April 2001

From University of Wisconsin-Madison

UW-Madison team to build next-generation 'quantum' computer

MADISON -- A working quantum computer could be so powerful that it would solve in seconds certain problems that would take the fastest existing supercomputer millions of years to complete.

Seeking this "Holy Grail" of computing power, an interdisciplinary team of engineering and physics researchers at the University of Wisconsin-Madison plans to use silicon germanium quantum dots to build the foundation for a new generation of computers.

From a competitive field of more than 30 submissions, the U.S. Army Research Office chose to fund the UW-Madison researchers with a three-year, $1.2 million grant to combine their unique tools and talents in the development of a semiconductor-based quantum gate or qubit.

At the center of the invisible atomic world of quantum computing is the quantum dot, a nanometer-scale "box" that holds a distinct number of electrons. The number can be manipulated by changing electrical fields near the dot.

A quantum computer would use these dots to take advantage of a quantum phenomenon known as superposition, in which, for example, an electron would have its spin state both up and down at the same time. Where a classical computer uses an on or off state to represent bits of information in the "zeros" and "ones" of binary code, a quantum computer uses the superposition as qubits.

With superposition, a qubit is in neither the zero nor the one state before being measured, but exists as both zero and one simultaneously. The spin state of the particle is determined at the time it is measured. Quantum theory holds that particles that have interacted are connected or entangled in pairs through the process of correlation. Determining the up or down spin state of one particle affects the spin state of its entangled pair. Even more astounding is that the entangled particles retain their connection no matter how great the distance between them. It's something Einstein called "spooky action at a distance."

All of this together means that a quantum computer could perform massively parallel calculations enabling certain "hard" problems, like encryption, to be resolved in mere seconds.

UW-Madison's team consists of physics professors Mark Eriksson and Bob Joynt; Max Lagally, the Erwin W. Mueller Professor and Bascom Professor of Surface Science, and principal investigator Dan van der Weide of Electrical and Computer Engineering. Also involved in submitting the proposal were postdoctoral researcher Mark Friesen, physics theory, and staff scientist Don Savage and graduate student Paul Rugheimer, materials growth.

The team will combine advanced physics theory, silicon-germanium heterostructured materials, low-temperature and high frequency measurements to build an elemental piece of a quantum computer, called a solid-state Controlled-NOT logic gate.

Creating this item will be an achievement in itself, but it is the team's approach that is a breakthrough. A useful quantum computer will require a chain of thousands of qubits. Other approaches have formed qubits using nuclear magnetic resonance or by trapping individual atoms in a vacuum but have been limited by the inability to link together large numbers of qubits.

The UW-Madison team's process uses new science and existing technology similar to complementary metal-oxide semiconductor (CMOS) technology. That means if one qubit can be made, the process likely could be scaled to make and link qubits by the thousands. The researchers predict their success could result in the first useful quantum computer in 10 to 30 years. The team has already disclosed its approach to the Wisconsin Alumni Research Foundation for consideration of a patent.

"That is what is so exciting," says Eriksson. "Here we are building a new type of quantum dot that hasn't been made before, and if we can do this successfully, the infrastructure is out there so that the technical community should be able to run with this."

The team attributes its success in winning the competitive grant to its novel approach, their unique mix of intellectual expertise and specialized facilities found on the UW-Madison campus. While related research efforts might focus on theory, materials growth or experimentation alone, the UW team is situated to integrate its new approach with existing results and theory into a working result.

"This has been an unusually strong and collaborative team effort right from the beginning," says Lagally, a professor of materials science and engineering.

"It's really an outgrowth of MRSEC (Materials Research Science and Engineering Center) directed by Tom Kuech," Lagally says. "The fact that we have this excellent collaboration in materials and the physical sciences made us successful in the Nanophase Hiring Initiative."

Both van der Weide and Eriksson were among the first cluster hires of the Madison Initiative, a public-private partnership to improve research, teaching and outreach by UW-Madison. "The nanophase hiring is allowing us to explore the future of computing," Lagally says.

This article comes from Science Blog. Copyright 2004

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