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Counterfactual Computation

  1. Feb 23, 2006 #1


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    I hope this news isn't moved to another forum - many of our readers would be interested in this:

    Quantum computer solves problem, without running

    "It seems absolutely bizarre that counterfactual computation – using information that is counter to what must have actually happened – could find an answer without running the entire quantum computer," said Kwiat, a John Bardeen Professor of Electrical and Computer Engineering and Physics at Illinois. "But the nature of quantum interrogation makes this amazing feat possible."
  2. jcsd
  3. Feb 23, 2006 #2
    Hi Kea

    I suspect this is like the following analogy. You have four envelopes and three dollars. You put the three dollars each in an envelope, leaving one empty. You send them to your friend in Minnesota (Thanks!)

    I open one, then another, then another. The last one is unopened, but I know what should be in it, even though I don't break the seal.

    How is things in NZ? I am longing for Spring. I wonder if Marcus is tickling the peach blossums yet?

    Hope you are well....

  4. Feb 23, 2006 #3


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    Hi Richard

    Nice story, but the quantum world is even weirder than that! If you had intercepted all the mail by robbing the LA post centre, you would still come to the same (classical) conclusions. :smile:

    We are enjoying a nice summer here. Balmy days full of sunshine. Sorry, I'm too poor to send you donations, but I might potentially be sending you money!
  5. Feb 23, 2006 #4
    Hey, that is great! I have to suppose that it will be some of those quantum bucks? The ones where you don't have to rob the LA post centre to get what is in there?

    Actually, despite my poor-me philosophy, I am really very comfortable. My cabin is warm, the night is mild, and there is tea and roast potatoes.

    Did you see that FQX is getting ready to publish their request for proposals? They now say they will have it on the table by Monday 27 February. I suppose they mean this year.

    BTW, I see that Nature 23 Feb has stories on this same topic, quantum computing I mean, and the new findings. I don't have a subscription but I get the content alerts.

    Last edited: Feb 23, 2006
  6. Feb 23, 2006 #5


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    Ah, you mean http://www.fqxi.org/about.html . Yes, I see. Not sure I want to apply.
  7. Feb 24, 2006 #6
    I see the popularity of ghost like interpretations has no limits... :yuck:
    If people cannot come up with a straight, well done, experiment which demonstrates the necessity of non-locality once and for all, it is of course indispensible to launch even more irrational theories to convince the public. :biggrin:
  8. Feb 24, 2006 #7


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    Emoticons apart, are you denying the content of the experiment?:confused:
  9. Feb 24, 2006 #8
    I do not know all the details of the experiment; but what I am saying is that if somehow the computer is running without pushing the button, then a more rational *explanation* can certainly be found. I was just making jokes about how far people are willing to accept a certain type of explanation. No wonder consciousness crackpots claim a legitimate existence in the physics community; banning them would mean that you try to define something like common sense crazyness. The latter being an impossible task.
  10. Feb 25, 2006 #9


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    That explanation is in a popular article, so it might have been a bit over the top. But the scientific paper is in this week's Nature. I haven't had access to that but I would bet the explanation there (a) supports at least generally what the physicists told the reporters, and (b) Uses standard QM to do it. I don't know if you consider that a "rational explanation" but if you don't, if you're selling some alternative to standard QM, then you should be up front about it.

    (Added) By standard QM I mean the formal structure, not any particular interpretation of it, except that "quantum superposition and uncertainty are features of the world."
  11. Feb 25, 2006 #10


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    I thought the experimenters were showing the particle-wave state of the algorithm itself could be used to predict certain computational outputs. Because quantum states are dynamical, quantum algorithms are always resident in a quantum computer. They therefore have a signature that can be used to predict the outcome of certain computations they are programmed to solve. . . at least I think that was the point of the article.
  12. Feb 26, 2006 #11
    **That explanation is in a popular article, so it might have been a bit over the top. But the scientific paper is in this week's Nature. I haven't had access to that but I would bet the explanation there (a) supports at least generally what the physicists told the reporters, and (b) Uses standard QM to do it. **

    In nature there also appear papers about how the existence of six extra dimensions could be shown to be true. :uhh: I am humbly waiting for a detailed paper on the arxiv.


