Slowed light handed off

Eight years ago Lene Hau and colleagues at Harvard University demonstrated the ability to slow a light pulse in a gas of atoms to 17 m/s. Quantum interference effects among the atoms, the light pulse, and a second, coupling light beam slow and spatially compress the light pulse as it enters the atom cloud; the pulse’s amplitude and phase information get coherently imprinted on the atoms. A few years later, the team showed that if the coupling beam is turned off, the light pulse is actually halted, but its imprint on the atoms remains. If the coupling beam is turned back on, the original light pulse could be reconstituted and sent on its way. Now Hau’s group has added an extra layer to the story: passing the encoded light-pulse information from one atom cloud to another. She and her colleagues begin by halting and extinguishing a light pulse in a Bose–Einstein condensate of sodium atoms. The amplitude and phase of the light pulse become imprinted on a pulse of atoms that is ejected from the condensate in the same direction as the incident light pulse. As seen going from left to right in this shadow image, the “messenger” pulse travels to a second sodium BEC some 160 microns away, from which the signal is revived as a light pulse. This feat, say the researchers, may find application in quantum information processing and controllable optical delay lines. ( N. S. Ginsberg, S. R. Garner, L. V. Hau, Nature 445 , 623, 2007. http://dx.doi.org/10.1038/nature05493 )