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Theorists Make Quantum Communications Breakthrough
KentuckyFC writes "One of the cornerstones of modern physics is Claude Shannon's theory of communication, which he published in 1948. If you've ever made a phone call, watched TV, or used a computer, you've got Shannon to thank for describing how information can be moved from one place in the universe to another using an idea called the channel capacity. But nobody has been able to develop a quantum version of this theory. So physicists have no idea how much quantum information can be sent from one point to another. Now two American physicists have made an important breakthrough by proving that two quantum channels with zero capacity can carry information when used together. That's interesting because it indicates that physicists may have been barking up the wrong tree with this problem: it implies that the quantum capacity of a channel does not uniquely specify its ability for transmitting quantum information (abstract). And that could be the idea that breaks the logjam in this area."
"two quantum channels with zero capacity can carry information"
Feynman once said that nobody understands quantum mechanics, and this is why.
I'm not sure how useful this is. The summaries seem to say that if you take two or more channels that have a signal to noise ratio of zero, there's some potential for binding them into a useful channel, but there's no indication of what kind of recovery rate there can be gained from this. Is this just error-correction applied to an extreme?
Am I the only one who's worried that we keep getting 'news' from papers published on ArXiv, which is not a peer-reviewed source?
Just saying, it needs to be taken with a grain of salt.
You just got troll'd!
I think that Khashishi has got the essence of the 0+0>0 thing here. I haven't completely penetrated the noise in the Smith/Yard ArXiv article yet, but I'd bet my money that it boils down to this:
Take two channels in each of which all bits are completely random, and independent of the information that you wish to send. Let each bit of your information determine the correllation or anticorrellation of corresponding bits in the two channels, by introducing a quantum constraint between them before their actual random values are determined. Then, as in Khashishi's description, the xor of the two random channels is the message.
The only difference I detect in Smith/Yard vs. Khashishi is that they use quantum trickery to make the whole thing look symmetric. Neither of the random channels predates the other. Each one, evaluated singly, appears to be completely independent of the encoded message. In Khashishi's description, the time sequence in the construction of the two random sequences makes one of them seem a priori random, and the other to be a one-time pad encoding of the message, while in the Smith/Yard article you can't tell which is which.
It seems more like a meretricious way of telling a causal story about a well-known phenomenon than something truly "essentially quantum."
Mike O'Donnell http://people.cs.uchicago.edu/~odonnell/
http://arxiv.org/PS_cache/arxiv/pdf/0807/0807.4935v1.pdf
Interesting, but the paper seems to have a nasty habit of simply redefining what "capacity" means in a quantum context, to basically, "Well, if we have two interacting channels, one changes the other to have non-zero capacity." And if I interpret it that way, it simply rewords the problem to be different from the original interpretation. Also, there's a significant amount (even for an arxiv paper) of speculation present (which is interesting!). From the paper: Nonetheless, each channel has
the potential to \activate" the other, effectively cancel-
ing the other's reason for having no capacity. We know
of no analog of this effect in the classical theory. Per-
haps each channel transfers some different, but comple-
mentary kind of quantum information. If so, can these
kinds of information be quantfied in an operationally
meaningful way? Are there other pairs of zero-capacity
channels displaying this effect? Are there triples? Does
the private capacity also display superactivation? What
new insights does this yield for computing the quantum
capacity in general?
One "classical" analogy is that of orthogonally-crossed polarizers, which, upon insertion of another polarizer with principle axis somewhere between that of the originals, will allow light to shine through where none was before.
The real question in my mind is whether this allows for FTL communication, or whether nature conspires against that once again?
I believe it's Bell's inequality that prevents information from traveling faster than light. But each of these channels does NOT transmit information, if the paper is to be believed.
So, does that mean they could somehow be used with entangled photons or whatever to transmit information faster than light?
The zero-point energy of an quantum mechanical harmonic oscillator is 0.5\hbar\omega > 0. Well, spoiled as i am, TFA can't surprise me anymore :)
I knew I was going to be a geek for life when at a young age I read a button on a soda machine as "Hi-Res" Root Beer.
Tiller's Rule: Never use a word in written form that you've only heard and never read. You will end up looking foolish.
The recent Amateur radio mode called WSPR ('whisper') can work with a signal around 27 dB below the noise (SNR of -27dB). This site records contacts between hams worldwide in real time. Most activity is on 30m.
Tiller's Rule: Never use a word in written form that you've only heard and never read. You will end up looking foolish.
That reminds me of a lesson an information theorist taught me:
If you want to be useless to people, you can't simply feed them *wrong* information, because once they realize you always give them wrong information, you become *more* useful to them, because they can simply invert everything you say (i.e. assume it's false), to extract useful information.
So, to be useless, you have to keep giving, not *wrong* information, but *random* information -- sometimes true, sometimes false, so they can't extract any "signal" out of you.
He was unemployed at the time.
Information theory is life. The rest is just the KL divergence.
Shannon's theorem about channel capacity doesn't make sense at all for quantum channels anyway. Shannon's theorem specifies channel capacity by voltage switching rate and bandwidth of the electromagnetic signal. Quantum links don't send information by changing voltage, and I believe it could be argued that they don't send information by an electromagnetic signal at all!