- News Home
17 April 2014 12:48 pm ,
Vol. 344 ,
Officials last week revealed that the U.S. contribution to ITER could cost $3.9 billion by 2034—roughly four times the...
An experimental hepatitis B drug that looked safe in animal trials tragically killed five of 15 patients in 1993. Now,...
Using the two high-quality genomes that exist for Neandertals and Denisovans, researchers find clues to gene activity...
A new report from the Intergovernmental Panel on Climate Change (IPCC) concludes that humanity has done little to slow...
Astronomers have discovered an Earth-sized planet in the habitable zone of a red dwarf—a star cooler than the sun—500...
Three years ago, Jennifer Francis of Rutgers University proposed that a warming Arctic was altering the behavior of the...
- 17 April 2014 12:48 pm , Vol. 344 , #6181
- About Us
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.