February 18, 1999

In a Major Breakthrough, Danish Physicist Slows the Speed of Light




When light travels through empty space, it zips along at a speed of 186,171 miles a second -- the highest speed anything can attain, even in principle. A moonbeam takes only a little over one second to reach Earth.

But a Danish physicist and her team of collaborators have found a way to slow light down to about 38 miles an hour, a speed exceeded by a strong bicyclist.

The physics team, headed by Dr. Lene Vestergaard Hau, who works concurrently at the Rowland Institute for Science in Cambridge, Mass., and at Harvard University, expects soon to slow the pace of light still further, to a glacial 120 feet an hour -- about the speed of a tortoise.

"We're getting the speed of light so low we can almost send a beam into the system, go for a cup of coffee and return in time to see the light come out," Dr. Hau said in an interview.

The achievement, by Dr. Hau, two Harvard graduate students and Dr. Steve Harris of Stanford University, is being reported on Thursday in the journal Nature. Physicists said it had many potential uses, not only as a tool for studying a very peculiar state of matter but also in optical computers, high-speed switches, communications systems, television displays and night-vision devices.

One of the most desirable features of the apparatus that the researchers built for their work is that it does not transfer heat energy from the laser light it uses to the ultracold medium on which the light shines. This could have an important stabilizing effect on the functioning of optical computers, which operate using photons of light instead of conventional electrons. A switch using the system could be made so sensitive that it could be turned on or off by a single photon of light, Dr. Hau said.

The medium Dr. Hau and her colleagues used in slowing light by a factor of 20 million was a cluster of atoms called a "Bose-Einstein condensate" chilled to a temperature of only fifty-billionths of a degree above absolute zero. (Absolute zero is the temperature at which nothing can be colder. It is minus 273.15 degrees on the Celsius scale, minus 459.67 on the Fahrenheit scale and zero on the Kelvin scale.

Dr. Hau's group reached an ultralow temperature in stages, using lasers to slow the atoms in a confined gas and then evaporating away the warmest remaining atoms. The temperature they attained, one of the lowest ever reached in a laboratory, was far colder than anything in nature, including the depths of space.

Bose-Einstein condensates (named for the theorists who predicted their existence, Satyendra Nath Bose and Albert Einstein) were first prepared in a laboratory four years ago and became the objects of intense research in the United States and Europe. They owe their existence to some of the rules of quantum mechanics.

One of these is Werner Heisenberg's uncertainty principle, which states that the more accurately a particle's position is known, the less accurately its momentum can be determined, and vice versa.

In the case of a Bose-Einstein condensate, atoms chilled nearly to absolute zero can barely move at all, and their momentum therefore approaches zero. But because zero is a very precise measure of momentum, the uncertainty principle makes the positions of these atoms very uncertain. In a condensate, as a result, such atoms are forced to overlap each other and merge into superatoms sharing the same quantum mechanical "wave function," or collection of properties.

It was such a superatom, made of a gas of superpositioned sodium atoms, that provided Dr. Hau and her associates with the optical molasses they needed to slow light down.

Beginning their project last spring, the group tuned a "coupling" laser to the resonance of the atoms in their condensate, shot the laser into the cold cluster of atoms and thereby created a quantum mechanical system of which both the laser light and the condensate of atoms were components. At this stage, the system was no more transparent than a block of lead, Dr. Hau said.

The next step was to send a brief pulse of tuned laser light from a "probe" into the condensate, at a right angle to the coupling laser, in such a way that the laser-condensate system interacted with the probe laser. Under these conditions about 25 percent of the probe laser light passed through the "laser-dressed condensate," but at an astonishingly slow speed.

The light that emerged from the apparatus, not visible to the naked eye, was only 25 percent as strong as the light that entered, but detectors found that it had roughly the same color.

The speed of light is reduced in any transparent medium, including water, plastic and diamond. Glass prisms and lenses, for example, slow light by differing amounts that depend on the thickness of the glass. The slowing of light causes the bending by which lenses focus images.

But the reduction of light speed in a laser-coupled Bose-Einstein condensate works in an entirely different, quantum-mechanical way. Not only is the speed brought to a crawl, but the refractive index of the condensate becomes gigantic.

Refractive index is a measure of the degree to which a medium bends light. The refractive index of the condensate created by Dr. Hau's group was about 100 trillion times greater than that of a glass optical fiber.

Although Dr. Hau said it might take 10 years before major applications were developed, the huge refractive index of the condensate, which can be precisely controlled, may make it a basis for "up shifting" devices that increase the frequencies of light beams from the infrared end of the spectrum up through visible light to ultraviolet. Possible applications include ultrasensitive night-vision glasses and laser light projectors that could create very bright projected images.

Laser-condensate combinations may also lead to ultrafast optical switching systems useful in computers that would operate using one light beam to control another light beam. Such a system could function as an optically switched logic gate, replacing the electronic logic gates computers now use.

Slow light could also be exploited in filtering noise from optical communications systems, Dr. Hau said.

Dr. Jene Golovchenko, a physics professor at Harvard familiar with Dr. Hau's work, commented, "She has worked long and hard on this, and now she's really hit a home run."


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  • Nature, an International Journal of Science (registration required).

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