Physicists Bring Light to a Crawl

Aaron J. Hand

Light travels in a vacuum at 186,282 miles a second. Experiments at the Rowland Institute for Science have slowed that speed 20 million times, essentially allowing light to travel along a typical suburban throughway without getting a speeding fine. If the researchers have their way, light could race a tortoise -- and lose.
The physics team -- led by Lene Vestergaard Hau, a researcher at Rowland and at Harvard University -- used a Bose-Einstein condensate to slow laser light down to 38 mph. Detailed in the Feb. 18 issue of Nature, the experiment sends a coupling beam through an ultracold cloud of sodium atoms, turning it into an electromagnetically induced transparent medium through which a pulsed probe beam can pass (without the coupling beam, the atoms would absorb the probe pulse). It's the second light pulse that is slowed down as it travels through what Hau describes as "a bizarre refractive index medium."
The experiment uses a green argon-ion laser to pump a ring dye laser, which -- at 589 nm, where it interacts best with sodium -- feeds every laser beam within the experiment, including a third imaging beam.
Unlike the slowing of light that occurs because of refractive index changes, this extreme slowing is induced by a quantum mechanical effect. A steep variation in the refractive index -- created and controlled by the coupling laser when it combines with the dense, slow-moving atoms in the Bose-Einstein condensate -- is used to achieve the remarkably slow light propagation. (The condensate begins to form at 450 nK, but the researchers have cooled the cloud to lower than 50 nK, Hau said.)
The researchers are getting new lasers in the laboratory that should enable them to slow the speed of light to 1 cm/s, Hau said, matching the speed of sound in the condensate. They will still use an argon-ion laser to pump a dye laser, but the new lasers will enable them to use sodium's D1 line, which should help do away with system loss.
If the system can be made practical, Hau sees tremendous opportunities to use this new optical medium to produce a range of optical devices, such as programmable optical delay lines, or optical switches that can be controlled by a very weak beam of light. "It's opening up doors to totally new regimes of optics," she said.