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The Slowness of Light
18 February 1999 7:00 pm
Physicists have used a clever apparatus to slow the speed of light to a startling 17 meters per second--about the pace of a Volkswagen bus chugging uphill. This optical tour de force, reported in today's Nature, relies upon a weird quantum-mechanical trick induced by cross-hatched laser beams, as well as ultracold temperatures that force atoms in a tiny gas cloud to a virtual standstill.
The key is an optical effect called "electromagnetically induced transparency," in which physicists make an ordinarily opaque vapor transparent by shining a precisely tuned laser into it. Electrons within atoms absorb light of a specific wavelength by jumping from one energy level to a higher one. But a laser can set up a quantum-mechanical interference that blocks the electrons from making the jump, allowing a second beam at the normally absorbed wavelength to zip through.
For light at that wavelength, the medium's "refractive index"--a measure of how much it slows and bends light--increases sharply. Ordinarily, the increase can't be observed, because the light doesn't pass through the material at all. Electromagnetically induced transparency, however, reveals the increased refractive index, and the resulting slowdown of the light waves. Scientists have used this method to make light travel 160 times slower through a cloud of lead atoms.
In the new project, physicists enhanced that trick by chilling a gas of sodium atoms to within 50 billionths of a degree above absolute zero. In such an icebox, the atoms cluster tightly and drift slowly in quantum lockstep, a bizarre state called "Bose-Einstein condensation" (Science, 14 July 1995, p. 152). The high density of the atom cloud and the quantum lockstep maximized the interference effect, and the refractive index increased. A team led by atomic physicist Lene Vestergaard Hau of the Rowland Institute for Science and Harvard University in Cambridge, Massachusetts, found that light moved 20 million times more sluggishly through the tiny condensate than it does through a vacuum. "It's like squeezing light pulses through a wall," Hau says. The light stalls so suddenly that the 750-meter-long laser beam gets compressed into a pulse just 0.04 millimeters long inside the condensate, she notes. Refinements could yield light speeds down to centimeters per second, Hau believes.
The research is "quite extraordinary," says Bose-Einstein condensation pioneer Eric Cornell of JILA, the former Joint Institute for Laboratory Astrophysics in Boulder, Colorado. "This is the first time electromagnetically induced transparency has made my mouth drop and say, 'wow.' " But practical applications are many years distant, he says, because they will require inexpensive ways of making condensates.