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Walborn and ’FTL’

  1. Aug 15, 2011 #1
    Walborn = ‘Double-slit quantum eraser’ with double slit and eventually quarter wave plates.
    http://grad.physics.sunysb.edu/~amarch/Walborn.pdf [Broken]


    p. 5 section V: ”Figure 2 shows the standard Young interference pattern obtained with the double slit placed in the path of photon s, without quarter-wave plates QWP1 and QWP2, and with POL1 absent from detector Dp .”

    So he just detects p and obtains anyway interfere with the 's'-beam.

    We should then just have a different type of measurement on p – which do not give s interference - and we have a 'communication system'.

    One possibility would be a PBS (0) - which forces p photons to choose between becoming horizontally or vertically polarized.

    p. 2 section I: “Because of their momentum, time, and polarization correlation properties, photon pairs generated by spontaneous parametric down-conversion play an important role in the experimental demonstrations of quantum erasure”
    “Because of their […] polarization correlation properties” the s-photons then should be vertically and horizontally polarized.

    The vertically p-photons has been reflected - which gives a half wave difference.
    and “Because of their […] time […] correlation properties” this gives the horizontally s-photons also a half wave difference .

    Is this the argument? - So we need only a half silvered mirror at p to ensure that s consists of two types with half-wave of difference?

    ‘FTL’ because of coincidence counter - but with a greater distance to double slit and ideal conditions it maybe could work without.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Aug 15, 2011 #2

    xts

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    In my view this experiment brings nothing new or important neither to physics nor to philosophy. It just may be decomposed to:
    1. "mystery of entanglement" - two photons are correlated. In this case we don't even have "Bell's mystery" - we may explain the results in terms of random series of H/V polarized pairs;
    2. "double slit mystery" - still mysterious, despite over 200 years since Young noticed it - photons coming through double slit form fringe pattern;
    3. "phase change" - not so mysterious - we may make an interferometer - as we change phase on one slit then fringe pattern shifts.

    There is no need for sound words like 'which path information', 'erasure', 'FTL', etc. We just made a filter able to filter out one fringe pattern from its shifted counterpart, which mixed together form a bulky spot. Pattern and anti-pattern are created by two sub-series of events, which differ by the phase shift between slits. Other arm (p) allows us to tell what is the polarisation (H or V) of incoming photon, thus to which of those sub-series particular event belongs.
     
    Last edited: Aug 15, 2011
  4. Aug 15, 2011 #3

    xts

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    To deprive the experiment from mystery, try something similar, but (almost) classical:
    Throw out the laser and BBO crystal, and put a light bulb in place of laser (very dim lightbulb, it should emit so little light that single photons should be distinguishable by our detectors) and Nicol's prism in place of crystal. Then made series of experiments: you turn prism to randomly chosen position of two such, that H polarised light goes to p and V to s or vice versa (don't note its position in the lab-book, that secret must be revealed by measurement in p!), then you turn on the lightbulb for a while (not too long, just to score a at least one click in both of p and s detectors, and note it as a coincidence event), then start again: choose randomly position...

    The outcome of the experiment will be exatly the same, as of Walborn's.

    Would you still say that 'measurement in arm p erases the which-path information in arm s' ?

    But it is too trivial, so it won't probably be awarded by publication in Phys.Rev. nor by grant for next similar experiment.
     
    Last edited: Aug 15, 2011
  5. Aug 15, 2011 #4

    DrChinese

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    No, it won't. You only get that result with entangled photons.
     
  6. Aug 15, 2011 #5

    DrChinese

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    You do NOT get a communication system from this setup. A reasonable person would assume that top scientists might have noticed such a property of the system and would already be collecting there prizes if there were such.

    But that doesn't happen. Instead, you can successfully transmit a bunch of random information faster than light. For whatever good that does anyone.

    Haven't we had this conversation previously? Seems so familiar. :smile:
     
    Last edited by a moderator: May 5, 2017
  7. Aug 15, 2011 #6

    xts

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    Would you accept a challenge, DrChinese?
    I bet a bottle of (it is A.Z.'s favourite, btw) Beerenauslese Rotgipfler 2005 from Herr Kamper's winery, versus bottle of young Californian Chardonnay ;)

    Challenge: Would you demonstrate calculations showing the results differ?

