## 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.

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