### Reducing 'noise' in quantum computing is more difficult than first believed

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.”

This is not a big setback for quantum computing, he said.

“This was not the only path for doing it,” he said. So many years had been spent trying to solve the problem that many scientists already were beginning to suspect that it could not be done. “It has already been factored in” to much of the research, he added.

Quantum computing uses subatomic particles, rather than binary bits, to carry and manipulate information. While a traditional bit is either on or off, a 1 or a 0, a quantum bit (or qubit) can exist in both states simultaneously. Once harnessed, this superposition of states should let quantum computers extract patterns from possible outputs of huge computations without performing all of them, allowing them to crack complex problems not solvable by traditional binary computers.

But noise in these computations is a problem because quantum computing does not allow bits of data to be copied for error checking, as traditional computing does. Transversal gates were supposed to solve this 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.

How close are we to practical quantum computing?

“I don’t expect there to be a quantum computer in the next 10 or even 20 years that can do things a classical computer can’t do,” Eastin said. “I may be wrong.”

The development of quantum computing has the advantage of being able to draw on experience from classical computing. At least researchers now know about the need for error correction.

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.

## Reader Comments

## Thu, Apr 9, 2009 rrtucci

What non-zense. Rewriting software doesn't take 100 years!