Light frozen in place
Technology Research News
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
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
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
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
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).
Funding: Government; Private
TRN Categories: Physics; Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "Stationary Pulses of
Light in an Atomic Medium," Nature, December 11, 2003
December 31, 2003/January 7, 2004
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