Atom Lasers Get More Laserlike

An "atom laser" could make measurements of length and time with unprecedented accuracy or even build microscopic structures atom by atom. But the few atom lasers built so far emit more of a blob than a beam, propelled by gravity, so it can only be directed straight down. Now researchers report in tomorrow's Science that by carefully nudging the atom cloud at the heart of an atom laser with light, they have produced an atom beam that is far more like a laser beam.

The active ingredient in atom lasers is a Bose-Einstein condensate, a trapped vapor of atoms cooled down to a temperature near absolute zero. Without the jostling produced by thermal energy, the atoms all condense into the single lowest quantum mechanical state. The first laserlike beam of atoms whose wave properties were "coherent"--identical and in step, just like the light waves in a conventional laser--was built at MIT in 1997 (Science, 31 January 1997, p. 637). The condensate is held together by a magnetic field, but if the atoms are slightly tweaked with a pulse of radio waves, they ignore the field, and a burst of atoms in the same coherent quantum state drops out of the trap.

To make a beam that could be aimed in other directions, William Phillips of the National Institute of Standards and Technology in Gaithersburg, Maryland, and his co-workers tried kicking the atoms out using a technique called Raman scattering. When a photon scatters off molecules in a fluid, it sometimes exits as a photon with a slightly longer wavelength, leaving behind a tiny bit of its energy. The researchers induced this effect with a laser beam. This produced a narrow, tightly focused atomic beam--as well defined as the beam from a laser pointer. And although the atom laser is not fully continuous, the emitted atom pulses overlap enough to form a nearly continuous beam.

Having an atomic ray gun with laserlike precision opens up a whole host of applications: Better atomic clocks and high-tech meter sticks can be made. And with such a beam, says Keith Burnett of Oxford University, "you'd be able to have the same control over matter that you have over light." The longer term dream is atomic holography. Just as a conventional hologram mixes beams of photons to create a three-dimensional image, so an atom hologram could combine beams of atoms to build a 3D solid object. Such a technique might one day be used to grow nanostructures for integrated circuits or biotechnology.