Physicists at Harvard University have created a pulse of light that contains photons, is compressed to fit within several centimeters of space, and does not travel. The finding builds upon earlier demonstrations of “stored light” by halting actual photons, not just their signature.

“We demonstrated how to hold a light pulse still without taking all the energy away from it,” says Mikhail D. Lukin, assistant professor of physics at Harvard. The research by Lukin, graduate student Michal Bajcsy, and research associate Alexander S. Zibrov of Harvard’s Department of Physics is described in the Dec. 11 issue of the journal Nature.

In the present experiment, which follows a theoretical proposal published last year by Lukin and Harvard graduate student Axel André, researchers fired a short signal pulse of red laser light into a sealed glass cylinder containing a hot gas of rubidium atoms illuminated by a strong control beam. While the pulse was traveling through the rubidium gas, they switched off the control beam, resulting in the storage of a holographic imprint of the signal pulse on the rubidium atoms.

Instead of using a single control beam to re-create and release the signal pulse, as was done in earlier experiments, the Harvard team used two counterpropagating control beams. Besides re-creating the signal pulse, the two control beams generate a standing-wave pattern of dark and bright regions. This light pattern makes the atoms behave like a stack of tiny mirrors. As the re-created signal pulse tries to propagate through this medium, the photons bounce back and forth in such a way that the overall pulse remains frozen in space. The pulse can be released again by switching off one of the control beams.

“Our work combines several known techniques to develop a novel tool for manipulating and controlling light propagation,” Bajcsy says. “Besides being fundamentally interesting from a pure science point of view, such ‘light control’ may have practical applications in information technology.”

The present work may yield new approaches to enhance interaction between faint light pulses, which could help process information carried by light pulses. An example of this would be quantum information processing – a powerful theoretical approach that uses single photons’ or atoms’ quantum states to store information. Today’s computers store information in electronic combinations of zeros and ones; bits represented by quantum states of single photons can carry far more information far more efficiently – at the speed of light.

“To take advantage of this, however, you need to be able to make photons interact controllably,” Bajcsy says. “The problem is that photons normally don’t interact with each other. By keeping the light pulse stationary, we gain time for the rubidium atoms to act as a catalyst and mediate interactions between photons.”

The research could also pave the way toward purely optical communications systems, which would eliminate the devices now necessary to turn information carried by optical fibers into electronic signals and back again. Such conversions are now necessary to create computer images, telephone messages, and other electronic communications. However, Lukin says that the present research is just another step toward efforts to control light and that further work is needed to determine if it can aid such applications.

The National Science Foundation, Defense Advanced Research Projects Agency, David and Lucile Packard Foundation, Alfred P. Sloan Foundation, and Office of Naval Research funded this research by Bajcsy, Zibrov and Lukin.