Good Vibrations For Computations

Physicists are developing acoustical "phonons" for computers of the future.

Gabriel Popkin, Contributor

(Inside Science) -- Electronic computers, like the one you're using to read this story, are a fantastically successful technology, having grown in just 70 years into a bedrock of the global economy. But within the next decade, experts say computer and personal electronics designers will start to reach the physical limits of how small and fast such devices can get.

Some physicists are now trying to put a new spin on the technology, by building computers that would store, move and process information using vibrations in solid materials.

The key is finding ways to select and control these vibrations. Typically, the atoms in a solid jiggle around chaotically, like a choppy sea during a storm. But in some materials under certain conditions, internal vibrations can resemble a more ideal, orderly wave, in which atoms would move back and forth at a well-defined rate (frequency) and amplitude (the maximum distance from their rest location). Physicists describe the energy of these atomic vibrations as being carried by particles called phonons. Phonons, in turn, could be used to represent information, like the 0s and 1s of conventional computing.

A phonon-based computer is at least a decade or two away, but the field is growing rapidly. So far, however, researchers studying aspects of phonon computing have worked mostly in isolation, said Sophia Sklan, an MIT physicist who recently published a paper in the journal AIP Advances describing much of the work that has been done.

"These fields don't really talk to each other that much," she said. "Part of what I wanted to do was to bring them together and put them all in one place."

A need for speed

Not only are modern silicon-based components close to their smallest possible size, heat is also a problem. As more electrical components are packed into a small space, it becomes harder to prevent them from overheating.

In order to build smaller, faster and more powerful computers, engineers need to change their strategy. Physicists are exploring various possibilities, from using light to fluid to magnetic properties, in addition to vibrations in solid materials.

It sounds trippy, but the basic concept behind phononic computing is straightforward.

Acoustic phonons of different amplitudes could represent the 0s and 1s of computing. They would correspond to sound waves of relatively low frequency, beginning with pitches within the range of human hearing, and beyond, up to around a million cycles per second. Very high-frequency phonons -- up to billions of cycles per second -- typically transport heat within a material. By harnessing those phonons, a computing device could use the heat generated by traditional electronic computers to do additional calculations, said Sklan. Currently this heat is simply a waste product.

Building blocks

Much work remains before such ideas become reality. Scientists seeking to develop the field of phononic computing have started by trying to design and build devices that act like the building blocks of electronic computers — diodes, transistors, memory and so on.

A phononic diode, for instance, would allow vibrations to travel in one direction only, similar to how an electric diode is a one-way conduit for electric current. In 2004, physicist Baowen Li at the National University of Singapore proposed a way to make a phononic diode by joining two dissimilar materials so they would conduct heat in one direction but not the other. In 2006 a group led by physicist Alex Zettl at the University of California, Berkeley reported they had built such a diode using tiny structures called nanotubes.

Li's group has also designed a phononic transistor, which would use a small thermal signal to control a larger heat flow. The transistor and diode are both crucial to implementing the "logic gates" that underlie digital computing; these gates deliver a specified output based on one or more input signal and can be built up into complex programmable circuits. However, Li's phononic transistor and logic gates have not yet been implemented in experiments.

Another option is to implement phononic logic without traditional diodes and transistors. Hiroshi Yamaguchi at NTT Basic Research Laboratories in Kanagawa, Japan has designed logic gates using materials called nanoelectromechanical resonators, which can convert electrical signals into motion and vice versa. In addition to implementing digital logic, such devices could take electronic information and store it in mechanical vibrations, leading to phononic memory.

In a different approach, Pierre Deymier at the University of Arizona in Tucson has designed specialized crystals that can manipulate incoming sound waves to yield predictable output. Such "phononic crystals" are structures with one material forming a three-dimensional framework inside another, designed to vibrate at certain frequencies and not others.

Devices like Deymier's and Yamaguchi's are so far too large and slow to compete with today's computers.

"We work on large things first because they're easy to make," said Deymier. But the scientists say that these advances can show the way forward and prove that, at least in principle, phononic computing is possible.

Going quantum

In additional to building components resembling those of conventional computers, researchers like Deymier and Li are seeking to use phonons to make a new kind of device: a quantum computer. In quantum computing, information can be represented as 0 or 1, but also as a combination of 0 and 1. This possibility provides a major advantage for certain kinds of problems that require doing a large number of calculations simultaneously. A quantum phononic computer could help speed up analysis of large datasets, for example, said Sklan.

To harness phononic quantum information, scientists are studying the possibility of using nanotubes, which can vibrate at different frequencies simultaneously. But sustaining those sorts of organized vibration involves major challenges. Thus far researchers can make them last only for tiny fractions of a second, and even then only at very low temperatures.

Charles Tahan and colleagues at the Laboratory for Physical Sciences in College Park, Maryland have taken a different tack, and designed a scheme that uses phonons to transfer quantum information stored in other forms.

Because of these difficulties, Sklan thinks that a future phononic quantum computer will most likely be used in conjunction with other kinds of computing devices, perhaps to store information that's been processed in another way.