Molecular Computers? Munich/UD Researchers Report First Glimpse Of Artificial Molecule In Action

"We developed a technology for probing artificial molecules that allowed us to see a phenomenon analogous to Rabi oscillations [pronounced RAH-bee], which are actions observed in real molecules," says Rogert H. Blick of the University of Munich, lead author of a paper scheduled to appear in the July 13 issue of the journal, Physical Review Letters. Blick performed the work at the Max Planck Institute for Solid State Research in Stuttgart, in collaboration with Daniel W. van der Weide (say WHY-deh), an associate professor of electrical and computer engineering at the University of Delaware.

Observations of artificial molecules in motion could help bring researchers one step closer to making the 21st-century dream of biocomputers a reality, says van der Weide, one of 20 scientists in 1997 to win a National Science Foundation Presidential Early Career Award for Scientists and Engineers. "Human beings walk around with this incredible bioprocessor in their heads," he notes. "The brain uses very little energy, and yet it has billions of exquisite interconnections. This is why humans and animals can perform complex tasks such as pattern recognition, which is very difficult for traditional, semiconductor-based computers."

Someday, explains van der Weide, technologies based on simple biological systems such as molecules and membranes might help researchers develop, for example, an auxiliary, biologically based processor, capable of recognizing hazardous materials or identifying spending patterns on credit applications.

Such technologies remain elusive for now, Blick emphasizes, but the Munich/UD researchers--including Rolf J. Haug of the University of Hannover and Karl Eberl of the Max Planck Institute for Solid State Research--were encouraged by their initial sneak-peek of artificial molecular activity. And, Blick says, the new detection system may help researchers overcome technological hurdles associated with artificial molecules, which currently function only at near-absolute-zero temperatures, and at relatively slow speeds.

Harnessing Particles in a Box

Artificial molecules are nothing new, van der Weide says. Researchers have been creating them for the past several years by exploiting a concept often described as the "particle-in-a-box problem," he says. When a particle is confined to tight quarters, he explains, it exhibits "quantized energies."

By capturing single or small quantities of electrons inside box-like structures known as quantum dots or "artificial atoms," Blick says, researchers can harness and manipulate quantized energies. Pairs of artificial atoms have been coupled to create artificial molecules. And, groups of artificial molecules eventually might be assembled to form basic molecular or biological systems, offering many intricate interconnections, which should promote a better understanding of real systems.

As conventional, semiconductor integrated circuits keep shrinking, van der Weide notes, the number of wire-based interconnections between individual components continues to increase, too, causing operational problems. "When you put millions and, someday, billions of these devices together," he says, "the heat load becomes enormous. A computer will cook itself, or at least slow down and malfunction."

Unfortunately, Blick says, no one has been able to investigate the behavior of artificial molecules in motion-until now.

Looking for Life

To detect signs of molecular activity in a semiconductor model, the research team set out to observe Rabi oscillations-the characteristic movement of electrons traveling back and forth between two points or molecules, a behavior first described by the late Austrian-born U.S. physicist Isidor Isaac Rabi. The resulting technique, van der Weide says, is analogous to conventional spectroscopy, used for studying real molecules by analyzing their energies based on observations of their spectra, which include colors.

Specifically, he says, the instrument prompts an electron moving between two artificial atoms to interact with high-frequency pulses of electromagnetic radiation, in the 2 to 400 gigahertz range, corresponding to the energy levels of the artificial molecule. The instrument is a dual-source spectrometer, incorporating a pair of integrated circuits for generating these short pulses, thereby "communicating with the traveling electron by interfering with it," van der Weide says. The two short-pulse sources are tuned nearly to the same frequency, so that their interference creates a "beat note" similar to what one hears when tuning a musical instrument with a tuning form, he explains. This beat note is at a low frequency, making it convenient for detection, yet it carries information about the high-frequency response of the artificial molecule, he says.

While the system is a far cry from a full-scale molecular computer or even a biologically based component, van der Weide says it may help set the stage for new computing strategies. Existing, semiconductor-based computers most likely will reach their speed and size limitations around the year 2020, Blick predicts. "If we want to keep moving forward," he says, "we need to investigate new options now."