Atom clouds ease quantum computing

By Eric Smalley, Technology Research News

Computers that use the internal properties of atoms to perform calculations promise to solve problems that will always be impossible for classical computers, which compute using electrical current running through transistors made up of millions of atoms.

One of the challenges of building a quantum computer, however, is controlling matter and energy at the level of individual atoms and photons. First, these particles are fantastically small. The difference in size between a hydrogen atom and a ping pong ball is about the same as the size difference between a ping pong ball and the Earth. Add the complication that particles vibrate and flit about and it's not hard to see why it's so difficult to isolate and control them.

Researchers at Harvard University, the University of Kaiserslautern in Germany, the University of Connecticut and the University of Innsbruck in Austria have sidestepped the problem with a scheme for building quantum computers out of clouds of atoms.

"We do not need to control atoms one by one," said Mikhail Lukin, an assistant professor of physics at Harvard University.

Atoms act like tiny tops that spin either clockwise or counterclockwise. These two spin states can represent the ones and zeros of computer logic. Researchers can flip the value of these quantum bits, or qubits, between one and zero by switching the spin of the atom with a laser beam or magnetic field.

Atoms also contain magnetic fields with North and South poles that, like ordinary refrigerator magnets, either attract or repel each other. In both refrigerator magnets and atoms, these interactions cause the magnetic field around each magnet or atom to stretch. Atoms with stretched poles interact more strongly with other atoms.

When these dipole atoms are polarized, or lined up magnetically, they form a dipole blockade, said Lukin. "The interactions are so strong that not more than one single spin can be flipped in an entire atomic cloud. In this situation an entire small atomic cloud can behave as a single quantum bit," he said.

These atomic clouds are easier to work with than single atoms, and the quantum states of the atom clouds last for several seconds, which is long enough to perform the thousands or millions of individual operations needed for practical computing. The quantum states of individual particles, in contrast, usually last only thousandths or millionths of a second.

A second challenge in making quantum computers is finding a way to transfer information from atoms to photons and back again in order to use the more mobile photons to transmit information. The larger target of a whole cloud of atoms should make this transfer easier to accomplish, said Lukin.

The atom cloud scheme can be used in a range of hardware that has been developed to corral individual atoms, including semiconductor devices and ions held in magnetic traps, according to Lukin.

A full-scale quantum computer is at least two decades away, according to many researchers in the field. "Whereas some minor applications could become technologically relevant within [a] five- to ten-year time-frame, a discussion of practical, full-scale quantum computers is very premature," said Lukin.

Even with the advantages of using clouds of atoms, the researchers' scheme may not lead to full-scale quantum computers because it uses light to link qubits, said Jonathan P. Dowling, supervisor of the quantum computing technologies group at NASA's Jet Propulsion Laboratory. "You have this limit that the light beams can't be any smaller than the wavelength of the light, and that's pretty big," he said.

Practical quantum computers would consist of hundreds of thousands or millions of qubits, said Dowling. "A scalable quantum computer, in my opinion, is not likely with these optical schemes," he said.

The scheme could be used for quantum communications repeaters, however, said Dowling. Repeaters, which boost fading communications signals, are what allow today's conventional communications lines to span long distances. Quantum communications, which carry information in specially prepared photons, would also require a series of repeaters in order to pass signals over long distances.

Lukin's research colleagues were Michael Fleischhauer of the Harvard-Smithsonian Center for Astrophysics and the University of Kaiserslautern in Germany; Robin Cote of the University of Connecticut; and Luming Duan, Dieter Jasch, Ignacio Cirac and Peter Zoller of the University of Innsbruck in Austria.

They published the research in the July 16, 2001 issue of the journal Physical Review Letters. It was funded by the Austrian Science Foundation, the European Union, the European Science Foundation, and the National Science Foundation (NSF).

Timeline:   5-10 years; Unknown
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Dipole Blockade and Quantum Information Processing in Mesoscopic Atomic Ensembles," Physical Review Letters, July 16, 2001


January 16, 2002

Page One

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Atom clouds ease quantum computing

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