Putting the brakes on light might have applications years down the road, but right now it’s a fascinating new physical property. To get there, physicists led by Lene Vestergaard Hau of the Rowland Institute for Science in Cambridge, Mass., used a tiny blob of supercooled sodium,activated with a laser at a particular wavelength. A second laser can then be fired through the normally opaque sodium; it passes through, but not much faster than a racehorse can run.
The experiment, reported in last week’s issue of the journal Nature, took advantage of a peculiar property called electromagnetically induced transparency. Every element has a certain color of light with which it interacts most strongly. Light at that exact wavelength would normally be totally absorbed. For sodium, the stuff these researchers used, it’s the yellow of common street lights. But a laser at a slightly different wavelength, called a coupling beam, can alter the quantum properties of the atoms so they no longer absorb that wavelength. The quantum seas part, and a laser at the wavelength that should have been absorbed passes through unfettered.
But changing the way a medium absorbs light also transforms the way light moves through it. All transparent substances slow down the light that moves through them–that’s why light seems to bend, or refract, when it passes from air to water. Electromagnetically induced transparency, says Stanford physicist Steve Harris, spectacularly alters sodium’s refractive properties. Light takes seven microseconds to cross a sample only eight thousandths of an inch long; in the vacuum of space, light would go more than a mile in the same time.
The sodium slows light the most if it’s in a special state, a quantum curiosity called a Bose-Einstein condensate. It’s created when atoms are cooled until they stop moving almost entirely. The atoms merge; the quantum wave functions that define them combine into one. ““You need a very pure gas, very cold, and the atoms in it can’t be colliding with each other,’’ says Eric Cornell, a physicist at the University of Colorado at Boulder and one of the makers of the first Bose-Einstein condensate. Chilled to just billionths of a degree above absolute zero, individual sodium atoms become a single, dense glob, atoms vibrating in lockstep.
Superslow light will remain a laboratory curiosity for some time to come. If they can slow photons down enough–Hau is shooting for about a centimeter a second–scientists could make images of the light as it passes by. In theory, it might be brought virtually to a stop, although it’s hard to know what you’d do then. Now, if only they could get it to go faster–that would be something.
