Stopping Light in Its Tracks
Stop that light! I want to get off! Nobody has ever
managed to ride a beam of light, although Einstein in his youth imagined
what it might be like to do so. Such
thinking eventually lead to the special theory of relativity. Although it is
still impossible for anyone to travel on a light ray, physicists
have managed to stop a laser beam in its tracks! This
amazing result
builds on earlier work by a research group led by Lene Hau
in which light was slowed to 38 miles
per hour.
Light in a medium travels somewhat slower
than 300,000 kilometers per second, its speed in a vacuum. This effect, known
as refraction, is responsible
for the bending of light in passing from one medium to another. In an
ultracold gas of sodium atoms, the index of fraction varies significantly with
the frequency of light. The gas is normally opaque, but when the sodium atoms are
bathed with secondary "controlling"
laser light of a particular frequency, the medium becomes
transparent and light can pass through it. Due to the
varying index of refraction, a primary laser pulse slows down
when it enters the gas. It also becomes greatly compressed. A
pulse with a length of one kilometer gets squeezed into a tiny unit one one-thousandth
of a meter in length. When the light pulse exits the region of sodium atoms, it expands
back to its original length zooming off at 300,000 kilometers per second again.
To bring the light to a standstill, Dr. Hau and her
co-workers turn off the secondary controlling laser light
after the impinging laser beam has
entered the ultra cold sample. The medium becomes opaque. One would think that
the laser pulse would be absorbed by the sodium atoms. Instead, the information about
the light wave is encoded in the atoms. When the controlling laser light
is turned back on, the atoms reconstruct the laser beam. It continues its propagation
though medium and then out the sample. If the time during which the beam is halted is
not too long, the shape of the reconstructed beam coincides to a high degree with the
original shape.
Imagine a train one kilometer in length
consisting of thousands of cars traveling at 300,000 kilometers per second. Suddenly,
the track becomes rough, causing the front engine and subsequent cars to come
to a screeching slowdown. With the train now moving at 38 miles per hour, the cars
pile up and are crushed to the size of an ant. Then
a magician snaps her finger and the train disappears but leaves an imprint on the
dirt between the tracks. With a second snap of the finger, the imprint vanishes while the
ant-sized train reappears and moves forward at 38 miles per hour again. Then the train
encounters smooth track. The engine speeds off at 300,000 kilometers
per second, and as subsequent cars enter the smooth track region, they too rapidly speed off
so that the entire accordion-like structure unfolds. Remarkably, little damage to
the train occurs.
"Impossible!" You say. Well, in the case
of a train, it is impossible because
the deformation of metal is an irreversible process. But light is more accommodating
and can be stretched or compressed more readily. Still, Dr. Hau's team has done what
seems to be magical. Why isn't the light absorbed and lost? How can the atomic information
reconstruct the beam? Why isn't the beam greatly distorted when it leaves the sample? The
answers to these questions reside in the miracles of quantum mechanics, the dynamics that
control the microscopic world of atoms and their interactions with light.
A major technological breakthrough has occurred. If low cost
means are found to manipulate light in the above manner, then computers and communications
will be revolutionized. One can imagine using light instead of electrons to perform
computational processes. Such photonic machines would calculate at many
times the rate of current computers.
To the Original Report on Slowing Down Light
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