|
|
|
|
 Mooney
Aircraft Corp. |
| Slower than a speeding
airplane. Using a hot rubidium gas
researchers slowed light signals to less than
the speed of this
airplane. | | | | |
Slowing light may be easier than you think. Physicists who
recently slowed a light signal to a glacial 17
m/s using the exotic state of matter known as a
Bose-Einstein condensate (BEC) received tremendous attention,
but now a team demonstrates how to slow light on the cheap.
They show in the 28 June PRL that an
ordinary gas of heated rubidium atoms can slow light signals
almost as well, to just 90 m/s--more than a million times less
than the speed of light in air or vacuum. The new concept
should make a wide range of advanced optics experiments more
feasible and could also be useful for devices.
George Welch of Texas A&M University in College Station
says he was "in awe" of the recent experiment, "but most
people don't make Bose-Einstein condensates in their
labs--that requires several hundred thousand dollars worth of
equipment." He and his colleagues realized that the basic
technique, known as electromagnetically induced transparency
(EIT), could be optimized to work with a less expensive
set-up.
In EIT a laser manipulates the quantum states in an opaque
cloud of atoms and makes them transparent to a narrow range of
wavelengths of light. According to electromagnetic theory,
this narrow transmission band leads directly to an index of
refraction that depends strongly on wavelength in this range,
although in a low-pressure gas it may not stray far from the
vacuum value of one. (The index of refraction gives the "phase
velocity"--the speed at which pure sine-wave light beams
travel.) Several years ago researchers demonstrated that the
wavelength-dependent phase velocity causes the light's "group
velocity"--the speed at which energy and all signals
travel--to slow by more than one hundred times. (More
explanation of this effect.)
Welch and his colleagues knew that by narrowing the
transmission band in their heated rubidium gas, the light
would slow even more dramatically. The group velocity went
down to 90 m/s --slower than many
propeller-driven airplanes--so that a pulse of light took one
fourth of a millisecond to traverse the inch-long gas cell,
rather than zipping across in a fraction of a nanosecond.
Welch says the long interaction time between the light and the
atoms can lead to extremely sensitive experiments in nonlinear
optics, where measurements of light interacting with matter
normally require very high intensity lasers. Although a BEC
can slow light signals even more than a rubidium gas, the
total interaction time with the BEC is less--at least with
current technology--because it fills such a small volume.
Many researchers in the field thought cold atoms, such as
those in a BEC, would be required to reach group velocities
this low, says Atac Imamoglu of the University of California
at Santa Barbara, but "this paper clearly shows that's not the
case." Under the extreme conditions of these experiments, a
single photon can deviate from normal behavior and interact
directly with another photon, a process Imamoglu says could be
used in a quantum computer. He says other applications are
also possible in developing communications systems that
operate entirely with light, without the need to convert to
electrical signals. Lene Hau of Harvard University, part of
the team that recently slowed light in a BEC, points out that
although hot gases are easier to produce, a BEC can slow light
"orders of magnitude" more and allows more flexibility in the
geometrical arrangement for future experiments.
Ultraslow Group Velocity and
Enhanced Nonlinear Optical Effects in a Coherently
Driven Hot Atomic Gas
Michael M. Kash,
Vladimir A. Sautenkov, Alexander S. Zibrov, L.
Hollberg, George R. Welch, Mikhail D. Lukin, Yuri
Rostovtsev, Edward S. Fry, and Marlan O.
Scully
Phys.
Rev. Lett. 82, 5229
(issue of 28
June 1999)
| | |
