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Entanglement Swapping and FTL Communication

  1. Jan 2, 2014 #1
    Recent experiments realized the thought experiment of Asher Peres on entanglement swapping. Here's the abstract.

    Couldn't Alice and Bob in the experiment receive information faster than light from Victor? Here's the set up:

    http://www.livescience.com/19975-spooky-quantum-entanglement.html

    Victor could use this scheme to send information to Alice and Bob. Bob and Alice would be in the same place and Victor could be a mile or 5 light years away.

    101100 = dog
    100110 = cat

    If Victor has 6 particle pairs with him, he can then send Bob and Alice the word dog or cat. Let's say he wants to send dog.

    Victor would do this. He would entangle the first pair, he would choose not to entangle the second pair, he would then entangle the next two pairs and he would choose not to entangle the last two pairs.

    When Bob and Alice check their particles pairs, they would see:

    quantum correlations, uncorrelated, quantum correlations, quantum correlations, uncorrelated, uncorrelated or 101100 which = dog.

    The beauty of this is causality will be preserved because nothing is actually moving through space-time faster than light.

    This is an experiment that could be carried out today with random number generators and atomic clocks. Why wouldn't this be FTL communications?
     
  2. jcsd
  3. Jan 2, 2014 #2
    Well...

    Exactly how would they "check" their particle pairs without using a classical information channel? (non-FTL)
     
  4. Jan 2, 2014 #3
    The problem is with assigning 0's and 1's to whether the pair Alice and Bob hold is entangled or not.

    How would know that Alice and Bob's photons are entangled? They might exhibit correlations that may make it look like they're entangled, when in fact they are not. I can't go into the technical details right now, but may be able to answer more fully tomorrow.
     
  5. Jan 2, 2014 #4
    To DennisN:

    This wouldn't be the case because you can still show the information was transferred faster than light with atomic clocks and random number generators. The Author of the paper even talked about communication through computers and quantum computers.

    To StevieTNZ:

    Of course Alice and Bob would know their photons exhibit quantum correlations. The whole idea behind the experiment hinges on Alice and Bob knowing if their particles showed quantum correlation or classical correlations. They can even see the quantum correlations before Victor even makes us choice to entangle his particle pair.
     
  6. Jan 2, 2014 #5
    Not quite. I would encourage you to read the Nature article itself. Especially when Alice and Bob measure in the H/V and 45/135 bases, and no bell-state measurement occurs on Victor's side. You'll find Alice and Bob's photons sometimes exhibit quantum correlations despite not being entangled. Remember, you're limiting yourself to 6 pairs of photons.
     
  7. Jan 2, 2014 #6
    To StevieTNZ:

    That's not the case. When Victor chooses to entangle his particle pair, the quantum correlation between Alice and Bob's particle pair is clearly identifiable or you couldn't even carry out the test. How would you know when Victor's choice cause quantum correlations between Bob and Alice's particle pair? Here's how the ended the article:

    On page 6 of the study this is clearly spelled out.

    One more thing.

    This pretty much sums it up.
     
  8. Jan 2, 2014 #7
    Okay:

    So we have a two pairs of entangled photons, as such that each is created in the state |H>|V> - |V>|H>.

    We send one photon from each pair to Victor, who either performs a separable state, or a bell-state measurement.

    Alice and Bob measure in the H/V basis. So does Victor.

    If Alice and Bob's photons are measured and both are found in |H>|H>, we know either that Victor performed a separable state measurement and got |V>|V> for his two photons; or that Victor performed a bell-state measurement and Alice and Bob's photons are now entangled in the bell-state |H>|H> - |V>|V> or |H>|H> + |V>|V>.

    How can we differentiate between the two without knowing what kind of measurement Victor made (either a separable or bell-state)? That requires a classical communication channel, and thus renders your FTL communication scheme invalid.
     
  9. Jan 2, 2014 #8
    What kind of information? Do you have a scientific source to support the words I put in bold? (a peer-reviewed paper?)

    Furthermore, you did not answer my question in post #2. Care to consider it? Exactly how would they "check" their particle pairs without using a classical information channel? (non-FTL). Determining correlations means comparing the results of two distant measurements. How do you compare the results without any classical communication?

    Not any faster-than-light communication. I can't find it neither in the article nor paper.

    Here's the paper:
    Experimental delayed-choice entanglement swapping
    http://arxiv.org/abs/1203.4834

    Download it and search for e.g. the words "faster" and "communication".
     
    Last edited: Jan 2, 2014
  10. Jan 2, 2014 #9
    Both of you guys seem to be making the same point which doesn't apply. You say they would have to use a classical information channel but that doesn't mean we can't determined if Bob and Alice received the message from Victor faster than light by using atomic clocks.

    It will not take Bob and Alice 5 light years to check for quantum correlations so the point is mute. Entanglement has already been measured using atomic clocks. We can know if Bob and Alice are receiving the information faster than light.
     