    The only they (Walborn et co.) used of entanglement is a perfect correlation in predefined, fixed base. So it doesn't differ from 'classical' correlation based on common history.
     
    Last edited: Aug 15, 2011
  8. Aug 15, 2011 #7

    Cthugha

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    Yes and no. Thermal light has the tendency to bunch which indeeds mimics some of the properties of entangled light and you can indeed get increased coincidence count rates in coincidence count experiments (which by the way even made it to PRL, see "Two-Photon Imaging with Thermal Light" by A. Valencia et al., PRL 94, 063601 (2005)). While being not exactly the same experiment, DCQE and thermal ghost imaging are very closely connected.
    Scarcelli and Shih did a myriad of experiments on that stuff.

    However, the difference is: You get similar results with a lightbulb, but the visibilty of the interference pattern is of course not as good as using entangled photons. It is not a surprise that it is reduced to the maximum amount allowed by the inequalities that distinguish classical and non-classical light so that everything stays classical from the DCQE point of view.
    From the imaging point of view it is the same. You get non-classical superresolution when using entangled photons and the commonly allowed resolution when using thermal light.
     
  9. Aug 15, 2011 #8

    xts

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    I believe it would be even better, as lots of experimental noise would be reduced.
    I may repeat the challenge thrown to Dr.C (I still have several bottles in my cellar) - show calculations!

    I am not speaking about any entanglement-like coincidences. Just, perfectly correlated in a fixed base (eigenstate) of H/V, stream of independent single photons. They produce perfectly matching pattern/anti-pattern.

    EDITed>>>
    I am not claiming that entanglement may be explained by 'common-history-correlations' or other hidden local variables. No. Just contrary.
    But this particular experiment does not violate Bell's inequality and is equally explainable in terms of common history.
    Walborn does not use entanglement in a "Bell-sophisticated" way - he uses it just to obtain one-to-one correlation between orthogonal polarisations in a fixed base. The same as a lightbulb and Nicol's prism...
     
    Last edited: Aug 15, 2011
  10. Aug 15, 2011 #9

    Cthugha

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    That depends on the circumstances. For the same mean count rate, quantum light performs better than thermal light. However, the mean intensity is of course higher for thermal light. However, you have a large noise background as thermal light is not perfectly correlated which provides an intrinsic background noise source even on the low-light level limit.

    You can find a whole page of boring calculations in the appendix of O’Sullivan et al, "Comparison of the signal-to-noise characteristics of quantum versus thermal ghost imaging", Phys. Rev. A 82, 053803 (2010).

    edit: I thought I understood what you mean, but now I do not see what you are getting at. When you are indeed assuming independent photons you will usually get no interference pattern at all because such experiments are usually carried out in a manner that the light at the double slit does not produce an interference pattern on its own anyway. If the photon in the other arm is completely independent of the other photon, no pattern will arise.
     
    Last edited: Aug 15, 2011
  11. Aug 15, 2011 #10

    xts

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    OK. I won't bet good wine in challenge on supremacy of candle light over BBO crystals regarding detector noise :uhh:
    Anyway, as I played a bit with quantum cryptography devices, their single-photon sources are solid lasers attenuated to a bit less than one photon per clock tick. Even N.Gisin uses BBO crystals only on Sundays :rolleyes:

    EDITed>>>
    N.Gisin is a co-owner of Id-Quantique - the company manufacturing QKD devices. He is also (at first!!!) recognised physicist in quantum optics, it was he, who performed lots of experiments extending Aspect's one.
    Sundays in a Quantum Engineer’s Life: http://arxiv.org/abs/quant-ph/0104140v1
     
    Last edited: Aug 15, 2011
  12. Aug 15, 2011 #11

    xts

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    I mean that photons in p and s arms are totally independent by means of entanglement, phase, frequency, time, or whatever else you like. They just have the opposite linear polarisation - both went through the same Nicol's prism, and one got reflected in one direction, the second in other direction.
     
    Last edited: Aug 15, 2011
  13. Aug 15, 2011 #12

    Cthugha

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    Yeah, I know. Nevertheless, I am the kind of guy for which every day is sunday. I second what the physicist in Gisin's paper says ”weak
    pulses are not good, because they may contain two photons”. Using faint light is ok if you know what that means and where the fundamental limitations are, but very often people new to quantum optics have the misconception that decreasing the light intensity really gives you single photons. That is not the case. No antibunching, no quantum light - even if that Clavis2 thing from idquantique uses attenuated laser light.

