Quantum current closer to computing

By Kimberly Patch, Technology Research News

One way to significantly improve computers is to use something other than the presence or absence of electric current to signal the ones and zeros that form the binary logic of computing. One promising alternative takes advantage of the quantum nature of electrons.

Spintronics is an emerging field that uses the spin of electrons to represent ones and zeros. Electrons spin in one of two directions, up or down, which is roughly analogous to a top spinning clockwise or counterclockwise.

In theory, these two states of an electron would allow for ultra low-power conventional computers and would provide the means for moving information within and between quantum computers. Proposed schemes for quantum computers use atoms or subatomic particles to represent ones and zeros and use quantum mechanics to check every possible answer to a problem at the same time.

In practice, there are many details to be worked out.

In order to use electron spin to signal a one or zero, the spins of a group, or current of electrons have to be aligned, and this collective spin must survive the electrons' transfer from one transistor to another and then last long enough to be useful.

Researchers from the University of California and Pennsylvania State University have moved spintronics a significant step forward by demonstrating that it is possible to efficiently move a current of electrons, with their collective spin intact, from one semiconductor material to another. In addition, the research shows that the spin state can be made to last as long as 100 nanoseconds, which is long enough to work for traditional computing.

"We have shown that spin lifetimes can exceed 100 nanoseconds and can be transported over distances exceeding 150 microns. In both cases, this exceeds the time and length scales used in today's technology," said David Awschalom, a physics professor at the University of California at Santa Barbara.

That this was fairly easy to accomplish surprised even the researchers. The implication of the results is that it should be possible to fabricate spin transistors, said Awschalom.

To investigate how practical using electron spin for computing could be, the researchers measured the spin of a current of electrons that was moving from a gallium arsenide semiconductor to a zinc selenide semiconductor. "I thought that this would be the simplest laboratory in which to test the basic idea: an atomically clean interface between two well-studied semiconductors," said Awschalom.

The researchers started by using polarized laser beams to create in a layer of gallium arsenide a reservoir of electrons whose spins were aligned. Ordinarily, only a small number of electrons from this reservoir would cross the barrier to a layer of zinc selenide and their spins would become random within a few hundred picoseconds, or trillions of a second. The researchers found that applying an electric field increased the number of electrons crossing the barrier by 40 times and also boosted the lifetime of the spins to usable levels.

Ultimately, the researchers hope to use electron spins for high-density information technology and fundamentally new methods of information processing like quantum computation, said Awschalom. If practical quantum computers can be built, they would be phenomenally fast for solving certain problems like cracking codes and searching large databases.

The experiments are something of a milestone in the spintronics field, said Jay Kikkawa, an assistant professor of physics and astronomy at the University of Pennsylvania.

In the experiments, the spin of the electron acts as an identification tag, said Kikkawa. "Its response to a magnetic field reveals the electron's magnetic history, which, in part, includes how long spins have spent in different layers," he said.

The researchers use this information to distinguish among several different channels within a spin current flowing across an interface between materials. "It's a very clever trick that one could never pull off in a purely electrical system," Kikkawa said. This is because electrical current consists of electric charge and the spins of its electrons are random.

Electron spins could be used in computing within the decade, Awschalom said.

Awschalom's research colleagues were Irina Malajovich of the University of California at Santa Barbara, and Joseph J. Berry and Nitin Samarth of Pennsylvania State University. They published the research in the June 14, 2001 issue of the journal Nature. The research was funded by the Defense Advanced Research Projects Agency (DARPA), the Office of Naval Research (ONR) and the National Science Foundation (NSF).

Timeline:   < 10 years
Funding:   Government
TRN Categories:  Semiconductors and Superconductors; Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Persistent Sourcing of Coherence Spins for Multifunctional Semiconductor Spintronics," Nature, June 14, 2001 were.


September 5, 2001

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