https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing

https://phys.org/news/2013-01-quantum-strategies-capacity-optical-channels.htmlTo my mind, quantum computing researchers should still heed an admonition that IBM physicist Rolf Landauer made decades ago when the field heated up for the first time. He urged proponents of quantum computing to include in their publications a disclaimer along these lines: “This scheme, like all other schemes for quantum computation, relies on speculative technology, does not in its current form take into account all possible sources of noise, unreliability and manufacturing error, and probably will not work.”

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.

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"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.

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