In a tiny atomic ellipse, images of atoms have a habit of showing up in two places at once, raising new possibilities for nanoscale computing.
In 1990, Don Eigler of the IBM Almaden Research Center used the tip of a Scanning Tunneling Microscope (STM) to spell out "IBM" with individual atoms, demonstrating for the first time the ability to build structures at the atomic level. A few years later his group showed how electrons could be trapped inside Stonehenge-like rings of atoms called quantum corrals, which they assembled from individual atoms. In their most recent achievement, announced in the journal Nature in February, Eigler and collaborators Hari Manoharan and Christopher Lutz have shown that elliptical quantum corrals can be used to project an image of an atom across a copper surface. Since images by their very nature carry information, this demonstrated a new method of information transport that may someday become a building block of nanometer-scale computer technology.
The surfaces of copper and a few other metals harbor a two-dimensional sea of electrons. According to quantum mechanics, the electrons in this ocean behave like waves. In the early 1990s, when Eigler and his group used an STM to study these electronic waves as they traveled across a copper surface, the researchers observed all the phenomena that are visible in the wave tanks of freshman physics labs: interference, reflection, standing waves and more. In their latest experiments, Eigler and his colleagues have gone a step further by building a structure to focus the electronic waves and to form images.
The Almaden experimenters chose for their structure an ellipse, among the virtues of which is the ability to focus waves. If one were to drop a stone at one focus of an elliptical pond, the spreading ripples would be reflected by the walls and would reconverge on the other focus; all the waves would have traveled the same distance from focus to focus and so would arrive in phase, reinforcing one another.
To form quantum images, Eigler's group used the tip of their STM probe to nudge a couple of dozen atoms, one by one, into the form of a tiny elliptical pond (or corral) about 150 angstroms across on a smooth copper surface. Instead of dropping a stone, they placed at one focus of their ellipse an atom of cobalt. Cobalt's orbital electrons have a magnetic moment that interacts with the electrons in the surrounding sea. The interaction is weak, and at high temperatures it is overwhelmed by the thermal jiggling of the electrons. But as the temperature is reduced, the interaction of the magnetic moment of the cobalt atom with the electrons begins to dominate, distorting the electron sea in its vicinity. The phenomenon is known as the Kondo effect and may be observed by mapping the electron sea near the cobalt atom with an STM probe. But when Eigler's group did the experiment inside their elliptical corral, they observed something more. Not only was the sea of electrons near the cobalt atom distorted, b
ut when they scanned the other, empty focus, they observed a nearly identical distortion, a perfect image of the electron sea around the distant cobalt atom: a quantum mirage.
"Transporting information through a quantum mirage is fundamentally different from sending information through a wire the way we normally do," says Eigler. In a computer chip, information travels as a flow of electrons through wires, causing the chip to heat up. IBM's new copper chip wiring technology helps reduce such heating. But using a quantum mirage to transport information would sidestep the problem altogether. "There is no power dissipation in the transport channel," Eigler explains. While the mirage works only over short distances, more and more information will have to travel on such a tiny scale as microelectronics shrinks to nanoelectronics. "I'm eager to determine whether we can turn this into a useful application for computing on a small scale," Eigler says.
Bruce Schechter, a freelance writer who lives in Brooklyn, New York, is the author of My Brain Is Open: The Mathematical Journeys of Paul Erdös.
