Matter makes waves
Mar 19, 1999
Physicists' dreams of building an atom laser - an intense, highly directional, coherent beam in which all the atoms have the same wavelength, just like the photons in a laser beam - has moved a step closer to reality. A team of researchers from the US and Japan, led by Bill Phillips of the National Institute of Standards and Technology in Maryland, has extracted a highly directional beam of sodium atoms, all with a well-defined momentum, from a Bose-Einstein condensate (Science 283 1706). And working with researchers from Israel, Phillips and co-workers have demonstrated "four-wave mixing" - a standard technique in nonlinear optics - with matter waves for the first time (Nature 398 218). Meanwhile, Theodor Hänsch and co-workers in Munich have produced a continuous beam of rubidium atoms for a tenth of a second - the longest time yet (Phys. Rev. Lett. to be published).
Phillips and his colleagues produced a focused directional output from the Bose-Einstein condensate by firing two laser beams into the condensate. One beam adds energy to the atoms, while the second stimulates them to emit photons and drop down to a lower energy state. As the photons of the second laser beam have slightly less energy than photons from the first beam, the atoms gain a small amount of energy. This energy is converted into momentum, which pushes the atoms into a given in a direction.
To create the four-wave mixing, Phillips and his team again started with a Bose-Einstein condensate. This time, however, they pumped a series of Bragg laser pulses into the atom cloud. Each pulse pushed some of the atoms out of the condensate. The pulses created three wave packets, which immediately started to overlap and produced a new wave packet with its own unique momentum.