Quantum strategies fail to improve capacity of quantum optical communication channels

Jan 31, 2013 by Lisa Zyga report

(Phys.org)—Quantum techniques have been demonstrated to offer improvements in areas such as computing, cryptography, and information processing, among others. But in a new study, researchers from IBM have proven that no quantum trick – no matter how complex or exotic – can improve the capacity of a type of quantum channel that serves as a building block of quantum optical communication systems. Although the result is somewhat surprising and a bit disappointing, it will help guide scientists to explore other ways to enhance channel capacity.

The researchers, Robert König and Graeme Smith at the IBM TJ Watson Research Center in Yorktown Heights, New York, have published a paper on the limits of quantum techniques for improving channel capacity in a recent issue of .

As the researchers explained, channel capacity is operationally defined as the maximum achievable , measured in bits per channel use, and it provides an ideal way to gauge performance. The maximum capacity of classical communication channels can be determined by a simple formula derived by the famous mathematician and engineer Claude Shannon in the 1940s. But Shannon's formula does not account for quantum effects, such as . In the intervening decades, many researchers have thought that could provide ways to increase the capacity of bosonic Gaussian channels, which would improve their speed and overall performance.

"Almost any communication link using is well described as a Gaussian channel for a wide range of parameters," Smith explained to Phys.org. "A Gaussian channel is basically a simple model for describing how a propagating is corrupted as it propagates. A good example of this is communication in optical fiber.

"Researchers thought that quantum effects could improve the capacity of Gaussian channels because there are examples of more complicated channels (though somewhat contrived) where entangled signal states can be used to boost the capacity of the channel. Also, in terms of actually proving limits on the possible size of such an effect, there were huge gaps between the best known achievable rates and the best upper limits on the capacity. That gap looked like an opportunity."

However, efforts to improve channel capacity with quantum effects have fallen short. In the new study, König and Smith have finally shown why by providing the first mathematical proof showing that quantum strategies are essentially useless for increasing channel capacity; although the proof doesn't rule out some very small capacity increases, they would be too small to care about.

To reach this conclusion, the researchers considered the concept of entropy, which is a measure of a channel's noisiness and closely related to capacity. They mathematically showed that, when two quantum signals combine at a beamsplitter, then no matter what state each signal contains, the beamsplitter always increases entropy. Through their calculations, the researchers could determine an upper bound on the channel capacity that no quantum effect can improve upon. The results suggest that current technologies for increasing capacity in bosonic Gaussian channels are working at near optimal efficiency.

"Our results tell us, for all practical purposes, what the actual capacity of electromagnetic channels is and how we can come close to achieving it," Smith said. "The classical Gaussian noise channel was considered explicitly in Shannon's original work precisely because of its relevance for practical communication systems. Since many practical channels (such as optical) are well-described by the quantum Gaussian noise model, we know the best ways to go about using these resources. I should say also that the mathematical technique we developed, the quantum entropy power inequality, has a classical counterpart that has many applications in information, statistics, and deep mathematics. Our quantum entropy power inequality may have similar further applications to information theory and mathematics."

In order to keep pace with the explosion of data and delivery of bandwidth, researchers will have to turn to other strategies for improving communication technologies, such as using multi-mode fiber, multi-core fiber, or multi-level modulation, rather than . König and Smith also plan to further investigate how the mathematical technique can be applied to other areas.

"I want to find more applications of the quantum entropy power inequality to the theory of quantum communications, perhaps in the network setting," Smith said. "It would also be great to find some appropriate mathematical generalizations and extensions and see where they lead. Like I said above, the classical version of the inequality has lots of unexpected connections to different parts of mathematics. I think the upper bound of the can save people a lot of time by preventing them from looking unsuccessfully for quantum transmitters for this important channel."

Explore further: Step lightly: All-optical transistor triggered by single photon promises advances in quantum applications

More information: Robert König and Graeme Smith. "Limits on classical communication from quantum entropy power inequalities." Nature Photonics. DOI: 10.1038/NPHOTON.2012.342
Also at arXiv:1205.3407 [quant-ph]

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1.8 / 5 (5) Jan 31, 2013
Scotty, don't split the bea...!
2 / 5 (3) Feb 01, 2013
Good ol' Claude strikes again. And a valuable insight into how fundamentally interconnected the notion of information is with even the most fundamental concepts we know of.
1 / 5 (3) Feb 01, 2013
The paper (from arxiv) contains this rather refreshing statement: "Overall, we find for a wide range of practical channels that good old classical modulation of coherent states can't be substantially improved upon with quantum tricks."
1 / 5 (3) Feb 01, 2013
Scotty, don't split the bea...!

True, but they at l(e)ast admit quantum physics is BS.
1 / 5 (4) Feb 01, 2013
lol Many succumb to taking what they envision or imagine seriously.
Paving possible inroads to a 'Rome' labeled science.
A 'Rome' still under construction.
Construction hats advised.
1.7 / 5 (6) Feb 01, 2013
researchers from IBM have proven that no quantum trick – no matter how complex or exotic – can improve the capacity of .. quantum optical communication systems. Although the result is somewhat surprising and a bit disappointing
The quantum computers are potentially fast but very noisy and operate at low number of qbits. Whereas the classical computers are slower (at least in principle) - but their reliability and precision is incomparable.

I already explained here many times, that the computational power of quantum computers cannot beat the power of classical computers at the moment, when the power of classical is already limited with Heisenberg's uncertainty principle. And you even don't need to pay expensive researchers for such an understanding...

But the mainstream science proponents who are naively believing in quantum computer hoax are downvoting me here again and again, simply because they cannot face even the most trivial logic like the religious tribal people.
1 / 5 (2) Feb 08, 2013
'Noise' is for quantum science what 'junk' DNA once had for a meaning in biology. In both cases prematurely viewed.