Stored light altered

By Eric Smalley, Technology Research News

Controlling interactions among individual particles of light and matter could give rise to phenomenally powerful quantum computers and devices that provide perfectly secure communications.

Quantum computers will need to transfer information stored in photons, which are easy to transmit, and atoms, which is easier to use for calculations.

Researchers at the Harvard-Smithsonian Center for Astrophysics have taken their second step this year toward this goal. In January, they brought a light pulse to a halt inside a chamber of gas atoms, stored an imprint of the pulse in the atoms and then reconstituted the pulse. Now they have figured out how to alter the light information as it is stored in the group of atoms.

This is possible because the process preserves the phase of the stored light, said Phillips. "The phase of the light is transferred onto the phase of the atoms and back to the light during the light storage process," he said.

This phase information can represent the ones and zeros of computing.

The phase of a lightwave corresponds to its position in the cycle between the crest and trough. Individual photons also contain wave phase information.

An atom's phase is different. It is "related mathematically to the phases of a child's top or a gyroscope as it rotates on its axis and precesses," said Phillips. If you set a top spinning on a post, then tip the top onto its side, instead of falling off the post it will hang there sideways, rotating, or precessing, around the post. The phase of a precessing top is its position in the circle it makes as it travels around the top of the post.

The researchers found that the phase information of the light pulse remains stable and accessible when it is imprinted in the atoms: if the light pulse is in one phase when it is stored in the atoms, the pulse remains in that phase when it is restored.

This makes it possible to change the phase while the pulse information is stored. "We can apply a magnetic field to our atoms during the storage process to shift the phase of the atoms," which in turn changes that phase of the reconstituted light, said Phillips.

So far the researchers have only stored ordinary light beams using the technique. However, demonstrating control over the phase of the light opens the door for using the technique to coax the quantum properties of particles to do computing.

Being able to store and manipulate particle properties like phase paves the way for building devices that store and transmit this quantum information. Quantum repeaters, for example, could restore the quantum information in photons, which begins to destabilize after traveling 10 kilometers or so through fiber-optic communications lines. Like repeaters in conventional computer networks, quantum repeaters would make it possible to send quantum information over much longer distances. Phillips' Harvard colleague Mikhail Lukin and researchers at the University of Innsbruck in Austria have designed a quantum repeater based on the light storage technique.

Many researchers say it is likely to take decades for full-blown quantum computers to become practical. It may be possible to use quantum information for cryptography sooner, however, said Phillips. "The light storage technique could prove useful as part of a quantum repeater in such a system. I would be surprised if the techniques involved in stored light moved out of the academic lab and into the development lab in less than five years, though," he said.

The researchers' next step is using the technique to store the quantum information from a single photon, said Phillips.

Phillips' research colleagues were Lukin, Alois Mair, Jean Hager and Ronald L. Walsworth of the Harvard-Smithsonian Center for Astrophysics. The research was funded by the National Science Foundation (NSF), the Office of Naval Research (ONR) and NASA.

Timeline:   >20 years
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Phase Coherence and Control of Stored Photonic Information," posted on the arXiv physics archive at; Technical paper, "Long-Distance Quantum Communication with Atomic Ensembles and Linear Optics," posted on the arXiv physics archive at


November 14, 2001

Page One

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Disappearing links shape networks

Stored light altered

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Evolution optimizes satellite orbits


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