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
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
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
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
TRN Categories: Quantum Computing and Communications; Integrated
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
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