Researchers Briefly Bring Light Beam to a Dead Stop

In a feat akin to catching lightning in a bottle, researchers have been able to reduce the speed of light from 186,000 miles per second to zero, trapping light beams for short periods of time before allowing them to burst forth again at full speed.

The achievement does not break any laws of physics, but it does illustrate the mysterious, bewildering world of quantum physics, where things are not always what they seem and where physicists often do the seemingly impossible.

The discovery, announced Thursday by two teams of researchers, represents a major step toward the development of so-called quantum computers, which would manipulate the intrinsic physical characteristics of atoms and be orders of magnitude more powerful than today’s supercomputers.

“It’s very exciting stuff,” said physicist Aephraim Steinberg of the University of Toronto. “It makes it possible to store information carried by light for much longer periods of time than was thought possible in the past.”

The speed of light is constant in a vacuum, but it can be slowed as it passes through glass, water and a variety of other materials. If it were not, we would not have magnifying lenses, rainbows and a host of other phenomena. But glass or water slows the speed of light only by about a third, and physicists have been trying to retard it much more radically.

A major advance occurred two years ago, when a team led by physicist Lene Vestergaard Hau of Harvard University shined a pulse of laser light through a cloud of super-cold sodium ions and slowed the light to about 38 mph. In effect, the sodium cloud acted like thick molasses that grabbed photons of light and slowed their passage by a factor of more than 10 million.

Now Hau’s group and a team led by Mikhail D. Lukin and Ronald L. Walsworth of the Harvard-Smithsonian Center for Astrophysics in Cambridge have independently gone that achievement one better. They have brought the light pulse to a complete stop before allowing it to burst forth once again.

In the experiments, a pulse of light half a mile long enters a chamber that is only a fraction of an inch wide; it is slowed so drastically that all of the beam enters the thin chamber before any begins to exit.

The process is similar to cartoons in which a bulky person runs behind a very thin tree, disappearing for a few seconds before reemerging on the other side.

Both research groups used clouds of metal atoms cooled to a few-millionths of a degree above absolute zero. Lukin’s group used rubidium and Hau’s used sodium.

Such clouds would normally absorb any light beam shining on them, destroying any information contained in the light. But using a precisely tuned laser called a coupling beam, researchers can alter the quantum states of the metal atoms to allow them to transmit the light, a phenomenon known as electromagnetically induced transparency.

While the light is passing through, however, it interacts with the spin state of the atoms, a phenomenon that slows its passage. The spin state of an atom is like a tiny bar magnet with a specific orientation in space. Changing that spin state is akin to pointing the magnet in a different direction.

In the new experiments, to be published later this month in Nature and Physical Review Letters, researchers shut off the coupling beam while all of the light pulse was in the chamber. They could halt the beam for as long as a millisecond before turning the coupling beam back on and allowing the light pulse to reemerge at full speed.

A millisecond may not seem like much, Steinberg said, but light moving at its normal speed would travel 186 miles in that period.

What actually happens to the photons of light during this period? In essence, they disappear, and the information they contained is stored in the spin states of the metal atoms. When the coupling beam is turned back on, that information is turned back into a light beam identical to the original in all its characteristics and information content, but somewhat weaker.

“Are they actually stopping the light? That is a semantic question,” said physicist John Preskill of Caltech. “Fundamentally, what we should be thinking about is not whether it is photons or atoms, but whether it is information, which can be encoded in a variety of forms.”

Steinberg noted that the same process was involved in earlier experiments that simply slowed light drastically. “We were confident in calling [the slowed photons] light then,” he said, but if the photons have stopped, is it still light? Steinberg thinks it may be.

Although the technology demonstrated in the two experiments is far from being used commercially, it gives hope to scientists attempting to build the next generation of computers, which would work with light rather than electrons.

Calculations performed using light could occur much more rapidly, but making such calculations would require that the information contained in a light pulse be stored briefly at one or more points during the computer’s operation. That has proved to be a difficult problem to overcome because of light’s intrinsically elusive nature.

But Hau and Lukin-Walsworth have now shown that it is possible. The rest, as theorists like to say, is simply engineering.

Physics “doesn’t get any more interesting than this,” according to Eric A. Cornell of the University of Colorado in Boulder.