Computing with charged quantum-dot arrays.

To add two digits, a conventional computer chip must shuffle vast numbers of electrons through an array of transistors. Devices requiring far fewer electrons for such operations offer potentially faster computation with greatly reduced size and energy consumption.

Now, a team of researchers has proposed a novel scheme for harnessing individual electrons, confined to tiny structures known as quantum dots, to encode information and perform addition and other operations involving computer logic. Craig S. Lent, P. Douglas Tougaw, and Wolfgang Porod of the University of Notre Dame in Indiana described the scheme at a workshop on physics and computing held last month in Dallas.

The research builds on recent advances in the fabrication of microscopic semiconductor "boxes," whose walls keep electrons confined to a small region of the material. Researchers have been able to detect and, to some degree, control the number of electrons trapped in these quantum dots (SN: 2/20/93,p.118). New experiments have also shown that electrons can tunnel from one quantum dot to another if the two dots are sufficiently close together.

Lent and his colleagues base their scheme on a unit, or cell, consisting of five quantum dots. With one dot in the center and one at each corner of a square, electrons can readily tunnel between any two of these sites.

Supplied with two electrons, a cell settles into one of two possible configurations in which the electrons occupy dots on opposite corners of the cell (see diagram). These orientations represent two distinct cell states.

Strung together into rows and other patterns, such cells serve as the building blocks of the logic circuits required for computation. The cells are sufficiently far apart that practically no electron tunneling occurs between dots of adjacent cells, but electrical forces between the cells enable one to influence another.

Thus, it's possible to create a "binary wire" made up of a row of cells, all initially in the same state. Switching the cell state at one end of the string causes a chain reaction that flips the states of the remaining cells one by one.

Lent and his coworkers have also created more complicated configurations that correspond to various logical devices. Their calculations and computer simulations, which involve quantum theory, suggest that such devices would function as required.

The researchers are now looking into the feasibility of fabricating these quantum-dot arrays. "We think we can make this work," Lent says.

How well the resulting devices function will depend on how easy it is to set the initial states of the cells and to detect a device's final output states. Moreover, it isn't clear yet what influence stray electrical charges in the surrounding material and other defects may have on these arrays.

Nonetheless, this scheme represents an interesting case of potentially useful quantum computing that exploits the emerging technology of quantum-dot fabrication, the researchers conclude. On nanometer scales, coding information in quantum-dot states provides an attractive alternative to the standard approach of using currents and voltages.