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Quantum eraser and super luminous communication

  1. Aug 26, 2012 #1
    Let say we take each of the entangled photons and send one of them to receiver and one to transmitter located light years away. On receiver side photon is passing through double slit and hitting the screen. On transmitter side there is movable light stop placed at the same optical distance from the source as receiver double slit. Without the transmitter light stop, receiver sees interference pattern on the screen, since each photon is free to choose which slit to go through. With light stop the transmitter photons are localized and measured in the plane that corresponds to the receiver slit and therefore localize their entangled partner position determining which slit receiver photon is going through. If my understanding holds this means that there is no interference at the receiver and effect takes place instantaneously. Right ?
     
  2. jcsd
  3. Aug 27, 2012 #2
    Don't fully understand this yet. However I am hoping to hear something interesting.

    Perhaps the answer is:

    there is never an interference pattern, even if no noise, in [edit] entangled photons....
     
    Last edited: Aug 27, 2012
  4. Aug 27, 2012 #3
    assembly i describes has double slit (not present in original quantum eraser) so there is no reason to expect lack of interference if photons or their entanglement partners wave function have not collapsed yet. But if one side has been measured then interference seems to disappear ....
     
  5. Aug 27, 2012 #4

    DrChinese

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    As it happens, entangled photons do not produce interference patterns. The usually explanation is that the source is not coherent (because it does not have a point source).
     
  6. Aug 27, 2012 #5

    Cthugha

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    Having a double slit is not sufficient to see an interference pattern. You need a double slit and a light field where the coherence length is longer than the distance between the two slits. Entangled light is necessarily very incoherent, so you do not get an interference pattern of noticeable visibility under conditions that allow to preserve entanglement.

    edit: DrChinese beat me to it. I should really type faster and stop making breaks while typing.
     
  7. Aug 27, 2012 #6
    DrChinese and Cthugha is lack of interference of entangled photons inherent phenomenon or it results from technological limitations of generating apparatus ?

    After all, you could see interference fringes from a candle if slits are far enough from source and one use sensitive detector. Similarly, one could restrict photon generation area to the size that will produce detectable fringes (at the cost of intensity, perhaps).

    On the other hand if entangled photons doesn't interfere because that is their nature, then inserting light stop on the transmitter path somehow ahead of receiver slits will destroy entanglement and produce "regular" photons which should restores fringes. This again could be exploit for on/off communication teletype. Or maybe not, tell me why.
     
  8. Aug 27, 2012 #7

    Cthugha

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    Thisi is inherent. One can also show that single-photon interference and two-photon interference (as seen in experiments on entanglement) are complementary (Abouraddy et al., Phys. Rev. A 63, 063803 (2001)).

    Yes you can increase spatial coherence by increasing the distance between light source and slits. This even works for light from a PDC crystal. However, by doing so you destroy entanglement.The reason is simple. Momentum entanglement means roughly speaking that the emission angles are correlated. Therefore you need some minimal spread in possible emission angles. If you filter out just one of them for example, the light will not violate Bell's inequality anymore because you will only detect the same set of angles in every run. If you reduce the spread of angles, light becomes more coherent, but you destroy entanglement. If you increase the spread you get entanglement, but lose coherence. There is a small intermediate regime where you can get both, but only with heavily reduced visibility.
     
  9. Aug 27, 2012 #8

    DrChinese

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    I drink a lot of caffeine in the morning. :biggrin:

    (Plus I knew what you would answer anyway...)
     
  10. Aug 27, 2012 #9

    Cthugha

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    It is evening here, so no caffeine for me. That must be the bad excuse, ehem reason. :wink:

    Help, I am predictable. But that question indeed comes up so often that it might be sensible to write a FAQ entry. On the other hand I am not sure whether it is that easy to formulate it in a "waterproof" manner. At least I suppose FAQ entries should be of higher quality than the average post.
     
  11. Aug 27, 2012 #10
    good posts DrChinese, Cthugha.

    just to clarify, is the below correct?

    entangled photons don't produce single-particle self interference

    however they do interfere with each other.....this is known as two-photon interference

    and this interference can happen even without the (entangled) photons having to meet at a same point in time-space

    in other words: two-photon interference can happen non-locally
     
  12. Aug 28, 2012 #11
    another clarification:

    Is this behavior affected by what is happening to the entangled partner ? I mean, if entanglement is destroyed at some point in time is photon regaining its single particle interference skills ?
     
  13. Aug 28, 2012 #12
    yes surely........ the photon regains it's single particle interference properties.....after "dis-entanglement"
     
  14. Aug 28, 2012 #13

    DrChinese

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    No it does not - for its partner anyway. An action at Alice does not change what Bill sees in any way Bill can detect himself (i.e. without more info from Alice).
     
  15. Aug 28, 2012 #14
    Thats about summarizes all the attempts to circumvent speed of light while building communication device. Glad to learn that, since I start to be afraid of retrocausality.
     
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