IBM scientists have discovered a way to transport information on the atomic scale that uses the wave nature of electrons instead of conventional wiring. The new phenomenon, called the 'quantum mirage' effect, may enable data transfer within future nanoscale electronic circuits too small to use wires.
"This is a fundamentally new way of guiding information through a solid," said IBM Fellow Donald M. Eigler, IBM's lead researcher on this project. "We call it a mirage because we project information about one atom to another spot where there is no atom."
As computer circuit features shrink toward atomic dimensions - which they have for decades in accordance with Moore's Law - the behaviour of electrons changes from being like particles described by classical physics to being like waves described by quantum mechanics. On such small scales, for example, tiny wires do not conduct electrons as well as classical theory predicts. So quantum analogs for many traditional functions must be available if nanocircuits are to achieve the desired performance advantages of their small size.
IBM's new quantum mirage technique may prove to be just such a substitute for the wires connecting nanocircuit components.
The quantum mirage was discovered by three physicists at IBM's Almaden Research Center here: Hari Manoharan, Christopher Lutz and Eigler. They reported their findings in the cover story of the February 3 issue of Nature, a prestigious international scientific journal published in London. They used the same low-temperature scanning tunneling microscope (STM) with which Eigler and Erhard Schweizer first positioned individual atoms 10 years ago, spelling out the letters I-B-M with 35 xenon atoms.
To create the quantum mirage, the scientists first moved several dozen cobalt atoms on a copper surface into an ellipse-shaped ring. As Michael Crommie (who is now a professor at the University of California-Berkeley), Lutz and Eigler had shown in 1993, the ring atoms acted as a 'quantum corral' - reflecting the copper's surface electrons within the ring into a wave pattern predicted by quantum mechanics.
The size and shape of the elliptical corral determine its 'quantum states' - the energy and spatial distribution of the confined electrons. The IBM scientists used a quantum state that concentrated large electron densities at each focus point of the elliptical corral. When the scientists placed an atom of magnetic cobalt at one focus, a mirage appeared at the other focus: the same electronic states in the surface electrons surrounding the cobalt atom were detected even though no magnetic atom was actually there. The intensity of the mirage is about one-third of the intensity around the cobalt atom.
"We have become quantum mechanics - engineering and exploring the properties of quantum states," Eigler said. "We're paving the way for the future nanotechnicians."
The operation of the quantum mirage is similar to how light or sound waves is focused to a single spot by optical lenses, mirrors, parabolic reflectors or 'whisper spots' in buildings. For example, faint sounds generated at either of the two 'whisper spots' in the Old House of Representatives Chamber (now called Statuary Hall) in the US Capitol Building in Washington, DC can be heard clearly far across the chamber at the other whisper spot.
"The quantum mirage technique permits us to do some very interesting scientific experiments such as remotely probing atoms and molecules, studying the origins of magnetism at the atomic level, and ultimately manipulating individual electron or nuclear spins," said Manoharan. "But we must make significant improvements before this method becomes useful in actual circuits. Making each ellipse with the STM is currently impractically slow. They would have to be easily and rapidly produced, connections to other components would also have to be devised and a rapid and power-efficient way to modulate the available quantum states would need to be developed."
The IBM scientists have built and tested elliptical corrals up to 20 nm long with the width as little as half that. (A nanometre is the size of five atoms placed side-by-side.) The electron density and intensity of the mirage depends on the quantum state, not the distance between the foci.
For more information on IBM Research see www.research.ibm.com
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