Way devised to stop light

TEXAS A&M University physicists have devised a way to stop light, an accomplishment that could help develop super-fast computers, called quantum computers.

"Slowing down light revolutionizes modern nonlinear optics," says Olga Kocharovskaya, associate professor of physics at Texas A&M. "Utilizing the nonlinear properties of light in a medium is usually not easy because it requires the use of lasers of very high intensity, and such lasers in many cases are not available or destroy the medium. Now by slowing down light, we have another way to realize these nonlinear properties without using very intense beams."

Texas A&M physicists Olga Kocharovskaya, Yuri Rostovtsev and Marlan Scully, describe the method to stop light in an article published in the the journal Physical Review Letters.

The slowing down of light is related to the phenomenon of Electromagnetically Induced Transparency (EIT) introduced by Kocharovskaya in 1986 and experimentally observed the first time in 1991 by Steve Harris, physicist at Stanford University. The EIT phenomenon allows an optically thick, opaque medium to be transparent to the light wave of a probe laser by means of another driving laser.

Since 1991, many experiments have been set up to slow down light, but none has ever stopped light completely. The recently published work of Kocharovskaya, Rostovtsev and Scully shows how light can be stopped.

Light is slowed down by sending it in a glass cell filled with gas. The interactions between the photons of light and the gas atoms create a coupled photon-atom system, called polariton. The more photons and atoms interact, the more the polariton slows down. Light is a component of the photon-atom system and thus slows down as well.

In previous attempts to stop light without using EIT, the laser beam was mainly absorbed by the gas atoms, and could not hold up in the cell.

"Instead of one laser, we use two lasers, a probe laser and a driving laser, so that both probe and driving light waves go through together without absorption, keeping the same intensity as they entered," says Rostovtsev. "In a sense, the cell gas is transparent to the lasers,hence the name Electromagnetically Induced Transparency (EIT) to describe this phenomenon."

When two laser beams are sent to the cell, light from one of the lasers interacts with the atoms, which absorb and reemit light continually, because the second laser prevents them from absorbing light without reemitting it back to the same probe light wave (as would be the case if only one laser is sent to the cell). The many interactions between light and atoms ultimately slow down the speed of light propagation through the medium.

Interesting experimental results have been published recently on the use of slow light for information storage by two teams of physicists, one led by Mikhail Lukin (Texas A&M former graduate student) and Ronald Walsworth, both of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the other by Lene Vestergaard Hau of the Rowland Institute for Science and Harvard University, both in Cambridge, Mass. They explore EIT properties of stationary atoms.

Now Kocharovskaya and her colleagues show that light can be completely stopped. They use the EIT phenomenon to selectively use atoms in a hot gas cell which are moving towards the laser beam instead of being at rest.

"To reduce the velocity of light to zero, or even to turn it back, we tune the frequency of the driving laser to the resonance with the atoms that move in a direction opposite to that of the laser beams," Kocharovskaya says. "This results in an effective drift of light backward. "Although the thermal speed of atoms is only 350 meters per second, which is much less than the speed of light in the vacuum c=300,000 km/s, this effect is very strong since most of the time photons are captured by atoms," she says.

An experimental setup that would provide the first evidence of light stopping is now being mounted in Texas A&M by physicist George R. Welch.

"One of the most interesting applications of light slowing down or stopping is quantum computing," says Scully. "It may then be possible, by using a driving laser, to control and manipulate single photons as new forms of bits (1 or 0) in quantum computers, and make further progress in the field of quantum computation."

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