Logic scheme gains power

By Eric Smalley, Technology Research News

Researchers from the University of Notre Dame have pushed an alternative computer chip architecture a step forward by finding a way to refresh the short-lived signals the scheme uses to represent the 1s and 0s of digital information.

The architecture promises to provide computer circuits that are faster and use much less power than today's transistor-based chips.

The researchers' quantum dot cellular automata scheme carries out the logic of computing using the positions of individual electrons trapped in quantum dots rather than large numbers of electrons flowing through transistors. The drawback to having no current flow through a device, however, is current cannot be used to boost signal strength.

Key to the plan was figuring out a new way to boost fading signals.

Like conventional chips, the researchers' device must refresh its signals because electronic circuits are not 100 percent efficient. "Digital signals lose part of their energy as they move along," said Ravi Kummamuru, a researcher at Notre Dame. "Just like a voice signal cannot be heard between two points a mile apart unless it is repeated or amplified every 100 meters... a digital signal has to be amplified at regular intervals along a circuit," he said.

The researchers' instead found a way to use the computer clock signal to restore signal strength, said Kummamuru. A computer clock sends a signal through all of a computer's circuits to synchronize them, which makes it possible to construct complicated, sequential logic operations using combinations of relatively simple circuits.

Under the researchers' architecture, electrons can be trapped in quantum dots, individual molecules, or metal tunnel junctions. Quantum dots are microscopic specks of semiconductor material, and metal tunnel junctions are circuits that contain infinitesimal gaps.

The researchers tested their architecture using tunnel junctions with aluminum islands that trap electrons. Each island was about 50 nanometers wide, or the width of 500 hydrogen atoms, and several thousand nanometers long.

The researchers arranged four islands into a square and used the resulting cell to trap a pair of electrons. The Coulomb force, which causes particles that have the same type of charges to repel each other, forced the negatively-charged electrons into opposite corners of the square. The two possible opposite corner positions can represent the binary numbers 0 and 1.

Quantum tunneling allows the electrons to switch corners, changing the state of a cell from 0 to 1 or 1 to 0. Tunneling is a weird quantum phenomenon in which an electron disappears and reappears on the other side of an otherwise impenetrable barrier.

The Coulomb force also causes a cell to affect the state of the cell next to it. This allows a bit of information to be transmitted through a line of cells. The researchers placed an additional island between each pair of islands in the cell to latch each bit. When an electron is trapped in a middle island, it blocks the electrons in the corner islands from switching corners, said Kummamuru.

The clock signal triggers the latch, and the latch keeps the electrons from tunneling for longer than a clock cycle, meaning the bits of information survive long enough to be affected by the next clock cycle.

The researchers measured the energy flow through a latch, and found that the power gain was enough to trigger the next latch in turn, and thus restore the entire system's logic, according to Kummamuru. "In quantum dot cellular automata devices, which use neither transistors nor supply lines, power gain can still be achieved by using power from the clock signal," he said.

In previous work, the researchers figured out how to use various arrangements of cells to make digital circuits that carry out the basic logic of computing. Quantum dot cellular automata logic has counterparts for all of the basic digital logic elements available in traditional transistor-based systems, Kummamuru said.

Circuits formed by tunnel junctions, quantum dots or molecules can be considerably smaller than circuits formed by the transistors in today's chips, and the smaller they are, the more efficient they are; the opposite holds for conventional transistors, said Kummamuru. "Quantum... devices will improve as they become smaller," he said.

The scheme faces several challenges that must be overcome before practical devices are possible, said John C. Lusth, an associate professor of computer science at the University of Arkansas. "While I have no doubt [the scheme] will work as the Notre Dame researchers envisioned, I do wonder about the utility of this approach," he said.

The major problem with using metal tunnel junctions is "there are wires running to every cell, so [the scheme] is size-limited just like conventional transistor logic," said Lusth.

Compounding the problem is that the researchers' device requires extremely low temperatures. Although the researchers "talk about molecular implementations [that] could possibly compute at room temperatures... the clocking logic as currently envisioned cannot be made that small without deleterious quantum-mechanical effects," he said.

It will be 10 to 20 years before practical devices can be made using the scheme, said Kummamuru.

Kummamuru's research colleagues were John Timler, Geza Toth, Craig Lent, Rajagopal Romasubramaniam, Alexei Orlov, Gary Bernstein and Gregory Snider. The research appeared in the August 12, 2002 issue of Applied Physics Letters. The research was funded by the Defense Advanced Research Projects Agency (DARPA), the Office of Naval Research (ONR), the W. M. Keck Foundation, the National Science Foundation (NSF) and Intel Corporation.

Timeline:   10-20 years
Funding:   Government; Private; Corporate
TRN Categories:   Integrated Circuits; Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "Power gain in a quantum-dot cellular automata latch," Applied Physics Letters, August 12, 2002


February 12/19, 2003

Page One

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Scheme smooths parallel processing

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Logic scheme gains power


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