    Well, this IMO is somehow nonsense: it is impossible to expect one single person to be on top of everything, and moroever, consider his/her alternative ideas to QM only viable if and only if he/she reproduces eighty years of QM results. For example: I was really willing to comment upon the Larsson papers (at least there is some original idea into it), which are on my laptop in the to read list, but I did not find any time since then and the list is growing faster than my reading of it is.

    For example, let me once and for all ask you this question: suppose someone would show that the full, stable atomic structure can be recovered from an entire classical theory, would you consider this the end of QM or not? Or, would you consider this a viable reason to investigate this path vigorously or not?
  13. Feb 26, 2006 #12
    Suppose someone would show that the moon is made of green cheese. Would you consider this the end of astronomy, or not? Or, would you consider this a viable reason to send cheese miners to the moon, so that we can feed our starving multitudes, or not?:surprised

  14. Feb 26, 2006 #13
    I always love it when quantum amateurs come up with an idiotic phrase like yours :biggrin: You clearly do not understand how you just might do that, despite of the several references/hints I have dropped once in a while. :frown: Physics is not about citation out of a book, but about a constant exploration to UNDERSTAND nature. Now, QM tells us that if we want to get the right results at the microscopic level, then we have to believe in magic. Therefore, any classical explanation for the very reasons why science impovered in the 1920 ties (atomic spectra, black body radiation) is IMO of major interest. As I said repeatedly and gave references accordingly, there exist hopeful - partial - results in this direction.
    Last edited: Feb 26, 2006
  15. Feb 26, 2006 #14


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    Don't be too hard on rt, careful, he was just making a light hearted analogy. The most interesting part of the subject article was the simulation of a virtual quantum computer using laser interference patterns. I was more intrigued by the methodology than the result.
  16. Feb 27, 2006 #15


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    They weren't trying to demonstrate non-locality, or validate QM to anybody. They're trying to do quantum computing. :grumpy:

    (Or... trying not to do quantum computing, I suppose. :biggrin:)
  17. Feb 27, 2006 #16


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  18. Feb 27, 2006 #17
    a visit to fairyland, and the circus

    I apologise, Careful, I know you are serious and don't deserve jibes. There is no reason for anyone to attempt to be humorous at your expense. But really, if you would, what is the difference between your statement and mine? Of course, if someone shows that the known features of partical physics can be demonstrated in a classical model, then it would be less than amature-ish to insist on maintaining quantum puzzlement.

    However, the facts seem to show otherwise. I cannot explain quantum effects such as are shown in benchtop experiments by classical means. Maybe we need a list so that we can consider them one by one, but starting from the early days, for example, there is the two slit experiment demonstrating wave-particle duality. I have not seen any classical treatment that shows how a single event involving a single photon can behave in that odd way.

    Now I will admit to you as part of my apology that the idea of magic is not entirely disreputable in my estimation. You see I am giving you stones to throw at me if you wish. Here: I have in the past and hope again in the future to visit undespoiled wilderness places where elves and other fair critters still reside.

    But even close to home, where magic is more often just a street trick, designed with the sole purpose of transfering wealth from the gullible to the crafty, there is a certain pedantic value to it. It is the mystery. How did they do that? Motivation to investigate and perhaps discover.

    It happens I am re-reading Penrose, The Road to Reality, in his discussion of imaginary numbers, in which he finds the word "magic" to be useful. Magic, in this sense, and in the sense which I prefer, means the demonstation of verifiable actions which seem to result in consequences which are forbidden under certain rules of behavior which we ordinarily find reliable. Something must be going on which we do not yet see.

    Now, I have given up this personal revelation just to show my honesty in that there are lots of things I do not understand, and I am aware that some of them at least are presented in a way meant to decieve me to my loss. I give you this because what I want from you is that you stop using the word "magic" as if it were something entirely reprehensible, in order that we may see what it is that you find reprehensible about the findings you slur as magical.

    I don't have a subscription to Nature and have not had an opportunity to closely examine the claims that are made at the top of this thread. I am vaguely aware of the idea of quantum computation and the seemingly magical things that photons can do. I put up the idea of the un-opened letter in my first post in this thread hopeing someone would be able to tell me how this experiment was more than that, but no one here who has read the actual report seems to be forthcoming on that topic. Probably it isn't really very interesting if you have better access to the research than I currently have. No one has taken time to show me how entanglement is different from the unopened letter trick either.

