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Entangled photons in bell experiment: transfer phase or angular momentum?

  1. Nov 7, 2012 #1
    Regarding the polarization correlation studies generated using parametric down conversion. All the studies appear to be done correlating the polarization of linearly polarized photons.

    Has any experiment been done showing the same effect with circularly polarized light?

    1) If this experiment has been done with circular polarized light, then that would mean that quanta of angular momentum are being transferred faster light.

    2) But if this experiment can only be done using linear polarized light I wouldn't find it as exciting. Because linear states are a super position of circularly polarized light. The orientation (s or p) depends on the phase between the different helicities. In which case, it's the phase that is propagating faster than light. Super luminal phase velocities aren't anything new or exciting to us.
     
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  3. Nov 7, 2012 #2

    DrChinese

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    I can change linear polarization to circular and they will still be entangled. I can do entanglement experiments with things other than photons.

    What effect do you think is happening faster than light? And why do you think that has anything at all to do with superluminal phase velocities? We are talking about a measurement on one photon which is outside the light cone of the other. There is nothing about phase velocities that has anything to do with the measurements.
     
  4. Nov 7, 2012 #3
    As far as I know, no one has experimental demonstrated non-local phenomena arising from entanglement on anything other than linearly polarized photons. Can you refer me to the peer reviewed articles reporting otherwise? Otherwise I'm gonna have to spend weeks sloging through the literature :P (see links below for where I'm starting)

    http://iopscience.iop.org/0034-4885/41/12/002
    http://prl.aps.org/abstract/PRL/v49/i2/p91_1 [Broken]
    http://rmp.aps.org/abstract/RMP/v74/i1/p145_1

    With regards to your questions: I would answer them, but I'm under the impression that they are completely rhetoric.

    From your questions I'm assuming you don't realize that circularly polarized light is an eigenstate of angular momentum and that linearly polarized light is a superposition.

    Furthermore it appears you don't realize that the phase of the coefficients in this superposition are what describe whether the polarization is linear vertical or horizontal.
     
    Last edited by a moderator: May 6, 2017
  5. Nov 8, 2012 #4

    DrChinese

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    I asked the questions to get better insight into what it is you are really asking. When you say "non-local phenomena", you could mean "violations of Bell inequalities". Or you could mean "violations of Bell inequalities under strict Einsteinian locality conditions". If you are saying the latter, then that has been done ONLY for photons. The issue of linear or circular is completely irrelevant to that experiment. I cannot assist you with that beyond what I say in the last paragraph.

    http://arxiv.org/abs/quant-ph/9810080

    Violation of Bell's inequality under strict Einstein locality conditions
    Gregor Weihs, Thomas Jennewein, Christoph Simon, Harald Weinfurter, Anton Zeilinger (University of Innsbruck, Austria)
    (Submitted on 26 Oct 1998)

    "We observe strong violation of Bell's inequality in an Einstein, Podolsky and Rosen type experiment with independent observers. Our experiment definitely implements the ideas behind the well known work by Aspect et al. We for the first time fully enforce the condition of locality, a central assumption in the derivation of Bell's theorem. The necessary space-like separation of the observations is achieved by sufficient physical distance between the measurement stations, by ultra-fast and random setting of the analyzers, and by completely independent data registration."

    If the former, this has been done for many quantum objects. Most scientists accept entanglement as such without having to demonstrate the higher standards of closing the locality loophole each and every time.

    To demonstrate entanglement on circular polarized photons, you would need to derive a Bell Inequality for those. I don't know that is possible, not really sure, but I assume it would require there to be different flavors of circular polarization other than left or right. I don't believe such exist.
     
  6. Nov 8, 2012 #5
    I'm feeling a little exasperated here for a number of reasons

    1) I don't think you're making an effort to understand my question.

    2) "Most scientists accept entanglement ......" I think you're missing the point of experimental validation.

