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Photon anti-bunching as necessary for BS entanglement

  1. Feb 26, 2016 #1

    The green dots represent photons in an anti-bunched state, squeezed light.

    The red dots represent photons in a semi-bunched state, laser light
    The blue dots represent photons in a bunched state, thermal light

    The complex setups used in entanglement experiments only squeeze light in amplitude, phase, photon number and filter out squeezed light from classical light before the beam splitter can perform entanglement.

    But if you are lucky, you just need two sources of squeezed light, which are usually non-linear crystals but single-photon sources will do too. Maybe in the future they will discover more ways to produce anti-bunched photons.

    You also need the photons from the two input beams to be indistinguishable in polarization, frequency, arrival time. Unless you want to use an artificial device that removes these properties composed of two special detectors (not any detector!) that can erase which-path information and a phase shifter.

    Do not try to contradict me this time because I have taken nothing out of context, I just found a published thesis which says the conditions for the input light to be entangled by a beam splitter

    Given that laser light is a mixture of classical light and squeezed light, a beam splitter could create entanglement with a very small probability, but to increase the probability of entanglement by a large factor, you have to use purely squeezed light
  2. jcsd
  3. Feb 26, 2016 #2
    The use of two single identical atoms and a beam splitter is enough to produce entangled pairs because atoms produce single-photon states, which are non-classical light squeezed in photon number.

    However, if you use many photons at the same time, you must filter out the bunched photons, which is done by a non-linear crystal.

    In practice, to create single photons that are indistinguishable from a laser source you need a setup with many elements to squeeze that laser light and filter out a single polarization, frequency. That is the role that the additional elements play.

    In theory or fantasy, you can imagine two ideal light sources which emit identical photons, one at a time, but this kind of source DOES NOT EXIST
  4. Feb 26, 2016 #3


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    Perhaps you have a reference so we could see that context. And perhaps... you have a question about it?
  5. Feb 26, 2016 #4


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    Also, I'd like a reference for this bit, as that contradicts what I always thought.
  6. Feb 26, 2016 #5


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    Really? I don't recall seeing a setup like that previously. I would expect a lot more to get a source of entangled pairs. Can you provide a reference on that too?
  7. Feb 26, 2016 #6
  8. Feb 26, 2016 #7

    Photons are more equally spaced (more delayed in their arrival time) in squeezed light than in a laser beam, and more equally spaced in laser beam than in thermal radiation

    It's about the Bose-Einstein statistics, which makes photons bunch together in pairs. These bunched pairs are filtered by nonlinear crystals
    Last edited: Feb 26, 2016
  9. Feb 26, 2016 #8
    Do I have to repeat that the additional elements just squeeze light and filter it from classical light? Photons cannot be entangled unless the beams that contain them are made indistinguishable, and that is the reason filters and polarizers are used.

    Single photon states are Fock states

    Fock states are non-classical, therefore can be used as input to beam splitter

    PDF version http://iontrap.umd.edu/wp-content/u...ton-pairs-from-2-remote-trapped-ions-2007.pdf
    Last edited: Feb 26, 2016
  10. Feb 26, 2016 #9


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    Be very, very careful when creating mental pictures like that. You can't strictly speaking talk about individual photons unless you have a Fock state. There is nothing preventing you from talking about average number of photons, but you can't really talk about "spacing" between photons in this case.
  11. Feb 26, 2016 #10
    It's not me but Wikipedia. Plus photon spacing means delay between their arrival times, not distance measured in meters
  12. Feb 26, 2016 #11


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    Wikipedia is not always correct.

    Its pretty obvious, like the process of creating polarised light in a polariser, the detail of what going on in creating entangled photons, requires some rather complex solid state physics. You will not do it via some simple mental picture.

    As I said, some rather complex physics.

  13. Feb 28, 2016 #12


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    As I have said repeatedly, you need a lot more to get entanglement. Your referenced experiment is no different. To select the entangled pairs from the entire universe, you need to have a classical signal go from one to the other. That is common for set ups such as this. It is only the pairs with very close arrival times are entangled. No way to know that until you compare information from both sides. I. E. post selection.
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