FIGURE 2.8 An atomic fountain. Atoms are held in optical molasses within a magnetic trap. A pushing beam launches them onto ballistic trajectories within a vacuum chamber. Radio frequency probes then excite the atoms from one energy state to another. (Adapted from Chu, "Laser Trapping of Neutral Particles," Scientific American, Feb. 1992.)

relatively dense beam of very slow atoms that can feed a continuous atomic fountain. "The Stanford funnel accepts atoms with a large velocity spread and cools them into a localized and collimated beam," he writes. "With our funnel, a beam of atoms was produced with a flux of 10⁹ atoms per second at a velocity of 270 centimeters per second and a temperature of 2 × 10⁻⁴ Kelvin."

The pursuit of ever-lower temperatures represents another area of current activity. This work relies on a subtle point: It is not the absolute values of atomic velocity that define their temperature but rather the velocity spread or range of variation about an average. Hence, to achieve ultralow temperatures, it suffices to greatly reduce this spread, even if the atoms continue to show a well-defined (and uniform) velocity. This work takes advantage of today's relatively long trapping times for atoms, which makes it possible to drive their velocity spread close to zero. Working with a colleague, Mark Kasevich, Chu reports producing a collection of sodium atoms with a velocity spread of only 0.027 centimeters per second, corresponding to an effective temperature of 2.4 × 10⁻¹¹ Kelvin.