Light frozen in place
By Eric Smalley
Researchers at Harvard University have trapped and held a light pulse still for a few hundredths of a millisecond.
The experiment extends previous research that showed it is possible to store a light pulse by imprinting its characteristics into gas atoms, and to reconstitute the pulse using a second beam. The Harvard researchers went a step further by briefly holding the reconstituted light pulse in place.
This type of control could be used to trap lightwaves long enough to interact with each other. This could improve data processing techniques that use light as an information carrier and also speed fiber optic communications, said Michal Bajcsy, a researcher at Harvard University. "Our experiment builds upon concepts and tries to develop new tools in an area that can be loosely described as manipulating light with light, or light-light interaction," he said.
The technique could also open up ways of manipulating single photons, a useful trait in quantum computing and communications, said Bajcsy. Quantum computers are theoretically many orders of magnitude faster than classical computers for certain types of large problems, including those that would render today's security codes useless. Researchers generally agree that practical quantum computers are least two decades away.
To trap the electromagnetic energy of light, the researchers fired a red laser pulse into a glass cylinder containing rubidium atoms. The key to freezing the light pulse after imprinting its information into the atoms is lighting the rubidium gas with a pair of control laser beams rather than a single beam, said Bajcsy. In addition to reconstituting the stored signal pulse, the control beams interfere to create a standing-wave pattern of dark and bright regions, he said.
The light pattern makes the atoms act like a set of mirrors, which traps the electromagnetic energy, said Bajcsy. "As the re-created signal pulse tries to propagate through the medium, the photons bounce backwards and forwards in such a way that the pulse overall remains frozen in space," he said. Switching off one of the control beams releases the pulse.
This works due to two properties of the interaction of light and matter: dispersion and absorption.
Matter bends, or refracts, light, and the particular types of matter refract different wavelengths, or colors, of light by different amounts. Refraction is responsible for the familiar bent-drinking-straw illusion. A pulse contains multiple wavelengths of light and when it travels through matter different degrees of refraction cause the wavelengths to disperse, or spread out. Dispersion causes white light to spread into a rainbow spectrum when it passes through a prism.
Dispersion also causes light pulses to slow down. The more a type of matter disperses light, the more its atoms absorb photons. Ordinarily, light pulses can be slowed only so much before they are completely absorbed. Recently researchers found a way to use strong laser beams to limit absorption while increasing dispersion, a technique known as electromagnetically induced transparency.
To produce this effect, researchers fire a strong laser beam into a vapor of rubidium atoms to overload rubidium's ability to absorb photons. Once the rubidium atoms can absorb no more photons, the beam propagates through the vapor as though the atoms weren't there. The researchers then send a weak signal pulse into the vapor, which does not absorb the photons but disperses the pulse enough to dramatically slow it. Gradually turning off the control beam slows the pulse nearly to a halt and imprints the pulse's shape and wavelength information into the atoms. Turning the control beam back on reconstitutes the pulse using the information stored in the atoms.
The Harvard researchers have gained more control over the light pulse by using a pair of control beams aimed from opposite directions to reconstitute the pulse in the midst of a standing wave interference pattern. The bright regions of the standing wave diminish the electromagnetically induced transparency, causing the atoms in those places to absorb photons. But by tuning the two control beams to make the bright regions very narrow, the researchers were able to cause the regions to reflect rather than absorb lightwaves, which trapped the pulse in place.
Unlike techniques that use microscopic mirrors to bounce pulses back and forth within small spaces, the researchers' approach doesn't lose any of the trapped light pulse, and so preserves the light's quantum information.
The researchers' next step is to use the technique to control interactions of fields that consist of relatively few photons, said Bajcsy. The method also opens the possibility of moving a frozen light pulse in space by carefully tuning the control beams, according to Bajcsy. It might also be possible to implement the method using dynamically controllable photonic crystals rather than rubidium gas and control lasers, he said.
Photonic crystals, which are used to guide lightwaves, are materials that contain regularly spaced air holes or rods of another material. The spacing in dynamically controllable photonic crystals can be changed on-the-fly.
Bajcsy's research colleagues were Alexander Zibrov of Harvard University, Harvard-Smithsonian Center for Astrophysics and Lebedev Institute of Physics in Russia, and Mikhail Lukin of Harvard. The work appeared in the December 11, 2003 issue of Nature. The research was funded by the National Science Foundation (NSF), the Defense Advanced Research Projects Agency (DARPA), the David and Lucile Packard Foundation, the Alfred Sloan Foundation and the Office of Naval Research (ONR).