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Harvard Quantum Teams Stop Light

By Jonathan H. Esensten, Crimson Staff Writer

Two years ago, researchers merely slowed light down to a pedestrian speed of 40 m.p.h. But now two Harvard research groups have independently brought light to a complete stop.

The latest experiment, which could pave the way for super-fast quantum computing, successfully brought light to a complete stop for half a millisecond. The normal speed of light is 186,000 miles per second.

The breakthroughs will be published in Nature next week and in Physical Review Letters on Jan. 29. Technology based on the discovery could be used in the future to make super-fast quantum computers or to send messages that are completely secure from hackers.

"The idea of quantum information is in its infancy," said David F. Phillips, an associate of the Harvard College Observatory at the Harvard-Smithsonian Center for Astrophysics and co-author of one of one of the studies. "We hope for wonderful things. Our imagination hasn't figured out what the possibilities are."

Phillips cautioned, however, that scientists are still "a long way" from using the discovery for practical applications such as quantum computing.

Phillips worked with the team led by Associates of the Harvard College Observatory Ronald L. Walsworth and Mikhail D. Lukin. Their paper will be published in Physical Review Letters. Another group, led by Associate of the Department of Physics Lene V. Hau, is publishing a similar study next week in Nature.

The new research is based in part on work by Hau published two years ago in Nature, in a study which brought light down to about 40 m.p.h.

The current study slowed light to a stop by shining it on super-cooled gaseous rubidium atoms. The atoms were bathed in laser light while a second beam was shined through them. By adjusting the intensity of the beams, the "fingerprint" of the second beam was imprinted on the electrons of the super-cooled atoms. In that state, the light was standing still.

That "quantum fingerprint" was then converted back into light by again shining a laser on them. Usually, any information about light is destroyed when its photons are absorbed by atoms. In these studies, the information was preserved.

"We're writing the information about the pulse into the atoms and at a later time we're reading the information out," Phillips said.

According to Phillips, the information about the light could in principle be stored for as long as a second. He said future research might focus on doing the reverse of the current study--reading the quantum states of atoms with light.

The ability to store and retrieve such quantum information would be an important step toward developing quantum computers, which unlike normal computers, could use bits that can exist simultaneously as zero and one and would be immensely more powerful than purely digital computers.

Hau could not be reached yesterday for comment.

--Staff writer Jonathan H. Esensten can be reached at

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