    3) It's bad practice to cite from ArXiv when a well known article is published in a reputable journal
    http://prl.aps.org/abstract/PRL/v81/i23/p5039_1

    The only reason I'm addressing your concerns is so that some one who knows a 3+ year grad student in quantum optics that happens to be reading this article doesn't think my question is sophomoric.

    As implied in the sentence above, I believe a 3+ year grad student in quantum optics would have the answer to this question at his finger tips.
     
  7. Nov 9, 2012 #6

    DrChinese

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    1. You have yet to make your question clear.

    There are many examples of entanglement, however not everyone accepts those as proof of non-locality. Rather, it is considered proof of "quantum non-locality". Are you questioning that?

    As I have said, I don't believe there are any experiments with circular polarized light violating a Bell Inequality, which was you original specific question. There are experiments involving circular polarized light and other quantum phenomena (GHZ comes to mind) but I don't think that really is the direction you are going. Again, I am trying to ferret out your question so it can be answered.


    2. I keep references handy to many experimental results. I pull those out as appropriate when there is some particular point that needs to be clarified. Your original citations were so old that it was hard to determine what level of discussion you want to start with.


    3. Around here, we usually cite the Arxiv version of articles as they are free to all readers. Your reference requires a subscription.

    If you don't want my assistance, perhaps someone else can understand what you are seeking. If you want to continue, then that is fine too. Meanwhile...

    As many times as I have read your OP, I keep returning to my basic point: linear vs circular in Bell tests does not make sense on any level I am aware of. Phase velocity is not relevant to discussions of quantum non-locality. And superluminal effects are not the overriding conclusion from Bell tests. Bell tests demonstrate that local realistic theories are not tenable. If you already reject local realistic theories because you believe QM is complete (answering the EPR question in the affirmative), then entanglement experiments will probably not be as exciting for you.
     
    Last edited: Nov 9, 2012
  8. Nov 9, 2012 #7
    If you don't have access to physics journals, I don't see how you expect to be able to contribute to this thread.

    Granted I may have a hard time making myself clear, but seeing as you haven't made any non-rhetorical or genuine requests to further explain my point, I see no point in continuing this discussion with you.
     
  9. Nov 9, 2012 #8

    Cthugha

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    Three points:

    Direct generation of photons in an entangled basis of left and right circularly polarized photons has been demonstrated for example in Nature 465, 594–597 (2010) by Salter et al. in terms of a cascaded decay. If I remember correctly the references inside also give hints at how to perform Bell measurements on such states.

    Annoying the science advisor who probably has the largest number of posts on entanglement in these forums is something only few people have achieved. Congratulations.

    The reason why you fail to make yourself clear is most likely that your claims 1) and 2) in your first post are simply and completely wrong (or if read in a benevolent way formulated in a very sloppy manner). Maybe it helps to read up on the meaning of linear polarization for single photons?
     
    Last edited: Nov 9, 2012
  10. Nov 9, 2012 #9

    Drakkith

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    This is an online forum. Requiring posters to have access to pay-for journals will substantially reduce the chances of actually having anyone answer your questions.
     
  11. Nov 9, 2012 #10

    Vanadium 50

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    Furthermore, some of us do not always have access where we are posting. I usually post from home, for example, but my journal access is through work.
     
  12. Nov 9, 2012 #11
    is there any energy loss (or energy required) to change the polarization?
    (say from linear to circular)
     
  13. Nov 9, 2012 #12

    DrChinese

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    The abstract from that reference:

