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Quantum entanglement , ftl

  1. Mar 1, 2013 #1

    I have a question suppose we have two entangled electrons , now we take them a sufficiently large distance away from each other so that the EM (light) wave would have to travel a certain distance/time.When one electron is measured at one place and the other at the other place they botch can then simultaneously know which one the other ones has.

    Now I imagine you could not send information with many entangled electrons as when upon measurement you can't predetermine the spin of the first electron so you can't for the second one too only when the first one is measured then you can know the second one.

    But if we would have multiple let's say 10 electrons entangled in 5 pairs.Now Alice is at one end and Bob at the other and they have wrote a code book in which it says that measuring the first electrons is A and the second B and the third corresponds C.
    Now they both have a copy of this book Now Bob at his end can measure let's say the second electron first so Alice now sees that the second electron is revealed for her as Bob has measured it already and so Alice can write down the first letter B, then Bob decides to measure the third one and Alice writes down C and then A.
    Now because in entangled particles when one is measured the spin of the other is know at the very instant but when large distance away could this then be a faster than light communication ?
    If it would happen randomly I guess no, but in this case they both have agreed on a code and the code is not in terms of particle spin as you can't predetermine it but rather the code is about particle order say which one comes first which second.?
    Just like typing on a keyboard but under every key there is a entangled electron with it's given number of order, at the other end there is a display which lights up certain letters that correspond to certain keys which were pushed and under them were certain electrons.
    I hope I explained the scenario as good as I could.
    Last edited: Mar 1, 2013
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  3. Mar 1, 2013 #2
    I have wondered about this experiment and I come to the following problem. How can bob tell the difference between his electrons before and after Alice measures one of her set? I wonder if a Elitzur–Vaidman bomb tester could be set up to serve such a purpose? I read a paper where they showed it was possible to achieve arbitrarily high efficiency for sensing the "bomb" with arbitrarily low probability of an interaction happening.

    Could a well designed bomb tester be able to distinguish a mixed state from an eigenstate?

    For example, an electron or beam of electrons prepared in a maximally mixed state of spin up and spin down should have the same cross section of absorption for right and left hand circularly polarized light, but if Alice's measurement causes Bob's electron to assume the state of spin up or spin down one, then there will be a difference in the cross section of absorption of the two polarizations.

    A modified version of Kwiat's apparatus could apply non-interactive circularly polarized light to Bob's electrons. Would it be able to tell the difference between the electron in a mixed state and an eigenstate? or would it somehow cause the decoherence of Alice and Bob's samples?

    If it did would the info get there at the speed of light or some other speed?
  4. Mar 1, 2013 #3


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    Enntanglement doesn't work that way. When Alice measures her particle and gets spin up, all she knows it that if and when Bob measures his, he'll get spin down. She has no way of knowing whether he's already made a measurement, will make a measurement in the future, or has dropped dead and never will make a measurement.

    Thus, Bob's measurement doesn't reveal ANYTHING to Alice.
  5. Mar 1, 2013 #4


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    No, not for an individual particle.

    For a large number of identically prepared particles, as with an idealized beam of particles, yes. But in this case Alice's and Bob's measurements would just tell them something about the source of the beam, how the particles were prepared. And there's no FTL there; they had to wait until the beam, traveling at less than c, arrives before they even have something to measure.
  6. Mar 1, 2013 #5
    @Nugatory well I was thinking something more like a flash light signal , when you turn the light on and off to another friend at a distance you too send information only one way you don't know whether he is there and sees your flashing light or maybe his having a good time drinking or whatnot.
    Same here Alice wants to say "THANKS" to Bob she now measures the electrons in such a way that they correspond to the letters to make the word "thanks" in the other side , I guess the problem then arises how does Bob"s apparatus tell when to turn on the letters that correspond to the measured electrons in Alices side , the apparatus should detect a sudden change in spin or something as Alice would reveal or lets just say measure her electrons in the right order to make the word appear from the electrons in Bob's side , Is there any such thing that could let's say flash on a LED when it detects that an electron suddenly changed it's spin,

    Even though I think I have an answer myself as if the other side would have an apparatus that detects or tries to detect a change in electron's spin it would cause decoherence of the quantum entangled state so even if those electrons were entangled the bob's measuring device would destroy the possibility to see something when Alice does the measurement on her side?
  7. Mar 1, 2013 #6


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    Keep in mind that all Alice and Bob ever see is a random string of Heads (+) or Tails (-). Hard to make out any kind of message from that.
  8. Mar 1, 2013 #7


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    No - there is no sudden change in anything that is detectable as the result of making a measurement on one side or the other. That's what I mean when I say "entanglement doesn't work that way".

    You can't destroy something that never existed, and that possibility never existed.

