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Quantum Information Can be Negative
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."
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.
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...
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.