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orbital angular momentum data transfer controversy

Discussion in 'Electronic Design' started by Jamie M, Nov 8, 2012.

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  1. Jamie M

    Jamie M Guest

    Hi,

    I was reading more about the orbital angular momentum data transfer
    technique, which apparently most people think is a bad idea:

    http://www.bbc.co.uk/news/science-environment-20217938

    New idea: Apparently there is no limit to the orbital angular momentum,
    ie. you can corkscrew a beam of light or electrons as much as the
    equipment allows and the beam will just have more OAM. If this is
    true, then maybe that gives a lot of bandwidth to send data potentially?

    cheers,
    Jamie
     
  2. I think one thing that the quantum club always forgets is the Shannon limit.
    Quantum computahs are limited because of noise, probably the reason
    we have never seen one do anything useful.
    Here it seems (I have just read the critical paper referred to) they try
    the same thing, and likely will die in noise the same way.
    Ken
     
  3. ....
    Now what is this? Are Pantelje and Tucker alter egos of the same
    ....err.. personality?

    Regards,
    Mikko
     

  4. http://vixra.org/pdf/1209.0107v7.pdf

    Abstract. We point out that the assumption that more than two
    spatially orthogonal farfield wave modes (the two polarization modes)
    can leave an antenna and propagate in free space violates the Second
    Law of Thermodynamics and is thus incorrect.
     
  5. Owen Roberts

    Owen Roberts Guest

    However the optical guys have demonstrated 16 QAM in free space, using
    8 closely spaced emitters.
    It has a potential for short distance optical links with very high
    bandwidths once the beams are merged, you get spatial patterning.

    Steve
     
  6. Apologies, a new internet SNAFU.
    Ken
     
  7. Sylvia Else

    Sylvia Else Guest

    The Shannon limit applies to a channel. Before you can apply the Shannon
    limit argument to show that some technique cannot provide an increase in
    the information carrying ability, you have to show that it does not
    create new channels, or if it does, that it does so at the expense of
    the carrying ability of the existing channel(s).

    Sylvia.
     
  8. Not sure that's topical.
    The (silly) english have vertical TV dipoles, but most have horizontal
    TV dipoles., which is a form of 'static' polarizations.
    Could a polarization be modulated?
    Consider a transmission dipole rotating at 1 Mhz, with that rate
    detected, a 2nd channel might rotate at 1.0001 Mhz though with
    each at an emission frequency that is equal.
    Ken
     
  9. rickman

    rickman Guest

    I'm not an expert at this, but doesn't the Shannon limit consider the
    bandwidth vs the signal to noise ratio? Your transmissions can modulate
    all they want. The question is what does this do to the transmitted
    power level? How do you rotate the carrier in two different ways at the
    same time in a single carrier? Using two carriers doesn't violet the
    Shannon limit since they would have twice the power total.

    Rick
     
  10. Jamie M

    Jamie M Guest


    Hi,

    I don't think OAM and polarization are the same things, ie. take for
    example a linear polarized source, in which the electric and magnetic
    fields always have the same 180degree phase offset. If you rotate
    that source (ie like rotating a flashlight axially) to give the light
    beam non-zero OAM, then the phase offset of the electric and magnetic
    fields will still be 180 degrees, ie. still linearly polarized.

    cheers,
    Jamie
     
  11. Sylvia Else

    Sylvia Else Guest

    No, but that doesn't mean that doing something entirely different in an
    entirely different scenario wouldn't create more channels.
    It's not a word game. Before you can use the Shannon limit you have to
    identify a channel. You can change the theory and call what it applies
    to an X, but then you have to identify an X.

    Sylvia.
     
  12. josephkk

    josephkk Guest

    Doubt it, i have seen them disagree as well.

    ?-)
     
  13. MrTallyman

    MrTallyman Guest

    Dead link.

    You are infinitely inaccurate. Hell, you can't even spell the word.

    Look into 16 APSK and 32 APSK.

    Would it have even been possible 20 years ago?
     
  14. Tim Williams

    Tim Williams Guest

    "Can't form fringes" according to what speed film? :)

    It's amazing how different, and how identical, light and radio are. It's
    also amazing how some people forget those basic facts.

    Tim
     
  15. It's hard to comment anything constructive without more
    specificly stated criticism. I think Shannon limit (whatever you
    specifically mean by that) is well known within QC community.

    Noise reduction is the bread and butter of the whole field,
    (i.e. getting the Johnson noise so low that the Callen-Welton
    tail becomes dominant), so it's hard to understand what exactly
    you think the QC community is overlooking.

