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Advance may lead to practical quantum computing

Photo of quantum computer developers van der Weide, Joynt,
                        Eriksson and Lagally

Clockwise, from top left: Associate Professor of Electrical and Computer Engineering Dan van der Weide, Professor of Physics Bob Joynt, Assistant Professor of Physics Mark Eriksson and Professor of Materials Science and Engineering Max Lagally (large image)

So powerful would be a working quantum computer that it could solve in seconds certain problems that would take the fastest existing "classical" supercomputer millions of years to complete.

A team of University of Wisconsin-Madison scientists is seeking a patent for their semiconductor-based device that can trap individual electrons and line them up, an advance that could bring quantum computing out of the gee-whiz world of scientific novelty and into the practical realm.

Professors Max Lagally (materials science and engineering), Dan van der Weide (electrical and computer engineering), Mark Eriksson and Bob Joynt (physics), have developed a new type of "quantum dot" device for holding electrons that can be scaled up to build a working quantum computer.

Made from tiny amounts of the same semiconductor materials used in today's computer chips, each quantum dot device contains just one infinitesimally small electron. When many of the devices are aligned, the electrons they house become usable quantum bits, or qubits, for computing.

"The first prerequisite to building a large computer is to have a lot of bits, and we think we have a way to get a lot of them," says Eriksson. "We've done some sophisticated simulations with this device that show the concept is very likely to work, and we're in the beginning stages of actually making the device."

Unlike the bits of classical, serial computers, which exist in either the 0 or 1 state, qubits can exist in more than one state at once. This elusive quality of their components frees quantum computers to calculate all the possible solutions to a problem simultaneously, instead of running through them one-by-one like their slower, serial counterparts.

This ability to "parallel process" means quantum computers hold tremendous number-crunching potential for certain tasks — such as highly sophisticated data encryption and code-breaking — that now defy even the most powerful computers.

The team's device uses layers of semiconductor materials and electrostatic forces — the same forces that build up when you scuff across a carpet in winter — to squeeze a single electron into place within each quantum dot. The design allows the alignment of a large number of dots, their captured electrons separated by a distance only one-one thousandth the width of a human hair.

"The structures we are building are unique combinations of successful designs," says van der Weide, associate professor of ECE and principal investigator on the UW grant. "For example, we confine charge carriers such as electrons both with metallic contacts as in conventional integrated circuits and with energy barriers grown into the material itself. It's like caging rabbits: if you don't have a roof over their heads, they can escape. The same thing turns out to be true of electrons."

Researchers worldwide are trying to find the best way to harness subatomic particles for quantum computing. In fact, others have realized success in stringing a few quantum dots together.

"People often talk about quantum computing in the future tense, but that's not really right — it exists today. People have solved simple problems with it, but in the future we want to address problems that can't be solved by any other means," says Eriksson.

With its potential for coupling hundreds of electrons, Eriksson believes the team's device could provide a quantum leap in that direction. "Our invention makes it more likely that quantum computing might actually be useful someday instead of a curiosity," he says.

Collaborators include postdoctoral researcher Mark Friesen (physics theory), staff scientist Don Savage and graduate student Paul Rugheimer (materials growth). A paper describing the technology can be found in a preprint archive at

A patent on the technology has been filed by the Wisconsin Alumni Research Foundation, a non-profit organization that manages the intellectual property of the UW-Madison.