A new spin on computing UC scientists suggest way to harness electrons for processors

From San Francisco Chronicle, Dec. 10, 2001
Carl T. Hall, Chronicle Science Writer

Some radically new ways of building computers are starting to take shape as scientists venture ever deeper into the weird realm of quantum mechanics.

A team of researchers at the University of California at Santa Barbara has taken a key step by suggesting for the first time a practical way to bring the elusive phenomenon known as "electron spin" under precise control.

Experts said it opens up a path toward a whole new style of computing, one that is expected to be particularly useful at performing calculations that stymie conventional machines, such as breaking complex codes and searching huge databases at lightning speed.

"We're trying to explore how to go about building real quantum devices," said David Awschalom, a physicist and director of the Center for Spintronics and Quantum Computation at UC Santa Barbara.

Although such devices are a long way off, experts say the basic scientific foundation is being laid for machines capable of exploiting the quirky behavior of matter at the scale of individual atoms and subatomic particles.

"Quantum computers are proving to be very difficult to build, for many reasons, but one of them is how do you get these little quantum elements to behave the way you want them to," said Mark Kubinec, a chemist at the University of California at Berkeley.

Awschalom reported the results of his latest adventures in the quantum world last week in the journal Nature. The experiments were among the first under a $1.2 billion research initiative launched by the state of California.

The high-profile effort, announced last December by Gov. Gray Davis, includes corporate partnerships and four new "Centers for Science and Innovation" being created at UC campuses throughout the state.

BUILDING RESEARCH PROGRAMS

Those overseeing the project admit that real products and jobs are likely to be many years away, but they insist the effort is not a frill. The state is relying on revenue-bond financing to maintain the project through the current recession and drop in tax revenues.

"We're trying to look far out on the horizon to see what kind of fundamental research programs should be built now in order to . . . keep California at the leading edge of the worldwide knowledge-driven economy," said Susanne Huttner, associate vice provost for research for the UC system.

Awschalom's work was conducted for the California NanoSystems Institute, a collaboration between UC Santa Barbara and UCLA. Among other goals, scientists hope to pioneer a new technology known as "spintronics" -- which some visionaries expect will take over where conventional electronics leaves off.

Today's computer circuits rely on manipulating current, moving streams of electrons through switches that carry out logic operations. Each individual bit of data, represented by a "0" or "1," corresponds to a positive or negative charge in the familiar binary code of microelectronics.

Millions of these switches, called transistors, make up the circuits found in computers. And data, such as a name or a date, is represented by streams of 0s and 1s.

Things work differently in a spin-based quantum computer. Each bit of information, known as a "quantum bit" or "qubit," is encoded by varying the orientation of electrons as they spin about their axes like tiny planets orbiting the nucleus of an atom.

This spinning can be "up" or "down" in the quantum universe, relative to a surrounding magnetic field. In fact, thanks to phenomena called quantum "entanglements" and "superpositioning," the particles can be said to spin in both directions at the same time, one particle's spin state influencing another's.

EINSTEIN CALLED IT 'SPOOKY'

Such phenomena seem to defy logic. Indeed, even Einstein had some trouble describing them, calling them "spooky effects at a distance."

"It's not spooky," Awschalom said, "it's just counterintuitive. It's difficult to conceptualize because it's so rare that you ever interact in everyday life with these kinds of quantum effects. You need to be a very small particle."

Although the true nature of things in the quantum world may defy easy description, the take-homemessage is not that hard to grasp: Flipping the direction of spin from an "up" to a "down" direction can be one way to encode information.

The problem is that electrons seem to spin with a mind of their own, wobbling and flipping in unpredictable fashion as the particles interact with one another and their surrounding environment.

Various schemes have been tried to impose some "coherence" on ensembles of spinning electrons. Some successes have been achieved, allowing for the advent of some spin-based data-storage devices already being used, but they don't require atomic-level precision.

The schemes devised so far are not good enough for quantum computers capable of carrying out complex instructions. Last week, Awschalom introduced a new idea, suggesting that spin might be controlled electrically after first trapping electrons in carefully layered sandwiches of semiconducting materials.

'WELLS' OF ELECTRONS

In essence, the researchers created pools of electrons in parabola-shaped pockets, just 100 nanometers across, like marbles in a cloth sack. Such structures are known as "quantum wells."

Applying a charge across the well pushes the trapped electrons from one material into the adjacent layer, which alters the spin. Researchers found they could manipulate the speed and orientation of the electrons without causing a lot of disruptions, gently raising or lowering the voltage much the same as sliding a dimmer switch on a light fixture.

That creates a blueprint for what would appear to be the first electrically controlled "spin gate,"analogous to the charge-based logic gates that make up conventional computer chips.

Such a simple control over quantum effects would allow manufacturers to adapt current technology to the quantum-based systems. Rather than relying on a lot of exotic manufacturing methods, the most important quantum properties would effectively be engineered into the materials used to construct the machines -- something of a Holy Grail for spintronic experimenters.

"It's a very important result," said Richard Hughes, a physicist and quantum-computing expert at Los Alamos National Laboratory in New Mexico. "He's shown there's a way to control spin electrically down to the level of single electrons. That's an essential ingredient in being able to do the logical operations you need for computing, with the bits of information represented by single electron spins."

Still, nobody is close to designing and building a working quantum computer. In fact, so many basic problems still need to be worked out that "it's extremely difficult, if not impossible, to even guess where the next discoveries are going to come from," Awschalom said.