It may be possible within the next ten years to build high-speed computers that run on small batteries rather than depending on a large and constant source of electricity.

As reported recently in Science magazine, researchers at the University of Notre Dame are working on a method of building circuits that need only a tiny initial electrical charge to transfer information. In a domino-like effect, the charge of the first cell in the circuit charges all the cells that follow. This could result in the development of tiny, powerful computers that run for extended periods of time on little power.

Today, microprocessors are composed of field-effect transistors (FETs), through which current flows. The presence of voltage represents a 1; the absence of voltage represents a 0. Thus, the transistor acts as a switch. FETs have maintained incredible performance despite great reductions in size, but miniaturization has its limits with this technology. An alternative approach that can compute at an ever smaller scale is quantum-dot cellular automata (QCA).

Diagonals rule
The basic concept of QCA is the creation of four quantum dots, little islands of metal that can capture electrons. When four of these islands are organized into a square and two extra electrons enter the square, the electrons repel each other--due to a phenomenon known as electrostatic repulsion--and position themselves across one diagonal or another. The diagonals correspond to the binary 0 or 1 just as the lack of electrical charge and the presence of charge correspond to 0 and 1 in existing microprocessor technology.

When a line of these cells is formed, polarization from one cell causes the neighboring cell to switch to that polarization. When organized in a line, these cells can transmit information by polarizing only the first cell, which then polarizes all the other cells. Information can thus be transferred without any electrical current coaxing it along. As a result, the process works using very little energy.

Scientists hope eventually to build a computing architecture using lines of cells like conventional wires in a microprocessor. The Notre Dame researchers put the theory into practice recently by creating a simple digital logic gate--a fundamental building block for microprocessors--using quantum dots.

Beyond Moore's Law
"Moore's Law will eventually run its course, and computer miniaturization as we know it may need to be replaced by something new," says Islamshah Amlani, a member of the Notre Dame research group, referring to the prediction that chip density will double every 18 months. "These new methods may provide an answer as quantum dots can theoretically be shrunk down to the molecular size. The advantage is that the new devices will actually to be able to perform the same things that we do today at a much faster speed and in much smaller packages. They will also save tremendous amounts of power with potentially 1,000 times less power dissipation than current devices which often require fans to cool down the chips," he says.

It'll take quite a while for quantum dot technology to land on your desktop. "We're a long way from this being a realizable technology," says Charles Smith, lecturer at the University of Cambridge physics department. "Right now, the circuits only work at just above absolute zero or -272.9 degrees Centigrade. The other major stumbling block is the degree of accuracy required to make these cells. If there's any slight defect and they're stuck in the wrong place, the whole process stops," he says.

Smith also points out that existing microprocessor techniques will continue to be used until at least 2010 or 2015. Smith is currently working on experiments similar to those at Notre Dame. As the director of a university start-up company called Cavendish Kinetics, he's developing technology that uses small, micromechanical switches, only a few microns square in size. The switches have applications in the mass storage market, particularly flash memory.

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