PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 521 January 18, 2001 by Phillip F. Schewe, James
Riordon, and Ben Stein
AT LAST! LIGHT BROUGHT TO A HALT. 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 http://www.aip.org/physnews/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:
http://focus.aps.org/v5/st19.html)
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.
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