Ripples in space: electrons make waves.

Drop a pebble in water and what happens? Smooth ripples glide to the water's edge in concentric circles. But what if those ripples hovered as standing waves, in motion yet apparently still?

In theory, electrons should form such standing waves when confined in a space of just the right size. Scientists knew roughly how this phenomenon should look, but they had never seen it happen. Now, they can watch quantum theory emerge in vivid color.

In the Oct. 8 SCIENCE, Michael F. Crommie and his colleagues at IBM's Almaden Research Center in San Jose, Calif., report building a quantum corral, a "round, two-dimensional box" that elegantly shows what quantum theory predicts -- namely, that electrons trapped in a flat, circular space will create standing waves at precise intervals. "Corrals let us actively shape electron wave functions," determining their spatial positions and energy levels, says Donald M. Eigler, and IBM physicist and coauthor of the report. "In this sense, a corral is a remarkable tool."

To build the structure, the scientists used a scanning tunneling microscope (STM) to individually place 48 iron atoms on a copper surface in a circle roughly 143 angstroms across. Then, using the STM again to sense electron behavior inside the corral, they detected "local densities," which appear as waves, at the very intervals predicted by quantum mechanics -- specifically, the Schrodinger equation for a particle in a hard-wall enclosure. The standing waves appear when iron atoms scatter the copper's superface electrons.

"Most scientists looking at this image will probably say, 'That's how it should look'," Eigler obseves. "This is not an unexpected result, but rather a unique visual confirmation of what Schrodinger's equations predict." By focusing surface electrons, corrals make possible deeper studies of electron behavior. "We can use corrals to learn much more about how electrons move as guided waves, about how they couple, and perhaps about quantum chaos," says Christopher P. Lutz, another IBM physicist involved in the research.

Quantum theory also predicts that, under certain conditions, electrons will move chaotically. To see quantum chaos, the researchers are building corrals with various shapes, such as grids, tubes, and even a "quantum stadium" -- a corral shaped like a running track. Measuring 140 angstroms by 300 angstroms, this flattened ellipse will, they hope, create an arena in which electrons can careen chaotically around, again enabling them to test behavior against theory. "The question is how a particle, which also acts as a wave, will behave in the stadium," says Lutz.