Quantum Information Can be Negative 445
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."
Yes it can (Score:3, Informative)
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.
Re:(-2)+(-3)=+1 (Score:1, Informative)
(-2)+(-3)=
-5
Now, if two people containing negative information multiply...
(-2)*(-3)
The result is positive. +6.
This provides a ray of hope for the observation that, "only stupid people are breeding", as noted by the famous song (not that famous, I guess. Cannot recall the song or artist). Eventually, things will come full circle.
Re:This is not news (Score:3, Informative)
I think Feynman thought of this first (Score:5, Informative)
Feynman presented it as just a different way of having quantum interference, from negative probability instead of complex amplitudes.
Re:(-2)+(-3)=+1 (Score:3, Informative)
(-2) - (-3) = 1
At least one (Score:4, Informative)
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...
It's your stock of entangled particles (Score:5, Informative)
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.
This paper proves nothing (Score:1, Informative)
However, I can't wait for the longer technical account. Is it common for Nature to publish exciting results without proof?
BTW, there seems to be a bit of confusion regarding negative entropy and negative information (or rather channel capacity). Entropy is a measure with respect to some coordinates. By changing the scale of the coordinates of a probability distribution, e.g. by changing its variance, you can change its entropy - even making it negative. However, channel capacity is always the same regardless of the coordinates used, as it measures a difference. Its like measuring voltage in a circuit.
Re:Affects black holes! (Score:3, Informative)
Re:At least one (Score:2, Informative)
I'm not blaming the authors. It isn't their fault people take it out of its strict scientific context and go wild.
But...mmmm, how shall I put this? When I read or review a scientific paper, I like to have some question or other answered definitively. We didn't know the value of X but now I've measured it and we do. No one could explain why Y followed on Z, and now here's a theory that does. That kind of thing.
I'm just not very fond of work that merely "raises interesting questions" or points out curious features of generally well-understood phenomena. I'm not saying such papers shouldn't be published or anything like that, but I see them as fairly low value.
Now, if everyone in quantum computation had heretofore believed strongly that quantum information could not be negative, and they had come along and proved it could, that might be well worth publishing. But I don't think that was the case.
Re:Yes it can (Score:3, Informative)
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.
negative information (Score:2, Informative)
Suppose there are 3 possible outcomes of an experiment: A, B, and C. A priori you know that A is 98% likely, B is 1% likely and C is 1% likely. Your uncertainty (i.e. entropy) about the outcome is quite low (because you are almost sure that the outcome is A). Now it is revealed that the outcome is not A. Your new probabilities are 50% on B and C. Your new uncertainty about the outcome is now much higher.
Being told that A did not occur thus has negative information because it increases your uncertainty (i.e. entropy) about the outcome.
I think I have this figured out... (Score:3, Informative)
In order to learn something, you have to make a measurement. Of course, in the quantum world, measuring a system will change it, so you are giving up what you know by measuring. It seems that in negative information situations, you are giving up your certainty in order to measure something, but your aren't learning anything in return. So your net 'gain' of information is negative.
Re:I think Feynman thought of this first (Score:3, Informative)
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.
Re:This is not news (Score:3, Informative)
I think another example would be religions.
Or the classic Billy Madison line "we are all now stupider for having listened to that" (not sure if I phrased that exactly but it's very close to that).
But you can look at pretty much anything in society and see this being used all the time. Look at how many people think things like Universal Health Care is bad, Schools need local control, taxes are always bad, paying off national debt isn't important, the media is a left wing conspiracy, nuclear power is bad, republicans are good with money , tax cuts jump start the economy etc etc...
(if you found a slant in there, well, sorry those are just the common ones going on today and have caused us to have the officials we have while the continue to screw up on the basis of people believing these things)
People just go with these things, but they would not be able to give you a solid answer as to why, they just hear it so much it sticks. Thats why we have pundents who go on the news and get air time to drive it into peoples heads even more. The more something is said the more it becomes fact even if it has no basis.
Look at myths and urban legends, people here it something so much for so long they just accept them even if common sense would prove it otherwise.
Quantum Conditional entropy (Score:1, Informative)
A more descriptive title (but more boring for the general public) could have been "Interpretation of Quantum Conditional Entropy".
Re:At least one (Score:3, Informative)
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.