    However, I continue in my perhaps naive belief that there is a natural, physical explanation for these events, and faith that I can understand it if I apply myself. I may be wrong on both counts, or either, but I do not find it productive or interesting to brand the events with a slur and so justify my ignorence. If these researchers have found yet another way to uncover the quantum mystery in a bench-top apparatus, I would gladly pay my quarter to enter their sideshow tent.

    And if you, dear Careful, have found out the secret of their machinations I would be grateful to you if you presented it here so that I can spend my nickles on soda pop instead. Rather, you seem to me to be a spoil-sport at the circus, muttering darkly that it is all a trick and that anyone who buys a ticket is a fool. Sadly I acknowlege that you are almost certainly right, but I should still like to experience the trick myself, and if possible, figure out how it is done, or, failing that, have someone wiser explain it to my understanding.

    Meanwhile, I am having what passes for fun under my thinning canvas, and I hope you are having fun also. And there is always the remote chance that we will discover something interesting.

    Thanks for your comments.

    Last edited: Feb 27, 2006
  19. Feb 27, 2006 #18

    Hans de Vries

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    In QFT there is an argument that a measurement at position A cannot
    influence a measurement at position B if it's outside the lightcone, see for
    instance Peshkin & Schroeder paragraph 2.4 at page 27. Causality requires
    every particle to have an anti-particle for the commutator to vanish.

    Regards, Hans.
  20. Feb 28, 2006 #19
    Obviously: some call QFT dynamically local (precisely for this reason) and kinematically non local (because of entanglement of states) - these are just words (and you have to give up local realism anyway)! Anyway, that was NOT the point: I was talking about an unambiguous EXPERIMENT which demonstrates this action at a distance once and for all and not about the sick theory itself.

  21. Feb 28, 2006 #20
    Hi R,

    Well, classical explanations are bound to be more involved but are nevertheless possible IMO. Now, I am not going to boast around here that I know how it all works since I am a serious person and the calculational work involved is tremendous (actually dr Chinese was once making unjustified fun about this). What I am trying to tell people is that IF you want to have a chance at solving problems, you better start out from a deep CLASSICAL understanding of it. It is very amusing for me to notice (and hard work to go through all of this), that understanding self interactions (as well electromagnetic and gravitational) classically is still a very hard partially open problem (the Lorentz dirac equation for the radiation back reaction is by no means correct - it relies upon some ad hoc assumptions about how the internal particle pressures are to behave). To give you some idea about this ``simple topic´´, here is some *useful* literature:


    Just to whet your appetite. Look: QM is useful (as I repeated many times) and is *by construction* bound to be correct on microscales; however it has a part which gives rise to magic on macroscopic scales (EPR type of experiments). It is precisely there where any sensible person believes QM to be wrong. The fact that nobody has convincingly demonstrated the MAGIC up to date despite of serious efforts strengthens me in that belief. So, what do you have to do? You have to understand the core reasons for the very construction of QM deeply from a CLASSICAL perspective. It turns out that the calculation of radiation back reaction and related issues is the very first oustanding issue in this quest (many smart people have gone that road before - with partial succes). Doing this you will also learn again why QED is still a ill behaved theory (this is already crystal clear at the level of the Lorentz Dirac equation) - Barut has written some illuminating stuff about that.

    This is why I despise this good news show about MACROSCOPIC ENTANGLEMENT so much: (a) it is not convincingly demonstrated (b) it is not at all at the heart of quantum mechanics; quantum mechanics = (sub)atomic microphysics which cannot explain the macroworld unless you go to some weird MWI stuff. It almost seems as if this show is an excuse to cover up the fact that we still do not understand classical Einstein Maxwell theory properly.

    Concerning my dark muttering and me being a spoil sport. This is kind of funny: you prefer monkeys in the circus (but then go to a real circus) above anyone who tells you that real insight has to come from hard work/ asking deeper questions about nature. The point I try to make is that you better figure first out what an elementary particle is before you start wondering about exotic many particle systems.


    Last edited: Feb 28, 2006
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