RUBIDIUM FOUNTAIN CLOCK. Atomic clocks keep time by counting the cycles of light in a microwave cavity tuned to correspond to the internal energy transition of atoms launched from a trap. The atoms never absorb light at precisely the same frequency, so clock accuracy can be enhanced by averaging over larger samples of atoms, or by watching the atoms for longer periods by chilling the atoms to make them more uniform. High precision is manifested in a narrow linewidth in the spectrum of microwaves absorbed by the atoms. In the process of cooling the atoms with lasers, however, there is a drawback; the quantum wavelengths of the atoms themselves increase at lower temperatures, giving them a larger "cross section" for scattering from other atoms, which in turn corrupts the measurement process.

Physicists at Yale (Kurt Gibble, 203-432-6365, kurt.gibble@yale.edu) have succeeded in reducing the scattering problem by a factor of 30 by using rubidium atoms (instead of the more commonly used cesium atoms) in a "fountain" setup in which cooled atoms are put into an excited state in one microwave cavity (which is slightly detuned so as to cancel the effect of atom collisions) and then sent upwards until, when they are nearly at rest at the moment gravity starts to drag them back down, their transition energy is measured in a second cavity. With improved precision the Yale researchers expect to achieve an accuracy of 1 part in 10¹⁶, which would result in the best timekeeping ability yet attained. (Fertig and Gibble, Physical Review Letters, 21 August /pnu/2000/; Select Articles; see figure at Physics News Graphics.)