For the first time, physicists in two separate laboratories have
effectively brought a light pulse to a stop. In the process, physicists
have accomplished another first: the non-destructive and reversible
conversion of the information carried by light into a coherent atomic
form. Sending a light pulse into specially prepared rubidium (Rb) vapor,
a group at the Harvard-Smithsonian Center for Astrophysics led by Ron
Walsworth (617-495-7274) and Mikhail Lukin (617-496-7611) has (1) slowed
the pulse's "group velocity" to zero and (2) stored its information in
the form of an atomic "spin wave," a collective excitation in the Rb
atoms. (A spin wave can be visualized as a collective pattern in the
orientation of the atoms, which spin like tops and hence act like tiny
bar magnets. "Spin" is merely the name for the tiny magnetic vector in
each of the atoms.) The atomic spin wave is coherent and long-lived,
which enables the researchers to store the light pulse's information and
then convert it back into a light pulse with the same properties as the
original pulse.
This new accomplishment in a simple system increases the promise for
quantum communication, which may someday be used to connect potentially
ultrafast quantum computers in a large network analogous to the
Internet. Usually photons (the quanta of light) are absorbed by atoms,
destroying the information carried by the light. With the present
method, in principle, no information in the light pulse is lost.
Previous efforts to slow light (such as Hau et al., Nature, 18
February 1999) have reduced the signal speed to about 1 mph (Update 472) by
using a process called electromagnetically induced transparency (EIT;
see Updates
37, 344
and Stephen Harris's article in Physics Today, July 1997).
Walsworth, Lukin and colleagues have gone the rest of the way to a
zero light-pulse speed by using a novel technique which was recently
proposed theoretically (Lukin, Yelin and Fleischhauer, Phys. Rev.
Lett. 1 May 2000; Fleischhauer and Lukin, Phys. Rev. Lett. 29
May 2000). The light storage experiment begins with the
Harvard-Smithsonian scientists shining a "control" laser beam into a
glass cell filled with rubidium vapor (about 70-90 degrees Celsius),
which puts the atoms into a conventional EIT state in which they cannot
absorb light in the traditional sense. The scientists then send in a
"signal" pulse of light which contains the information they want to
store. As the pulse enters the rubidium cell its propagation speed is
reduced to about 2,000 mph. Since the front edge of the signal pulse
enters the cell (and hence is decelerated) first, the pulse experiences
dramatic spatial compression: from several kilometers in free-space to a
few centimeters inside the rubidium vapor. The light in the vapor cell
interacts with the atoms (see figure at Physics News Graphics),
changing the atoms' spin states coherently and creating a joint
atom-photon system known as a polariton. (For a nice descriptions of
polaritons see Phys Rev
Focus, 26 April 2000.
The light-atom interaction causes the polaritons to act as if they
have
an effective mass; so one way to understand the signal pulse's
reduced speed is that the mixture with atoms, in the form of a
polariton, effectively weighs down the otherwise massless photons. Next,
the Harvard-Smithsonian scientists stop the signal pulse of light by
gradually turning off the control beam, which causes more atoms to be
mixed with fewer photons, thereby increasing the polariton mass and
further reducing the signal pulse's speed. When the control beam is
completely off the polariton is purely atomic, the light pulse is
effectively halted, and no signal pulse emerges from the glass cell
during the storage period. At this point there are no photons remaining
in the cell. The light does not go into warming of the atoms, as is the
usual case. Instead the photons are expended in the creation of the
atomic spin wave. Thus, the information that the light pulse carried
(all that one can know about the photons) is stored in the atomic spin
wave, waiting to be released as a light pulse that is in principle
identical to the incident pulse.
An alternative way to understand the slowing of light is to think of
the signal pulse as a wave made of many different components, each with
a different frequency. The Rb atoms bend or "refract" the individual
components of the light by different amounts depending on each
component's frequency. The vapor cell's frequency-dependent index of
refraction causes the component waves to add together in such a way that
the group velocity, the velocity of the composite pulse, slows
appreciably. The dimming of the control beam makes the vapor's index of
refraction more sharply dependent on frequency, and this serves to
reduce the group velocity further. The dimming causes the atoms to
become transparent to a narrower range of frequencies. But
simultaneously, the light wave (or more precisely, the combination of
light wave and atomic spin wave) is continually slowing down,
maintaining its shape but narrowing its range of component frequencies
so that the atoms are still unable to absorb it. After a relatively long
delay the control beam can be turned back on, reverting the polariton to
being a light wave by coaxing the atoms to emit the exact signal light
pulse that entered the medium.
In brief: (1) the length of a light pulse is compressed from
kilometers to centimeters in a properly-prepared rubidium vapor; (2) the
information carried by the light pulse is then imprinted upon the
ensemble of rubidium atoms in the form of long-lived spin waves; and (3)
the light pulse can later be read out on demand. This new light storage
method is robust because information is maintained in collective atomic
spin states, which are much less sensitive to dissipation, losses, and
quantum-computer-crashing decoherence effects than are excited
electronic states in atoms.
Scientists believe that the light storage method is quite general and
that the simplicity of its implementation is a big advantage. They even
speculate that the technique may be utilized in certain solid-state
materials. The Harvard-Smithsonian demonstration experiment is exciting
news for scientists worried about preserving the coherence of quantum
information transfer. With further work, this technique should allow for
the storage and transmission of photon quantum states useful for quantum
communication and computation. (Phillips et al.,
Physical Review Letters, 29 January 2001.)
Walsworth and Lukin say that a very similar result has been recently
obtained by Lene Hau's group (Harvard/Rowland Institute of Science) in
an ultra-cold atomic gas. In addition, an upcoming theory paper
(Kocharavskaya et al., Phys Rev. Lett., 22 January) discusses a
novel technique for making a light beam not only stop in its tracks but
reverse its direction; this effect could be useful for non-linear optics
applications.
Names of some experts: Atac Imamoglu, UC Santa Barbara, (805)
893-3262, atac@ece.ucsb.edu;
Marlan Scully, Texas A&M, (979) 862-2333; Stephen E. Harris,
Stanford, 650-723-0224, seharris@ee.stanford.edu;
George Welch, Texas A&M, (979) 845-7737, george@leona.physics.tamu.edu;
Olga Kocharovskaya, Texas A&M, 979-845-2012; Susanne Yelin, Harvard
Smithsonian, syelin@cfa.harvard.edu; Michael
Fleischhauer, Ludwig-Maximilians University, Germany. Journalists can
access the text of the PRL article at Physics News
Select.
