Apr. 13, 1999 A functioning logic gate -- the most basic element of digital computers -- that is based on a transistorless approach to computing called quantum-dot cellular automata (QCA) is reported by University of Notre Dame researchers in the April 9 issue of Science.
According to Greg Snider, assistant professor of electrical engineering, who headed the research team, QCA is an effort to bring information storage down to the molecular level. In QCA technology, digital data is encoded in the positions of only two electrons.
Conventional microelectronic technology has relied on shrinking transistors to produce increasingly smaller, faster and more powerful computers. But, because the laws of physics prevent conventional devices from working below a certain size, that method is nearing its physical limits.
QCA leapfrogs that barrier with an entity known as the "quantum dot," a tiny structure in which an electron can be confined, explains Snider. These quantum dots can be created and arranged into cells through microelectronic techniques, and in turn these cells can be lined up end to end to form "binary wires" or arrayed to form switches and various computer logic devices.
If successful, a future 1 cm square QCA chip could contain as many as 1 trillion devices, as opposed to the 6 million devices in the most advanced conventional chip. And since it does not rely on flowing electrons to transmit a signal, a large electric current is not needed and heat problems are avoided.
In an earlier demonstration of a basic cell reported in Science in August, 1997, a single electron was used for the first time to control the position of another electron.
In this experiment, the logic gate consists of a cell, composed of four quantum dots connected in a ring by tunnel junctions, and two single dot electrometers. The device was operated by applying inputs to the gates of the cell, and the logic AND and OR operations was verified by using the electrometer outputs.
The experiment demonstrated that the logic gate output characteristics conform well to theoretical simulations.
Other members of the research team include Gary H. Bernstein, professor of electrical engineering, and Craig S. Lent, professor of electrical engineering.
This research is funded by Defense Advanced Research Projects Agency (DARPA)/Office of Naval Research (ONR).
A new project recently was funded by DARPA to explore molecular-sized (rather than quantum-sized) QCA cells. These hold the promise of high-temperature operation and extremely high device density. This new project is a collaboration of research teams in Notre Dame's chemistry and electrical engineering departments.
Editor's note: A web page containing more information about Quantum-dot Cellular Automata is located at http://www.nd.edu/~qcahome/
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