Profs Perform Atomical ‘Magic’

Physicists at Harvard have discovered a “quantum-mechanical magic trick” by finding a way to stop a pulse of light in one part of space and make it reappear two tenths of a millimeter away, according to researchers at the University’s Hau Laboratory.

Led by Mallinckrodt Professor of Physics Lene V. Hau, the experiment involved firing lasers through two clouds of sodium atoms cooled to a few billionths of a degree above absolute zero.

At such cold temperatures the atoms’ motions are virtually halted, and they begin to behave more like waves than particles.

Featured in the Feb. 8 issue of Nature, the study showed that once a laser pulse is shot into one sodium cloud—known as a Bose-Einstein condensate (BEC)—it is signficantly compressed and deccelerated.

“The light pulse is slowed from 186,000 miles per second to 15 miles per hour, and is also spatially compressed from 1 kilometer to less than half the width of a hair,” Hau wrote in an e-mail.

From the beginning of the experiment, a control laser is continually fired through the BECs. When the experimental laser pulse enters the first BEC traveling in the opposite direction of the control laser, a matter imprint is made and both lasers are manually shut off.

Even with both lasers discontinued, the matter imprint continues to travel at 700 feet per hour through the cloud, exiting the first BEC, continuing on through free space, and ultimately entering the second BEC.

The control laser is then rekindled and the laser pulse is revived and exits the second BEC at its original speed of 186,000 miles per second, Hau explained.

According to Hau, who collaborated with research assistant Naomi S. Ginsberg and post-doctoral fellow Sean R. Garner on the study, matter is easier to manipulate than light, which implies many future applications of their discovery.

When the light pulse is in a matter state between BECs, “we can grab it with a laser beam—put it on the shelf so to speak—and later let it back on its way and revive it in the second BEC,” Hau said. During this “shelved” period, the matter wave can be manipulated by physicists and will preserve any changes when it is revived again as a laser.

In particular, these maniuplations can potentially be used in the processing and encoding of optical information, Hau said.

Data could be sent along optical fibers, much like what many internet service providers use to facilitate internet communication, and be manipulated in the process by using the sodium clouds.

According to Hau, the system could also be applied to quantum information networks.