In experiments reminiscent of “Star Wars” on a microscopic scale, two teams of researchers have for the first time succeeded in using the fine touch of a laser beam to bring a stream of speeding atoms to a halt. They did it by shining precisely turned laser light into the face of an onrushing beam of free, neutral sodium atoms.
These experiments open the way for trapping neutral atoms within pockets created by electrical or magnetic fields. Stationary or slowly drifting atoms may spend enough time within the traps to allow extremely precise measurements of their spectra — the frequencies at which the atoms absorb or emit light. Normally, any atomic movement tends to broaden, shift or blur these spectral lines. Until now, only ions (atoms with a net electrical charge) could be trapped and would stay put long enough for such exact observations to be made. “Right now, the technique we have is suitable for providing nearly stopped atoms for loading into magnetic or laser traps,” says William D. Phillips of the National Bureau of Standards (NBS) in Gaithersburg, Md. Phillips, Harold Metcalf of the State University of New York at Stoney Brook and their colleagues describe their method in the March 11 PHYSICAL REVIEW LETTERS.
In this technique, a laser beam, tuned to one of the resonance frequencies at which sodium atoms absorb light, is fired directly at a narrow beam of sodium atoms that initially have an average speed of about 1,000 meters per second. For eash photon absorbed, an atom slows down by a certain amount. It takes about 30,000 photons to bring a given sodium atom to rest. But, just as the pitch of a train’s whistle changes when the oncoming train slows down (the Doppler effect), so too the atomic resonance frequency shifts slightly as atoms slow down. If the shift is too large, the atoms no longer absorb laser light and stop slowing down. To compensate for this effect, the researchers use a magnetic field that gradually varies along the length of a solenoid through which the sodium beam passes. This magnetic field brings an atom’s resonance frequency back to a level at which laser light can be absorbed. “If the magnetic field changes too rapidly, then the atoms can’t absorb light fast enough to keep up with the change in the field and they go out of resonance,” says Phillips.
“We’ve carefully designed the field gradient so that that doesn’t happen. The deceleration of the atoms automatically adjusts itself so that it occurs at exactly the rate necessary to keep up with the change in the magnetic field.” A rival technique, reported in the same journal and developed at the Joint Institute for Laboratory Astrophysics in Boulder, Colo., also uses the idea of laser “cooling”–slowing atoms to speeds that correspond to temperatures well below I kelvin (close to absolute zero). However, instead of using a spatially varying magnetic field along the length of a solenoid to compensate for the Doppler effect, this group of researchers changes the frequency of the laser light. If an atom happens to absorb too many photons, it automatically stops absorbing until the frequency of the laser catches up. So far, both techniques have been tried only on sodium atoms. “No one would be serious about making a practical frequency standard with sodium atoms,” says Phillips.
HoweveR, “it might be possible to make a lot of very slow cesium atoms and to use them in very much the same way as they’re now used in atomic clocks.” John L. Hall and the Boulder group are more expansive. They report, “We expect that the near future will offer some delicious and bizarre advances in the art of manipulating atoms.”