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Entanglement and Some Unanswered Questions

  1. Jan 2, 2016 #1
    Unanswered to me at least. I have read many articles, forum discussions on quantum entanglement but never found anyone discussing these questions. Here they are:
    1) How do they create (mechanism) entanglement? or How 2 particles are generated entangled?
    2) What are the differences (properties) between 2 entangled and 2 non-entangled particles? Can we tell by looking at 2 particles if they are entangled or not without the 'entanglement test'?
    3) What are the 2 heaviest entangled particles generated so far? Electrons, Protons? Neutrons? Is there any limit on mass for entanglement?
    4) Is it true if polarization of one of 2 entangled photons is changed before detection, the other photon will also change its polarization?

    I'm really interested to know what makes 2 particles entangled. I think the answers to our entanglement problems lie in the process of entangling them.
  2. jcsd
  3. Jan 2, 2016 #2
  4. Jan 3, 2016 #3


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    Staff: Mentor

    Just about any interaction between particles will cause them to be entangled in some way for a while, so there are many different ways of producing entangled pairs. Google for "spontaneous parametric down conversion" and "calcium cascade photon pair" for two examples. Any serious report of an experiment involving entangled particles will include an explanation of how the entanglement was produced.
    There is no way of knowing whether a single particle is entangled. I have to measure both members of the pair and then compare the results, and even then it might just be a coincidence. Thus, I have to look at multiple pairs coming from my before I can conclude that the pairs it generates ate entangled.

    It's as if you and I were playing a game in which we both toss a coin at the same time and compare the results. In a single trial there's a 50% chance that we'll both get the same result, so there's nothing surprising if we play the game once and get that result. But if we do many trials and get the same result every time we'd be very surprised and would know that something weird was going on.
    No, but not for the reason you're thinking. The way quantum mechanics works, the polarization has no value before it is measured - and that is literally "has no value", not "it has some value but we don't know what it is yet". Thus, it is meaningless to talk about the polarization changing before measurement - there's no polarization to change until the measurement happens.
  5. Jan 3, 2016 #4
    Thanks. I now got some idea and some direction what type of article to look for.
    About polarization, after posting the thread I realized, a polarizer also performs some kind of detection.
    Then it must be true if one of the two entangled photons is detected for measurement, the entanglement is destroyed.
  6. Jan 3, 2016 #5
  7. Jan 5, 2016 #6
    More questions:
    I'm suspicious that quantum entanglement has entered into pop culture like Black Holes. The reason of my questions is online forums that deals with science are flooded with layman's interpretations of quantum entanglement.These layman are presenting entanglement as more mysterious than it really is. Another thread in PF on this subject also felt the same way as I'm being slowly convinced.

    Here is another question, anyone please answer:

    1) Two entangled particles are separated at distances, we'll call them P1 and P2. We'll perform first measurement only on P1. We have different instants of time t1, t2, t3..(t1<t2<t3..). Measurement on P1 can be made at t1 or t2 or t3..etc. A measurement at t1 shows spin is UP, but instead if we take measurements at t2 or t3, can the measurement show spin is DOWN? (Note, I understand measurement can be made only once).
    If the answer is YES, then I understand why entanglement is so mysterious.

    Now if we take a measurement on P2 at times T1, T2, T3.. etc, (T's are after a measurement is made on P1 with result spin=UP). Is it correct to say , doesn't matter when the measurement on P2 is taken, the result will always be spin=DOWN?

    2) What I'm suggesting is Binary properties of particles, UP/DOWN, LEFT/RIGHT, etc are not good tests for entanglement.
    Have they made any measurements on multi-valued properties, such as speed (momentum), etc? What are the results of speed of P2, AFTER speed of P1 is known?

    Btw, @stevie, @Jerom, thanks. The articles in those two links are very new, interesting and important to the questions I asked.
  8. Jan 5, 2016 #7


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    Gold Member

    1) The results of spin measurements on P1 and P2 are completely independent of time, time ordering, etc. Once either particle's spin is observed, that particle will continue to yield the same value in subsequent observations. If P1 is UP at T2, then expect an UP at T3.

    (Please recall that there is no meaning to unperformed observations when you speak about quantum systems. So trying to ascribe a value to spin at earlier T1 is not considered meaningful.)

    2) Binary properties are great tests for entanglement. But the real magic shows up when you measure P1 and P2 at different angles. That is when Bell's Theorem comes into play.

    On the other hand, there are many multi-valued properties (not binary) that evidence entanglement. Examples are momentum, time, frequency, energy, position. And there are actually many more, although these get very advanced very quickly. The technique is to develop a Bell Inequality around the observable, and then perform an experiment to obtain a value.
  9. Jan 6, 2016 #8
    I still think binary properties are not very convincing proofs.
    But I do not see why physicists are so mystified by quantum entanglement.
    What we know so far and the links given by Jeromy and Stevie draw a somewhat
    clear picture of entanglement.

    Entanglement occurs due to interactions between two quantum particles, which
    can generally be represented by superposition of 2 wavefunctions - creating a
    single new wavefunction.

    Physicists are puzzled, I think, by the stretching of 'a single wave function'. Jeromy's link
    shows existence superposition of particles .5m apart. It appears to me two entangled particles, separated by distance share the same wave function - a stretched wavefunction. So the question is

    1) Not how far a wavefunction can be physically stretched, but what physical or non-physical
    actions, (in between 2 separated particles) breaks the entanglement? Breaking take place
    in our everyday events.

    After reading the link by Stevie it occurred to me entanglements take palce in many forms in nature.
    2) Can it be that Earth-Moon (or similar pairs) are also entangled? The statement is not far fetched if one assumes 2 wavefunctions are replaced with 2 gravitational fields, a transition from 2 microscopic worlds into 2 macroscopic worlds.

    Well, I guess this necessitate closing of this thread, too much speculation, unless
    someone already has info to back it up.From now on, I'd not be puzzled by entanglement unless I hear something new.
    Thank you all.
  10. Jan 6, 2016 #9
    In my opinion it is not the nature of quantum physics which is puzzling but rather the "mechanism" which seems to contradict classical "mechanisms" on some points but mimics other classical behavior, i.e. wave nature.
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