For years, physicists have been slowing light down to a crawl, and even stopping it in its tracks, by shooting laser beams into cold gases known as Bose-Einstein condensates (BECs). Now, researchers have done the same thing in a much less exotic solid. The advance may one day lead to memory devices for computers that store information on beams of light.
In the late 1990s, physicists managed to slow light down to a poky 17 meters a second--below the top speed of a bicycle (ScienceNOW, [http://sciencenow.sciencemag.org/cgi/content/full/1999/218/1] 18 February 1999). They did this by creating sharp variations in how a BEC absorbs light across a narrow range of frequencies. Two lasers zapped the atoms in the condensate, causing their electrons to interfere with each other in ways that made it impossible for them to absorb photons of a certain frequency. This tiny "spectral hole" caused an unprecedented slowing of the light pulses.
Unfortunately, in solids, the laser-induced transparency trick makes too wide a spectral hole to slow light so dramatically, says physicist Phil Hemmer, now at Texas A&M University. To narrow the hole, Hemmer and colleagues used a third beam to increase the absorption inside the hole to create a smaller "antihole." The other two lasers also bleach a narrow anti-antihole of transparency inside the antihole, yielding a range of transparency narrow enough to slow light down to 45 meters per second, the team reports in the 14 January issue of Physical Review Letters.
The light can store information in ways that make it suitable for quantum computing, says team member Alexy Turukhin, a physicist at the Eatontown, New Jersey, branch of laser company JDS Uniphase, because the technique preserves the light's phase and amplitude. And although the yttrium crystal must be kept at a chilly 5 kelvin, it is still much easier to handle than a BEC. What's more, notes Lene Hau of Harvard University, light pulses shrink as they slow down, a property that might give scientists an efficient means of compressing information stored on light pulses.