Chip impurities make quantum bits

By Eric Smalley, Technology Research News

It's difficult to make a semiconductor computer chip that is pure. Usually for every few billion or so semiconductor atoms there's an unwanted atom of some kind. These impurities are little more than a nuisance to chip makers, but they could become the key to phenomenally powerful quantum computers.

Researchers based at the University of California at Santa Barbara have demonstrated that individual electrons associated with these impurities can serve as quantum bits, or qubits. The research opens a route to solid-state quantum computers that would be compatible with today's semiconductor manufacturing processes.

The researchers made qubits by firing intense, high frequency lasers at electrons of donor atoms, according to Mark S. Sherwin, a professor of physics at UC Santa Barbara.

A donor is an atom of a different element that has one more valence electron than the atom it replaces, he said. Electrons reside around an atom's nucleus in specific bands or orbitals; valence electrons reside in the outermost band.

"If a silicon atom substitutes for a galium atom, three of the silicon's four valence electrons will be tied up in bonds to neighboring [galium] atoms, but the fourth will be left over with nowhere obvious to go," Sherwin said.

The laser drives the electron from its ground, or low-energy, state to a higher energy state. The two states can be used to represent the 0 and 1 of binary computing. The electron then oscillates between the two states and during this oscillation the electron enters the quantum state of superposition in which it is in both states at the same time.

Quantum computers hold the promise of being faster than the most powerful possible ordinary computer for certain applications like cryptography and database searches. The power of a quantum computer comes from manipulating many qubits in superposition at once, thereby processing at the same time all the possible numbers those qubits represent.

The researchers are working on containing the donor atoms' electrons in quantum dots or other structures in order to preserve the electrons and separate them from each other, said Sherwin. In the current setup, the electrons can be freed with relatively little energy, and the number of donor atoms means the electrons are on average about 200 nanometers apart, which makes it difficult to address each one individually, he said.

The researchers also plan to drive the electrons to a different higher energy state because the one they used in the experiment is relatively unstable, allowing for only one oscillation between the high and low energy states, said Sherwin. The researchers will need the superposition of states to last long enough to perform the thousands of operations necessary to implement a quantum algorithm.

"Using the qubits in our present experiment, I don't think we could perform any quantum algorithms," said Sherwin. "We know things will get much better, but it is difficult to predict how much better."

It will be at least 10 to 20 years before practical quantum computing applications are developed, said Sherwin.

Sherwin's research colleagues were Bryan E. Cole, Jon B. Williams and B. Tom King of the University of California at Santa Barbara and Colin R. Stanley of the University of Glasgow. They published the research in the March 1, 2001 issue of Nature. The research was funded by the Army Research Office and the Defense Advanced Research Projects Agency (DARPA).

Timeline:   10 to 20 years
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Coherent manipulation of semiconductor quantum bits with terahertz radiation," Nature, March 1, 2001


March 14, 2001

Page One

Tools cut data down to size

Ribbons expand nanotech toolbox

Silicon cages metal atoms

Surfaces channel liquids

Chip impurities make quantum bits


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