Electron waves compute

By Eric Smalley, Technology Research News

Give an electron two paths to get to one location and it will usually take both. This fact of quantum physics plays a leading role in a computer architecture that could replace today's chip technology when it reaches its limits in a decade or so.

According to the laws of quantum physics, electrons are waves as well as particles. Like ocean waves, where two crests meet they reinforce each other and where a crest and trough meet they cancel each other out. Researchers at University of Missouri at Rolla have devised a scheme for using electron wave interference to represent the ones and zeros of digital computing.

Traditional electronic computers use combinations of transistors, which are tiny electronic switches, as the logic units that perform the binary arithmetic at the heart of digital computing. Electron wave computers would use networks of microscopic wire rings that form the two paths for the electron waves to follow, said Cheng-Hsiao Wu, a professor of electrical and computer engineering at the University of Missouri at Rolla.

"You do not need transistors to control the flow of charge if all the devices involved are very small and at low temperature," said Wu.

The researchers' proposal involves using modified forms of Aharonov-Bohm rings, which are used in basic physics research, to form the logic gates of computers. Aharonov-Bohm rings are circles of extremely thin wire and are commonly made several times smaller than a red blood cell. Due to their wave nature, electrons entering the Aharonov-Bohm rings travel in both directions at once, meeting -- and reinforcing each other -- at the other end.

Using a magnetic field perpendicular to the ring, researchers can speed up or slow down the electron wave traveling in one side of the ring, throwing the waves in the two sides out of sync and causing the waves to cancel each other out when they meet at the other end. The reinforced waves and the canceled waves could represent the ones and zeros of computing, according to Wu.

Aharonov-Bohm rings have an input and an output terminal. The researchers' scheme calls for making three- and four-terminal Aharonov-Bohm rings. Their work shows that three-terminal rings could be combined to form IF-THEN, XOR, OR, AND and INVERTER logic units. These logic units could, in turn, be combined to form half adders and full adders. A half adder adds two binary numbers but cannot carry, and a full adder includes the carry function.

A single, four-terminal Aharonov-Bohm ring could also be used as a half adder, said Wu. "It replaces eight transistors for the same function." And two connected four-terminal Aharonov-Bohm rings could serve as a full adder. "This replaces about two dozen transistors in traditional microelectronic circuits," he said.

In addition to the potential for making smaller, and therefore faster, computer circuits, electron wave computers could solve certain problems faster than even the fastest ordinary computer by examining all of the possible solutions to a problem at once, according to Wu.

Electron wave interference could be used to make massively parallel processing computers, he said. "Millions of inputs enter a large network [of rings] simultaneously with desirable outputs when the waves arrive at the output terminals. This is similar to optical computing."

Optical computers use light waves that reinforce and cancel each other out. Last year, researchers at the University of Rochester demonstrated an optical computer running a quantum search algorithm.

The electron wave scheme is an idea worth trying, said Ian Walmsley, a professor of experimental physics at the University of Oxford and a professor of optics at the University of Rochester. "The nice thing about electrons is that [their] wavelengths are inherently smaller than optical wavelengths, so the whole machine can be smaller. At present I see the advance as a technical one rather than a fundamental one," he added.

"It's a very neat idea but... completely theoretical," said Mike Lea, a professor of physics at the University of London. "I'd be quite skeptical about claims without at least some analysis of the likely practicalities based on real experiments," he said.

The researchers are working out the physics for larger networks of Aharonov-Bohm rings, said Wu. "I would like to convince experimentalists elsewhere to simply extend the original Aharonov-Bohm effect to three or four terminals. I promise nice results will come out of such a simple extension," he said.

Given that today's semiconductor technology is likely to reach its limits by the year 2015, researchers and engineers should have a good idea of how to build devices smaller than 10 nanometers by then, said Wu. At that point, electron wave computing could be a contender for the next generation computer architecture, he said.

Wu's research colleague was Diwakar Ramamurthy. They published the research in the February 15, 2002 issue of the journal Physical Review B. The research was funded by the university.

Timeline:   13 years
Funding:   University
TRN Categories:   Quantum Computing and Communications; Integrated Circuits
Story Type:   News
Related Elements:  Technical paper, "Logic Functions from Three-Terminal Quantum Resistor Networks for Electron Wave Computing," Physical Review B, February 15, 2002


April 3/10, 2002

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