  11. Jan 2, 2014 #10
    Well, we can get into the measurement problem which has bearing on your scheme.
     
  12. Jan 2, 2014 #11
    Alice and Bob, if they're people, won't know if they received a message FTL due to the reason I stated in my earlier post.
     
  13. Jan 2, 2014 #12

    bhobba

    Staff: Mentor

    Its impossible.

    To send information the person sending would need to know the outcome of his/her measurement so the person receiving it knows what was sent. Since QM says you cant do that its not possible to send information this way.

    Its utterly trivial once you understand that you can't predict the spin that will be observed.

    Thanks
    Bill
     
  14. Jan 2, 2014 #13
    At the end of the day, this could easily be tested using random number generators and atomic clocks today.

    You can set this up where Victor, Bob and Alice are 1 mile apart. Scientist have already clocked entanglement using atomic clocks. You determine the speed that light will carry a message from Victor to Alice and Bob. You then set up three words that could be sent.

    101100 = dog
    100110 = cat
    110001 = rat

    You then have a random number generator determine which word will be sent. You also have atomic clocks set up with Alice and Bob and one with Victor.

    To determine which word is being sent, you just need to check for quantum correlation which = 1 and when there's no quantum correlation it = 0.

    Quantum correlation occurs when Victor entangles a particle pair that's already entangled with the particle pairs Alice and Bob have. The thing that makes this possible is Alice and Bob's particle pair doesn't exhibit quantum correlation until Victor chooses to entangle his particle pair.

    So it's simple. When Victor wants to send a 1, he entangles his particle pair and then the particle pair of Alice and Bob will show quantum correlation. If Victor wants to send an 0, he doesn't entangle his particle pair and Alice and Bob will not find quantum correlation.

    I think people can't think past Relativity when it comes to FTL communication. This isn't violating anything because there's no information passing through space-time between Victor, Bob and Alice.
     
  15. Jan 2, 2014 #14
    So we have a two pairs of entangled photons, as such that each is created in the state |H>|V> - |V>|H>.

    We send one photon from each pair to Victor, who either performs a separable state, or a bell-state measurement.

    Alice and Bob measure in the H/V basis.

    If Alice and Bob's photons are measured and both are found in |H>|H> (a quantum AND classical correlation), we know either that Victor performed a separable state measurement and got |V>|V> for his two photons; or that Victor performed a bell-state measurement and Alice and Bob's photons are now entangled in the bell-state |H>|H> - |V>|V> or |H>|H> + |V>|V>.

    How can we differentiate between the two without knowing what kind of measurement Victor made (either a separable or bell-state)? That requires a classical communication channel, and thus renders your FTL communication scheme invalid.
     
  16. Jan 2, 2014 #15
    bhobba,

    You don't have to predict anything. You're just seeing if Victor made the choice to entangle or not to entangle. You're not trying to predict which spin will be observed.

    I direct you to the experiment posted in 2012:

    http://arxiv.org/abs/1203.4834

     
  17. Jan 2, 2014 #16
    Therefore we cannot attribute to any one measurement a 0 or 1 until we find out what measurement Victor performed.
     
  18. Jan 2, 2014 #17

    bhobba

    Staff: Mentor

    At the end of the day, it doesn't matter how you twist, turn, try this that, or whatever, since there is no way for the sender to determine what they want to send, there is no way to send information. All you have are correlations - but that is not enough.

    Some bright spark came up with a sneaky way of doing it if you could clone states. But guess what - they discovered a theorem that proves you cant clone states. Nature has conspired to respect relativity.

    Sorry but you do.

    Imagine you are the person receiving information. You get spin up or down. How does this relate to what the person sending it wanted to do - in other words if you got spin up exactly what did the person sending it do to ensure he/she got spin up?

    No referring to a paper. This is utterly basic - you must be able to answer it if you want to send information.

    Thanks
    Bill
     
    Last edited: Jan 3, 2014
  19. Jan 2, 2014 #18
    StevieTNZ:

    Asked and answered.

    Of course Alice and Bob can differentiate between the two even before Victor makes a choice. This is the delayed choice portion of the experiment. This is from the article:

    When Victor does perform a Bell-state measurement there's significant correlations between photons 1 and 4 in all three bases.

    In short, Alice and Bob can detect when Victor makes a separable state measurement or when he makes a Bell state measurement based on the strong correlations between photons 1 and 4 on all three bases or the absence of strong correlations in the +/- bases or R/L bases.

    Again, FTL communication can be achieved.
     
  20. Jan 2, 2014 #19

    bhobba

    Staff: Mentor

    But to determine a correlation you need to know what's sent.

    This is utterly trivial and obvious.

    If you cant see such can't be used to send information - shrug - there are some things people just don't get.

    Thanks
    Bill
     
  21. Jan 2, 2014 #20
    They only know that there are significant correlations in all three bases, when they know whether Victor performed a bell-state measurement or not.

    Don't believe everything you read. I disagree with
    and that is evident by my example I provided earlier, which equally applies to 45/135 and R/L bases.
     
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