Science's New Nano Frontier

The quest: To build supercomputers molecule by molecule

`Anyone who is not shocked by quantum theory has not understood it," said Niels Bohr, one of the 20th century's premier physicists. In the quantum world--the world of atoms, electrons, and other very tiny things--cherished notions of reality collapse. Bits of matter can exist in more than one place at the same time. Electrons tunnel through seemingly impenetrable walls. And some say quantum theory allows the possibility of multiple parallel universes.

All of this strange quantum behavior, which once mattered only to theoretical physicists, has major implications for the electronics industry. Semiconductors have shrunk to the point where the behavior of individual atoms and electrons will soon begin to matter. Quantum effects are starting to take over, changing all the rules. Unless chipmakers learn to harness quantum physics, growth in computer power and everything else that depends on chips will come to an abrupt end.

"FUN TIME." Around 2010, the width of lines etched into semiconductors will be narrowing to less than one-tenth of a micron. Electrical signals running through those circuits will have so few electrons that adding or subtracting a single one could make a difference--putting chipmakers squarely in the quantum world. "At that point, we better have a new technology ready to go into production," says Gary A. Frazier, manager of nanoelectronics at Texas Instruments Inc.

That's why scientists around the world are rushing to probe what University of Wisconsin materials scientist Max G. Lagally calls "the last great frontier in solid-state physics." Lagally and others have made tiny structures called quantum dots that can hold single electrons. They are so small that billions could dance on the head of a pin. Researchers have used quantum dots to fashion transistors that switch on or off with movements of a single electron. And they've concocted clever arrangements of quantum dots that could serve as the guts of tiny, powerful computers. "It's a fun time," says Yale University electrical engineer Mark A. Reed. "The field is exploding, and there's a lot of good work going on."

The current research involves long-range, basic science. "We have to understand the new physics, new properties, and new devices that might be possible at these smaller scales," explains IBM chemist Christopher B. Murray. Indeed, the quantum devices being built or contemplated today may never prove economically feasible. But the notion of building tiny structures molecule by molecule, incorporating advances in chemistry, physics, and materials science, is "bound to have a tremendous payoff," says University of Notre Dame physicist James L. Merz. The promise has led companies such as Texas Instruments, IBM, Hewlett-Packard, and Motorola to back major research efforts.

The main goal of such research is to control the movement of very small groups of electrons without running afoul of weird quantum effects. Quantum dots could make that possible. These dots are clumps of matter less than 20 nanometers wide--that's 20 billionths of a meter, or about the length of a string of 60 silicon atoms. If such a clump is engineered from atoms with just the right properties, it can turn a free-spirited electron into a caged canary. The electron can't escape without a precisely sized boost of energy from the outside.

This "quantum confinement" gives rise to some interesting phenomena. It makes possible the small, highly efficient lasers already in use in many CD players. These so-called quantum-well lasers are made of an ultrathin layer of semiconductor material sandwiched between two layers of another material. Electrons in the middle are trapped in a quantum flatland, able to move in only two dimensions. That makes it easier to pump energy into them in the way needed to produce laser light. The result: more light produced from less power.

At Bell Laboratories, owned by AT&T spin-off Lucent Technologies Inc., researcher Loren Pfeiffer is delving even deeper into quantum research. He lops off one more dimension from the flatland and makes lasers based on so-called quantum wires. Inside, electrons can move in only one direction. Quantum-wire lasers can emit light at power levels beyond the practical limits of quantum-well lasers. "That could prove an overwhelming advantage for communications," Pfeiffer says. These higher-power lasers may reduce the need for the expensive repeaters installed every 50 miles or so on telecommunications lines to regenerate laser pulses, which degrade as they travel through optical fibers.

Scientists expect more gains by moving from quantum wires to quantum dots. But the quantum-dot lasers that researchers have built haven't lived up to expectations. For various reasons, including the difficulty of making these clusters exactly the same size, "the dots are not as useful as people thought they would be," says researcher Gilberto Medeiros-Ribeiro at Hewlett-Packard Co.

The first challenge is to find a way to mass-produce quantum dots of nearly identical size. Two leading methods are to etch pillars in semiconductors and to deposit clusters on top of them (diagram, page 101). Some researchers are spurning these techniques in favor of chemistry. For instance, Louis E. Brus, a physical chemist at Bell Labs, has pioneered a method that grows quantum-dot crystals in a test tube, molecule by molecule. Using this technique, researchers have built light-emitting diodes that can be tuned to different colors.

Even more exotic are the quantum structures made from single organic molecules in James M. Tour's chemistry lab at the University of South Carolina. This approach offers the tantalizing prospect of packing trillions of molecule-size devices onto a square millimeter. That single millimeter would contain 10,000 times more transistors than now found in a PC.

The ability of quantum dots to grab and hold electrons is essential if computing functions are to be performed by movements of single electrons. Physicist Konstantin K. Likharev of the State University of New York at Stony Brook has constructed a model memory chip with quantum storage dots. Single-electron transistors "read" the contents of the dots by sensing the electrical field of a trapped electron. On paper, his design could store a terabit of data--that's a trillion bits, or 15,000 times more than existing chips can store--on a chip about the same size as those used today.

A number of research teams have crafted the single-electron transistors needed by Likharev's model. In 1993, for example, a Hitachi-funded lab at Britain's Cambridge University announced it had constructed an experimental single-electron memory device. Last July, it unveiled a single-electron logic structure.

Most quantum devices work at extremely low temperatures. Even the slightest heat makes electrons too rambunctious and swamps the tiny quantum effects. However, by carefully altering materials to give quantum dots a tighter grip on their electrons, a few electrical engineers, including Kazuhiko Matsumoto of Japan's Electrotechnical Laboratory and Stanford University's James S. Harris, have recently built single-electron transistors that work at room temperature. "When we were working at 1 degree Kelvin [minus 458F], most of the world thought we were really crazy," says Harris. "Now, the work is stimulating a lot of research."

Many problems remain to be solved. Switching speeds can be slow, and a single electron is easily diverted from its intended path by stray electrical energy. So instead of trying to design quantum imitations of today's computers, most scientists are hard at work concocting radically new approaches.

SCIENCE'S NEW NANO FRONTIER
SCIENCE'S NEW NANO FRONTIER
SCIENCE'S NEW NANO FRONTIER

SQUARE DANCE. At the University of Notre Dame, Craig S. Lent and fellow theorist Wolfgang Porod have come up with a scheme in which the basic building block is a square containing four quantum dots. When two electrons are added, they back into opposite corners. So the square has two possible configurations: electrons in the top left and bottom right corners, or in the top right and bottom left. This is just what's needed for a switch--and it can be flipped back and forth by the movements of electrons in neighboring squares. That means these squares can be arranged to duplicate all the logic functions needed for computing. So far, Lent's team has managed only to build pairs of dots to test the physics. But the initial results are "stunning," says Lent. "I was prepared for them not to work as well as they do."

Whatever technique proves to be the best way of building quantum chips, years of painstaking engineering lie ahead. "Quantum computing is so far out that, right now, it's just something fun to scratch our heads about," says R. Stanley Williams, head of Hewlett-Packard's new quantum effects lab. Still, researchers foresee a day when trillions of quantum dots could be stacked in layers on otherwise conventional slabs of silicon. That promises a supercomputer on a pinhead--making these exotic structures part of the hottest boomtown on the quantum frontier.