether 2 unentangled photons can be entangled with some kind of classical apparatus like an interferometer or a beam splitter. Generally, the answer is no because the photons are distinguishable. To be indistinguishable, they must usually be phase matched. To do that, you need all kinds of other gear. QED.
     
  13. Feb 22, 2016 #12
    If the photons are in superposition (their polarization unknown as stated in 1st post) and they interact in the beam splitter don' they become entangled? Also I've gotten the impression from my vague understanding that if two photons coincide eg. interact in some scattering process they become entangled...?
     
  14. Feb 22, 2016 #13

    Nugatory

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    They do, but only in the trivial sense that any interaction involving anything will lead to some degree of entanglement around some observable for some short period of time. In this and his other posts, sciencejournalist00 is asking for something much more specific: a coherent state in which the photons are entangled on one particular observable (polarization) and in which coherence will be maintained over a long enough period of time to do something interesting with the pair. There's a lot more to doing that than just grabbing two random light sources and pointing them at a beam splitter.
     
  15. Feb 22, 2016 #14

    DrChinese

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    You have answered your own question when you mention 2 photons interacting. How are they going to do that? They would need to arrive close together, so close that the arrival time would not distinguish them. Or alternately that their arrival times are "smeared out" and therefore overlap partially (and will only be partially entangled - as Nugatory is alluding to: "some degree of entanglement"). Etc.

    So what needs to be done to accomplish that? Clearly, that is the rub. To have a degree of entanglement that can be subsequently measured to exist: that won't really happen in practice without the complex setups. Such as the reference you provided, which is anything but simple. :smile:
     
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