Electronic Design

Quantum Mirage Hints At Possible Atomic-Scale Communication

Nanotechnology has the potential to make even seemingly impossible technological feats a reality. Advances in this field of ultra-small devices, circuits, and machines do not come easy, though. Researchers working at the small sizes that characterize nanotechnology face many barriers.

One such obstacle, communication between atomic-scale circuits, now has a plausible solution. Scientists at the IBM Almaden Research Center in San Jose, Calif., have developed a way to leverage the wave nature of electrons to transport information on an atomic scale.

This new phenomenon is known as the quantum mirage effect. It promises to enable data transfer within the tiniest nanoscale electronic circuits. These circuits will be so minute, even conventional wiring won't work. If this happens, the miniaturization of electronic circuits will transcend all expectations.

Three IBM physicists—Hari C. Manoharan, Christopher P. Lutz, and Donald M. Eigler—originally made this breakthrough. A low-temperature scanning tunneling microscope (STM) was imperative to their findings. Using this instrument, they were able to arrange several dozen cobalt atoms on a copper surface into an ellipse-shaped ring (see the figure).

This circle of atoms acts as a quantum corral. It reflects the copper's surface electrons within the ring into a wave formation. Quantum mechanics dictated the pattern. The researchers then tried altering the corral's size and shape. By doing so, they discovered they could determine its quantum states (the energy and spatial distribution of the confined electrons).

Once this was realized, they had to decide which quantum state would be appropriate for use. They opted for one that concentrated large electron densities at each focus point of the elliptical corral.

Placing an atom of magnetic cobalt at one focus caused a mirage to appear at the other focus. That is, 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 equaled approximately a third of the intensity around the cobalt atom. Its electron density and intensity varied depending on the quantum state.

Essentially, this quantum mirage effect is just a clever way to guide information through a solid. As Eigler, IBM fellow and lead researcher on this project, explains, "We call it a mirage because we project information about one atom to another spot where there is no atom. It's similar to the effect created when light or sound waves are focused to a single spot by optical lenses, mirrors, or parabolic reflectors."

To date, elliptical corrals up to 20 nm in length and widths less than 10 nm have been built and tested. That's awfully small—especially considering a nanometer is one billionth of a meter, or about 40 billionths of an inch.

According to Manoharan, "Working at such small sizes, the quantum mirage technique permits us to do some very interesting scientific experiments. These might include remotely probing atoms and molecules, studying the origins of magnetism at the atomic level, and ultimately manipulating individual electron or nuclear spins."

Researchers concede that applying this concept to actual circuits will require significant improvements. For starters, using an STM to make an ellipse is impractical and slow. A technique for easily and swiftly creating the ellipses must be devised. Connections to other components would also have to be created. In addition, a rapid, power-efficient way to modulate the available quantum states must be found.

Resolving these issues will prove increasingly important as circuit features shrink to atomic dimensions. On such a small scale, tiny wires don't conduct electrons as well as classical theory predicts. Basically, if nanocircuits are to achieve desired performance, quantum analogs for many traditional functions must become available.

Having a viable means of communication on a nanoscale opens the door to a host of new developments. As Eigler explains, "We have become quantum-mechanics engineers and are exploring the properties of quantum states."

For more information, visit the IBM Almaden Research Center web page at www.research.ibm.com.

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