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Electronic Design

Researchers Stop Light Dead In Its Tracks—And Start It Again

Photons are the fastest and most robust carriers of information, but they are difficult to localize and store. Yet experimenters have successfully brought a light pulse to a full stop, trapping it in a vapor of rubidium (Rb) atoms. The researchers then stored it for a controlled period of time and released it, finally sending it on its way.

This astonishing achievement was announced by David Phillips, Annet Fleischhauer, Alois Mair, Ronald Walsworth, and Mikhail Lukin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., in the Jan. 29 issue of Physical Review Letters. This development could vastly increase speed in computers and dramatically improve security in communications.

This "light-storage" technique is based on a recently demonstrated phenomenon of ultraslow light group velocity, which is enabled by electromagnetically induced transparency. During the experiment, the researchers effectively decelerated and trapped a light pulse in a vapor of Rb atoms, stored it for a controlled period of time, and then released it on demand. As they report in the article, they accomplished this "storage of light" by dynamically reducing the group velocity of the light pulse to zero.

The team first slowed and then stopped the light in the atomic Rb vapor at temperatures of 70oC to 90oC. In this medium, the light dimmed until it slowed and then stopped. By flashing a second light through the gas, the team brought the original beam back to life.

During the process, two beams of light strike a vapor of Rb atoms, altering them so they do not absorb light as they normally would. This lets the first beam imprint a pattern in their spin orientation. As the second beam is turned down, the first beam slows to a halt and virtually disappears as it struggles to alter the atoms' spin pattern. The first beam can then be revived as the second beam is turned back up (see the figure).

A considerable spatial compression is associated with slow light. With this compression, a signal pulse can be almost completely localized in the atomic medium. As the signal light propagates, the atoms are driven into a coherent superposition of hyperfine states that is strongly coupled to the light.

A key feature of this light-storage method is its nondestructive nature. Quantum computers could use this phenomenon to breeze through certain operations far faster than existing computers. Eavesdropping would be impossible, too. Light is needed to configure such large networks, but this is difficult without temporary storage of light. That's where this innovation may very well assume a major role.

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