Thursday, April 9, 2009

Transversal Encoded Quantum Gates May Be Impossible

Via -

From a theoretical point of view, work on quantum computing is moving along at a good clip.

The first classical computing machines were envisioned around 1800, long before the introduction of electronics, and it took about 150 years to produce a practical computer even though the theory had long been worked out, said Bryan Eastin, an information theorist with the National Institute of Standards and Technology (NIST).

“In that respect we’re doing pretty good, in that I expect we will have [a quantum computer] in less than 100 years,” Eastin said. “There are no theoretical difficulties, but there are a lot of painful technical difficulties.”

One of those difficulties—the problem of "noise," or errors in calculations introduced by stray energy — turns out to more difficult than thought. Eastin and NIST mathematician Emanuel Knill proved in a paper in the March 20 issue of Physical Review Letters that one promising technique for squelching quantum noise actually is impossible.

The technique, called transversal encoded quantum gates, seemed simple at first (at least to a physicist). “But after substantial effort, no one was able to find a quantum code to do that,” Eastin said. “We were able to show that a way doesn’t exist.”


Transversal gates were supposed to solve this [error checking] problem by preventing qubits that are going to be error corrected together from interacting, thus squelching the noise of errors. Similar gates have been designed for other purposes, but Eastin and Knill were able to show a mathematical proof that the structure of quantum space is not amenable to this particular technique.

With transversal gates ruled out, scientists now are free to move onto greener fields of research and come up with better solutions, Eastin said.


Now that he has finished off transversal gates, Eastin has a number of other research irons in the fire, such as quantum discord, a measure of non-classical correlation in quantum systems.

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