First, There Was Light—Until Harvard Physicists Stopped It

First, there was light. Then, Harvard scientists developed a technique to slow light down. Next, they figured out how to store light for short periods of time. Now, they have stopped light altogether and are attempting to “control” it.

A Harvard research group has successfully brought a light pulse containing photons to a standstill for 10 milliseconds, marking the first time actual photons have been held in a “frozen” state.

The ability to stop and manipulate light could be used in the future to build powerful, so-called quantum computers and to send secure electronic messages.

Graduate students Michal Bajcsy ’01, Axel P. André and Alexander S. Zibrov conducted the study under the direction of Assistant Professor Mikhail D. Lukin, head of a Quantum Optics group in the Department of Physics.

“We demonstrated how to hold a light pulse still without taking all the energy away from it,” Lukin said in a statement.

Speaking of the amount of time the photons remained at rest, Bajcsy said, “It might seem like a short time, but since light travels so fast, it’s not so bad.”

The normal speed of light is 186,000 miles per second.

This recent study, whose results were published in this week’s issue of the journal Nature, builds on the advancements made in recent years by other research groups at Harvard.

In 1999, Harvard Professor Lene V. Hau oversaw an experiment in which light was slowed to a speed of about 40 m.p.h.

Two years later, David F. Phillips, an associate of the Harvard College Observatory at the Harvard-Smithsonian Center for Astrophysics; Ronald L. Walsworth, another associate; and Lukin published a study describing how the team of researchers captured a “quantum fingerprint,” or holographic imprint, of a pulse of light in a super-cooled gaseous medium.

At the time of the study, Phillips said, “We’re writing the information about the pulse into the atoms and at a later time we’re reading the information out.”

The Quantum Optics group was able to “freeze” the photons themselves in a hot gas of rubidium atoms.

“The earlier experiments on light storage...stored only the ‘signature’ of the light pulses in a process somewhat similar to creating a hologram,” Bajcsy said. “Thus, during the storage time there were no signal photons present in the medium.”

“Our experiment on the other hand ‘traps’ actual signal photons inside the rubidium vapor in such a way that the signal pulse overall does not travel,” he said.

Using a technique called dynamic electro-magnetically induced transparency, the group fired a signal pulse of red laser light into a sealed glass cylinder containing the gaseous medium.

Instead of using a single control beam, as was done in the previous studies, two beams were used, Bajcsy said.

The interaction between the two beams within the vapor simulated a surface of tiny mirrors.

“As the recreated signal pulse tries to propagate through this medium, the photons bounce backwards and forwards in such a way that the pulse overall remains frozen in space,” Bajcsy said.

André, who published a theoretical proposal for the experiment with Lukin last year, said he was happy with the results.

“It’s very exciting,” André said. “It’s great to see that we can come up with these simple ideas of how to manipulate light, and it turns out that they are very useful for all kinds of things,” he said.

Currently, the team of physicists is trying to figure out how to move the light spatially through the medium, which would be a step toward building super-fast computers.

However, André said, “this prospect of quantum computers might be in the very distant future.”

The Quantum Optics group received funding for their research from the National Science Foundation, the Defense Advanced Research Projects Agency, the David and Lucile Packard Foundation, the Alfred P. Sloan Foundation and the Office of Naval Research.