    My main point, however, was: It is the fact that the photons in a thermal light beam are not statistically independent which gives you a coincidence count spectrum looking similar to the one in Walborn's paper.

    edit: Reply to second post. I suppose that would only work in situations where you would also see an interference pattern in one arm without coincidence counting, but not in situations where the light is too incoherent to produce an interference pattern in one arm alone. I mean - this is one of the major surprises of using entangled light. The pattern is only in the coincidence counts, but not in the single count rates. Imho it is not possible to reproduce that using independent photons.
     
    Last edited: Aug 15, 2011
  14. Aug 15, 2011 #13

    xts

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    Really? So explain really independent experiment like this.

    'short time' = such that probability of finding more than two clicks is less than 1%
    Perform billion times:
    1. select randomly (note the position) polarizer setting as H or V
    2. light up the candle
    3. wait for a short time and note if the click happened in arm 'p'

    Wait a fortnight, and move with your equipement to Australia

    Perform billion times:
    1. select the polarizer setting as H or V the same as in previous series
    2. light up the Australian candle
    3. wait for a short time and note if the click happened in arm 's'

    Perform correlation analysis.
    Does it differ from what I've shown, or what Walborn got?
     
    Last edited: Aug 15, 2011
  15. Aug 15, 2011 #14

    xts

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    No. The pattern is not seen. We have bulky blob.
    But you may use results from one (p) arm as a filter splitting the results from (s) arm into two classes. One class produce fringe, the second one produce anti-fringe. Mixing them - you have that blob.
     
    Last edited: Aug 15, 2011
  16. Aug 15, 2011 #15

    DrChinese

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    Mmmm, you are definitely talking my language with the wine. Certainly sounds like you have some nice stuff.

    Hey, I don't think we are talking quite the same thing... although maybe we are. The reason we see the main results in DCQE setup is because the photons are part of a single (entangled) system. You cannot expect to learn more about any component than the HUP allows - which explains why the outcome at either arm will be consistent. Now if your setup mimics that result, then the source photons must likewise be entangled in some basis.

    If you have a reference for a setup using thermal light, sure, I'd love to learn some more.
     
  17. Aug 15, 2011 #16

    xts

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    You must visit Hr.Kamper's winery - it is a fair chance to meet A.Z there ;)
    I don't agree. The whole experiment relies on one-to-one classical (anti)correlation: horizontal-in-p-arm <=> vertical-in-s-arm
     
  18. Aug 15, 2011 #17

    DrChinese

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    OK, let's decompose this. The prism is like a polarizing beam splitter (PBS). Fine. The light source is one which emits 2 photons around the same time, right? Well, you already have an issue because you are trying to say they take an identical path to the PBS, were created at indistinguishable times and are of *opposite* polarization. I don't see how that happens but I stand open for correction.

    Is this setup similar? Take a SINGLE Type I BBo crystal which outputs 2 photons of identical known polarization (and are therefore not polarization entangled). Shift Alice via wave plate so she is opposite to Bob. Bring them both back together again in such a way that they are going through the PBS at very nearly the same time. I say these are not polarization entangled although they might be entangled in another base. I say you cannot use polarization orientation (via the PBS) or any other polarization technique to get Walborn-like results.
     
  19. Aug 15, 2011 #18

    DrChinese

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    It is in Austria, I take it? I may need to plan a trip! Although I am partial to reds, I hope they might be found there too. Or maybe not...
     
  20. Aug 15, 2011 #19

    xts

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    Ouch?!
    So point a flaw in my explanation!
    Or present calculations showing that (classically probabilistic) mixture of H/V gives different results than you expect.

    Gumpoldskirchen - far southern suburb of Wien. In a range of metropolitan transport. One of the nicest places for niche wines...

    Austrian red wines are, well, how to say...., they cultivate some red grapes, esp. Pinot Noir (Schwarzburgunder), but for those I'd rather recommend Spain or New World, esp. South Africa...
     
    Last edited: Aug 15, 2011
  21. Aug 15, 2011 #20

    DrChinese

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    I was afraid that might be the case. At any rate, would love to join you for a glass of the white, perhaps after dinner, and enjoy the countryside!

    Next time I am that way, because my travel budget is a little soft right now. :cry:
     
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