Toward Quantum Computing

John Markoff published an interesting article in the NY Times regarding Hewlett-Packard's progress on building a quantum computer. My quantum friend, Josh Wolfe, and I have a standing bet on whether quantum computing will ever become a reality. I say yes. He says no.


SAN FRANCISCO, June 30 - Scientists at Hewlett-Packard said Thursday that they had developed a new strategy for designing a quantum computer composed of switches of light beams that could be vastly more powerful than today's digital electronic computers, which are constructed from transistors.

Quantum computers are machines based on the principles of quantum mechanics, a branch of physics that describes the quirky world of subatomic particles where both yes and no can simultaneously be true.

The potential of quantum computing is still controversial. To date, researchers have built small demonstration systems, but most scientists in the field believe that it will be more than a decade before a large-scale quantum computer can be built, and there is debate about the range of problems such a machine will be able to solve.

The transistors in today's digital computers hold information in binary units - either 0 or 1. In quantum computing, units of information called "qubits" can hold both 0 and 1 simultaneously. That capacity is the heart of the vast potential power of quantum computers.

For example, while an array of three conventional bits could hold only one of eight possible values at a time, a similar quantum array could contain all eight values at once. Moreover, computing capacity based on multi-qubit computers scales up exponentially, a fact that underlies the potential of quantum computers.

The new strategy for designing a quantum computer was outlined in an article researchers at Hewlett-Packard published in the May issue of The New Journal of Physics. On Thursday the company said it would receive as much as $10 million from the Defense Advanced Research Projects Agency of the Pentagon, known as Darpa, to design a prototype of the technology described by the researchers. The company said it planned to contribute about $7.5 million to the project.

The researchers said their idea was a potentially important advance because it may make it possible to assemble a quantum computing system out of very large numbers of light switches.

The Hewlett-Packard paper - written by Bill Munro and Tim Spiller of Hewlett-Packard Laboratories in Bristol, England, with Prof. Kae Nemoto of the National Institute of Informatics in Tokyo - explores the idea of using laser pulses to force the interaction of photons, which can contain quantum information.

Hewlett-Packard is assembling a research program at its laboratories in Palo Alto, Calif., to build a working prototype based on the paper.

"To perform a demonstration once will not be difficult," said Ray Beausoleil, a researcher at that lab. "To do it reliably and to do it in a way that will allow us to do quantum information processing, you have to be very careful."

Several quantum computing scientists said that the paper offered an interesting theoretical proposal but that there were significant obstacles to building a useful system.

"The paper is interesting, and is likely a real advance in terms of quantum computing using photons," said Umesh Vazirani, the co-director of the Berkeley Quantum Information Center at the University of California, Berkeley. "That said, optical quantum computing schemes are not regarded as the most practical alternatives."

Most researchers in the field say that the leading candidate among the competing technologies for creating workable quantum computers is based on trapped ions, which are charged atomic particles that can be confined and suspended using electromagnetic fields.

The Hewlett-Packard researchers are also looking for applications for quantum computers. For example, in the field of computer security, quantum computing may make it possible to detect eavesdropping or tampering with great certainty.

"We're trying to figure out the quantum computing equivalent of the hearing aid," said Mr. Beausoleil, referring to one of the earliest uses of the transistor, which brought that technology into the mainstream.

According to Dr. Vazirani, last year Darpa had considered financing an ambitious "moon shot" program for quantum computing research, but scaled back that program after some researchers warned that there was a high likelihood of failure.