    Regards,
    Mikko
     
  16. OK Phil, I'll play a bit. (I know jack about Optical Angular
    momentum.)

    So If I can put some spatial mode distribution on light, that is
    preserved as it propagates and is then discernable at the detector,
    doesn't that count as another channel?
    (I'm not saying it's cheaper than just running another fiber.)

    George H.
     
  17. 'Somebody' in the unmoderated group like sci.physics is
    not the Quantum Computing community. I'm almost sure
    you have talked to some amateur which has just read an
    article written by some popular science journalist without
    a clue.
    Yes of course. That is very well known by the QC community, but
    it may not be understood by an anonymous person posting to
    sci.physics.

    Agreed, there have not been that impressive results from Quantum
    Computers, but the reason is not that those guys wouldn't understand
    what noise is or what Shannon theorem says. The reason is that
    the task is HARD.

    You seem to be talking about quantum-gate based computers.
    The technique perhaps more likely to give some practical results
    soon is the Adiabatic Quantum computing, although it is
    not a "full" quantum computer. It just finds global minima in
    various optimization tasks.

    I was a couple of weeks ago in a conference in Portland, where
    Richard Harris from D-Wave explained their AQC hardware. Their
    AQC finds the global minimum of a N-dimensional Ising system
    with freely programmable couplings. Harris claimed that many
    problems can be cast in the form of Ising system minimization,
    including image recognition problems tackled by Google Inc.

    Their computation time scales as
    T[us] =5.84 N^2 + 65.5 N + 2E6 the function of the problem
    dimensionality N. This scaling does not come from a theory,
    but from the actual engineering of their functioning 128-qubit
    processor, the D-Wave One. For instance, the constant
    2-second -term comes from the time it takes the system to
    cool back to the 20 millikelvin operating temperature, after
    the couplings are programmed through RSFQ circuitry. The
    programming cycle heats the system to 200 millikelvins.
    Lockheed Martin bought one D-Wave One system
    recently, I suppose not because it is practical, but because
    they want to stay at the edge of the developments in the
    field.

    The classical supercomputers still perform the Ising
    system minimization faster then the D-Wave One,
    but the classical computational time increases ~exp(N),
    when using the fastest known Simulated Annealing or
    Iterated Taboo Search algorithms. Harris claimed that
    the break-even complexity is roughly N~2000, and that
    they are just about to roll out their 512-qubit
    D-Wave Two processor.

    The stuff above are from Harris' talk, it sounds plausible
    to me, but I haven't studied all the details.All that stuff
    is pretty new to me, D-Wave has been criticized in the
    past about not telling the details of their work.

    The rest of the drivel would take too long to comment,
    here's just one thing:
    You don't count Bardeen's work as physics?

    Regards,
    Mikko
     
  18. Jamie M

    Jamie M Guest


    Hi,

    I guess there is no room for OAM in the theory of electromagnetic waves
    if it really does propagate as transverse waves, and the typical way to
    get a corkscrew effect is to superimpose two orthogonal transverse
    waves that are out of phase to make the elliptical polarization, this
    causes the pseudo OAM by having the vector of the E field and M field
    from the two orthogonal transverse waves rotating.

    If physicists think there is such a think as fundamental OAM, they must
    think light doesn't propagate as a transverse wave then I guess, that
    would be interesting if light transverse waves could corkscrew I guess,
    maybe there is a way to do that.

    cheers,
    Jamie
     
  19. Guest

    OAM is not the same as superimposed orthogonal transverse
    polarizations, nor circular polarization.
    Typically, optical OAM beams are generated using fork holograms:

    http://upload.wikimedia.org/wikiped..._generation.png/550px-Hologram_generation.png

    For RF, one way is to take a dish reflector and cut it from center
    to edge, then displace one edge in the direction of propagation and
    the other edge "backwards" so to speak; IOW make it a sort of slice of
    a corkscrew:

    http://ej.iop.org/images/1367-2630/14/3/033001/Full/nj400111fA4_online.jpg

    from:

    http://iopscience.iop.org/1367-2630/14/3/033001/article


    Mark L. Fergerson
     
  20. Jamie M

    Jamie M Guest

    Hi,

    Those are neat, I think they are used in special electron microscope to
    give the electron beam angular momentum for allowing different types of
    sample measurements to be done, but the efficiency and amount of
    angular momentum is limited with that technique. I didn't know they
    can work on light too!

    cheers,
    Jamie
     
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