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Quantum Information Can be Negative

Posted by timothy on from the you-are-now-dumber dept.

nerdlygirl writes "In a development that would probably even puzzle Claude Shannon, information can be negative -- at least when the information is quantum. The discovery, by Horodecki, Oppenheim, and Winter, appears in the current edition of the leading journal Nature. If I tell you negative information, you'll know less. Apparently, researchers hope to use this to gain deeper insights into phenomena such as quantum teleportation and computation, as well as the very structure of the quantum world. More details can be found here and here A popular account of the article can be found on Oppenheim's homepage, and a free version of the article can be found in the arxiv for those of us without subscriptions to Nature."

8 of 445 comments (clear)

  1. Yes it can by Anonymous Coward · · Score: 3, Informative

    This article is not bogus.

    The concept of a "quantum eraser" is not a new one. Consider the classic double-slit experiment, where electrons are shot at a double slit and form an interference pattern on a screen which corresponds to the probability distribution of the particle's position. If you were to place detectors so that you knew which slit the particle went through, the interference pattern would disappear-- that is, there would be no uncertainty in the position (because obviously, you know which slit it went through). This is intuitive if you consider the interference pattern to be a probability distribution.

    However, if you were to place a 50/50 beam splitter in front of the detectors, the interference pattern would reappear! By destryong the which-path information, the interference pattern (uncertainty) is restored. Bizarre, but true.

    Google "quantum eraser" for more info.

    1. Re:Yes it can by Anonymous Coward · · Score: 3, Informative

      > The double slit experiment involves photons, not electrons.

      1) The experiment works with photons or electrons. The latter is more related to the experiment's most recent significance (re: the implications for quantum mechanics).

      > The paradox can be very simply explained to the lay person.

      2) Your ability to be trusted as an authority died per point 1), and the carcass is beaten by your subsequent ramblings.

      > It turned out that if one tested for particles, one got results
      > consistent with particles. On the other hand, if one tested for
      > waves one got results consistent for wave functions.

      3) Uh... speaking of a vague explanation that misses the point. Simply put: no. The experiment shows that what we thought of as "particles" exhibit wave-like properties, meaning that the "particle" model did not fully describe the nature of matter at a certain scale.

      > What is ultimately uncovered is an even greater and far more interesting
      > question: How can the results of a controlled experiment be affected by the
      > observer?

      4) This must be the #1 held misconception about physics. NO. What is ultimately uncovered is that there is no way to predict individual particle paths (observation notwithstanding) -- that a particle's characteristics (location & momentum) are (at best) described in terms of probability and are not individually predictable -- that existence is a big sea of probabilities and not certainties. The slit experiment represents empirical verification of that model of the (quantum-scale) universe.

      > Goggle the Internet for more info ;-)

      5) Yes, please do.

  2. Re:This is not news by sratbot · · Score: 3, Informative

    FTFA: "It sometimes seems that we become more ignorant after talking to certain individuals. Perhaps they are saying things which are confusing or untrue. Well, after getting negative information, you know less. But not in the same sense as someone who tells you lies are tries to bamboozle you. Remember, that we don't worry about the quality of information (whether it is true or false for example). We just concern ourselves with how much there is. So, if we know less after receiving negative information, the amount of information we have must actually go down. This obviously cannot happen classically, but let me try to explain why it can happen quantumly."

  3. I think Feynman thought of this first by mbone · · Score: 4, Informative

    I distinctly remember a lecture by Feynman at Caltech in the early 1980's where he talked about negative information (probability). I am sure I still have notes for it somewhere. Of course, you can never see negative information; any actual measurement has to have positive probility. But it can give quantum interference effects in measured quantities.

    Feynman presented it as just a different way of having quantum interference, from negative probability instead of complex amplitudes.

    1. Re:I think Feynman thought of this first by Anonymous Coward · · Score: 3, Informative
      Two things. First, can we observe negative information? Sounds to me like we still just observe nonnegative information. That hasn't changed. Appears to me that negative information is virtual which is quite in line with Feynman's points.

      What do you mean by observe? If you mean, what is the information of the things we see, then what you say sounds right. Because classical information is positive always. If you mean, can we tell that the information is negative, then it seems we can -- the authors show that if the information is negative you can "gain the potential for future communication".

      Second, single particles going through slits? Sounds like self entanglement, ie, the states of the particle going through two slits considered seperately are entangled with one another.

      No no no. The particle is only entangled with the slits when the slit records which one the particle went through. But this is exactly the case when all the probabilities are positive, because the particle behaves classically. When the particle shows interference, then it is not entangled with the slits.

      Feynman's use of negative probabilities (which is different to information!) was a calculational trick, and is really cool, but it is not what you and the parent seem to think it is.

  4. At least one by Quadraginta · · Score: 4, Informative

    Geez, don't be so cynical. After I got my PhD from Berkeley, with a dissertation in quantum mechanics, I taught the stuff to graduate students for five years or so. I've published QM papers in PRA and all that, too. So, yeah, I know what they mean. I'm perfectly qualified to review their Nature paper, if it comes to that, and I doubt I'm the only one like this reading /.

    I have to say I'm not especially impressed by the work, however. The frisson of defining information as negative emerges ultimately from a semi-deliberate muddling of the distinction between the definition of information in the quantum computing context and information as we use the word in daily life. This is not hard useful scientific discovery so much as the scientific equivalent of making an outrageous pun.

    But then I feel similarly about most of what's published in the Bell's Inequality, EPR paradox, quantum tele-whatever field. Getting cynical myself, maybe I am....bah, humbug...grumble...

    1. Re:At least one by quantfreak · · Score: 3, Informative
      I am also a researcher near to this field, and actually heard it presented at a conference a few months back. The reaction of the people I know (who actively do research in this area, more so than myself), was one of extreme excitement. I think perhaps you don't understand the result.

      Quantum information is not a question of interpretation -- it is well known what it is, in terms of communication theory. What these guys did, is prove how much communication was required to send information, if the receiver already had some of it. No one knew how to calculate this, much less knew that it could be negative. I can't fathom why you would consider this a matter of interpretation. It is a complex mathematical calculation.

  5. It's your stock of entangled particles by iabervon · · Score: 5, Informative

    The trick is that you can use quantum entanglement to have excess unspecified knowledge, which can be converted into specific knowledge. It's like being on a quiz show where you are given a certain number of times you can look up an answer. These bonuses have to count in your total knowledge (I know 100 facts, plus I can look up things twice). If someone tells you something, you get positive information. If you look something up, you get zero information (you trade a bonus lookup for a fact). If you look something up, and you already knew the answer, you get negative information.

    Now think about it as if someone else controlled the book. They can tell you things over the phone, and they can cause answers to pop out of the book. If they waste the book on something you actually already knew, your total information goes down, so the information in the transaction is negative.