    "An optical quantum computer, powerful enough to solve problems so far intractable using conventional digital logic, requires a large number of entangled photons1, 2. At present, entangled-light sources are optically driven with lasers3, 4, 5, 6, 7, which are impractical for quantum computing owing to the bulk and complexity of the optics required for large-scale applications. Parametric down-conversion is the most widely used source of entangled light, and has been used to implement non-destructive quantum logic gates8, 9. However, these sources are Poissonian4, 5 and probabilistically emit zero or multiple entangled photon pairs in most cycles, fundamentally limiting the success probability of quantum computational operations. These complications can be overcome by using an electrically driven on-demand source of entangled photon pairs10, but so far such a source has not been produced. Here we report the realization of an electrically driven source of entangled photon pairs, consisting of a quantum dot embedded in a semiconductor light-emitting diode (LED) structure. We show that the device emits entangled photon pairs under d.c. and a.c. injection, the latter achieving an entanglement fidelity of up to 0.82. Entangled light with such high fidelity is sufficient for application in quantum relays11, in core components of quantum computing such as teleportation12, 13, 14, and in entanglement swapping15, 16. The a.c. operation of the entangled-light-emitting diode (ELED) indicates its potential function as an on-demand source without the need for a complicated laser driving system; consequently, the ELED is at present the best source on which to base future scalable quantum information applications"

    And a related article:

    http://arxiv.org/abs/1103.2969

    "A practical source of high fidelity entangled photons is desirable for quantum information applications and exploring quantum physics. Semiconductor quantum dots have recently been shown to conveniently emit entangled light when driven electrically, however the fidelity was not optimal. Here we show that the fidelity is not limited by decoherence, but by coherent interaction with nuclei. Furthermore we predict that on 100\mu s timescales, strongly enhanced fidelities could be achieved. This insight could allow tailoring of quantum logic to operate using quantum dots in the fault tolerant regime."

    Of course, even with these articles it appears that the circular polarization vs linear is not really an important distinction (it's not mentioned). From the editor's summary of the 2010 article:

    "For optical quantum computation and related information technologies to fulfil their promise, they will require a source of entangled photons that can be delivered efficiently on demand. Existing entangled-light sources are laser driven, and involve bulky and complicated optics. Salter et al. have now developed a compact light-emitting diode with an embedded quantum dot that can be driven electrically to generate entangled photon pairs. Much simpler than its laser-driven counterparts, this ELED (entangled-light-emitting diode) device, based on conventional semiconductor materials, is a promising start point for the development of an entangled light source for quantum information applications."

    On demand entanglement! Cool. I presume that one would simply use a wave plate or similar if you specifically needed to have linear polarization for an application.
     
  14. Nov 9, 2012 #13

    DrChinese

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    Nope.
     
  15. Nov 9, 2012 #14

    Cthugha

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    You are of course right. It is absolutely not an important distinction. I mean: You could simply take light entangled in a linear basis and pass both beams through a quarter waveplate to get to a circular basis as well.
     
  16. Nov 9, 2012 #15
    I read somewhere that solar sails use the energy of the photons hitting them.

    here the photon is passing through (sort of striking) a quarter-wave-plate.

    must it not lose some, however infinitesimally small, amount of energy?

    ....maybe a very small fraction of a quanta?
     
  17. Nov 9, 2012 #16

    Drakkith

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    Solar sails work because upon absorption and reflection the photons impart momentum into the sail. The reflected photons lose this momentum and are redshifted slightly. (That's what I've been told here on PF at least)
     
  18. Nov 9, 2012 #17

    Cthugha

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    Why should it lose energy? It is not absorbed, not even scattered. It just travels through a birefringent medium. Photons can lose tiny amounts of energy in inelastic scattering processes such as Raman scattering, but these typically require absorption and reemission of that photon via a real or virtual intermediate state.
     
  19. Nov 9, 2012 #18
    what you have said above is correct, i think.

    however it's, perhaphs, interesting that energy can convert into a red-shift (phase change?) in time-space
     
  20. Nov 9, 2012 #19
    ok, thanks Cthugha

    can these tiny amounts of energy be less than a quantum?........even if we cannot directly measure that

    pardon my limited understanding of "the quantum"
     
    Last edited: Nov 9, 2012
  21. Nov 9, 2012 #20

    Cthugha

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    These are different photons, so speaking of "a quantum" is a bit pointless. Each emission and absorption process still results in emission or absorption of a single quantum of light, but absorption and emission happen in different modes of the electromagnetic field, so one sees a difference in energy. The difference in energy then corresponds to the difference of two energy levels present in the scatterer.
     
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