    Using some interpretations, you may speak in terms of "Bob's particle" "becoming entangled with" "Bob's measuring apparatus" (three different sets of quotes to identify three different dubious and ill-defined concepts); but that's just a way of thinking about it, not a definitive answer to anything. For example, you might amuse yourself wondering why we don't end up with Bob's apparatus, alice's apparatus, and two particles all tangled up together.
  9. Mar 1, 2013 #8
    those are not quotes I made them up at the moment when i wrote to try to make my concept more clear.I had a strong feeling that it would not work that way.

    My basic underlying thing was that you have three electron pairs let's say and their entangled a large distance apart , so the question was about if Bob measured the electrons in some order would the other side see that order.I guess they won't because for them to tell which electron is at what position they have to see the first one from that pair.
    Well I guess that pretty much answers the question that in order to tell which ones of those second electrons is corresponding to the first ones you have to have them both pairs at sight.
    I hope I'm right about this?

    @Nugatory well how do you mean not existed , quantum entanglement exists , so when you make a measurement of one of the particle positions the other ones position is revealed at the same instant , so who would be the first to make the measurement , he also would be the one who decides what position the other electron will have this is what I meant.
    Also by destroying i was thinking of the chance of one of the entangled particles having a sudden interaction with the surrounding medium as to destroy the entangled state.
  10. Mar 1, 2013 #9


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    Correct me if I am wrong, but I was under the impression that you don't decide what position anything has, you only choose what to measure. What state that particle is in when you measure it is purely random. Is that incorrect?
  11. Mar 1, 2013 #10


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    Nothing is "revealed" on Alice's side when Bob makes a measurement. Alice doesn't learn anything until she makes a measurement herself, and when she does neither she nor Bob know which one made the first measurement on the pair, they each just know what they measured at their particle.
  12. Mar 1, 2013 #11
    @Drakkith well that is just semantics , ofcourse we don't decide :D let's consider that a typo. :D

    @Nugatory , well I was thinking more like that when you have two entangled particles when you make a measurement on one you can assume that the other ones position will be the opposite of the one you just measured. Is that correct , I think so, otherwise the whole quantum computer idea is "bollocks"

    Well revealed was a poor choice in words.Ok i see the contradiction here , If Bob makes the measurement on the particle Alice can't know that and if Alice makes the measurement first then again Bob is left wondering but since those entangled particles are a far distance away from each other like in my scenario then they both would have to contact each other to know the result of the measurement.So were left with light speed conversation.
  13. Mar 1, 2013 #12


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  14. Mar 1, 2013 #13
    Another point that is interesting is that barring loopholes (unlikely), if the non-local effects observed in Bell-type experiments propagate at any finite speed, then non-locality could be exploited for superluminal communication (e.g. 'can't stay hidden'):
    Looking Beyond Space and Time to Cope With Quantum Theory

    Quantum non-locality based on finite-speed causal influences leads to superluminal signalling

    Full article posted in arxiv:

    One of the authors (Gisin) discusses other ramifications in a follow-up piece just posted in arxiv:
    Quantum correlations in Newtonian space and time: arbitrarily fast communication or nonlocality

    Lecture from lead author from Perimeter Institute:


    The just of the article (as I understand it) is that the hidden/private quantum signals that exist between entangled particles/systems cannot remain hidden if the speed of these "private lines" is anything less than infinite velocity/instantaneous. Since one prefers that signals remain hidden/private (so there's no violation of relativity), the more likely choice is that velocity is infinite but private (can't be used for signalling). This is possible if there's a preferred reference frame. What is still strange is the instantaneous (non-local) connection. Assuming a realist interpretation, some like Gisin, have argued that such non-local effects appear to be coming "outside" space-time. Of course, others argue that the "realist" assumption plays no role.
    Last edited by a moderator: May 6, 2017
  15. Mar 2, 2013 #14
    Well from what I have read and my own personal intuition I tend to think that the world is non local in quantum terms.

    But still it is interesting well the speed limit for the em field is c.But maybe that's because it is mediated by photons and they have to travel to bring the field and information that it may contain with it.
    Well the entanglement as I understand doesn't have a field or any tie between them.Well that's what they are trying to find out @Bohm2 how I understand it from your last post.
    Well basically if they don't find any tie between the entangled particles then that is not much of an answer , the only answer would be to find something or it stays the "spooky action at a distance"
    But if they find a link between those entangled particles than that link would be somehow similar to what the em wave is to ordinary matter.But that would bring some bizarre consequences.i think that would violate relativity.
    Because as much as I feel and understand if something far far away knows how to move according to the "other side" there has to be some sort of a link between them.Humans use phones radios and other stuff mediated by the em wave under that all to know that but at the particle scale the particles themselves if they are separate entities they have to have some kind of a link.
    An invisible string between them wouldn't violate relativity I guess but that's just a guess, i think we have no real proof for that.
    Last edited: Mar 2, 2013
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