FIGURE 2.7 Layout of apparatus for use of a magnetic field in slowing and trapping sodium atoms. The atomic beam comes from the left; the laser beam, which slows the atoms, comes from the right. A tapered solenoid applies a magnetic field that diminishes with distance, imposing a Zeeman effect that adjusts the atoms' spectral-line frequency to match the constant frequency of the laser. A pair of magnetic coils then trap the atoms, which have nearly zero velocity. From "Cooling and Trapping Atoms," by W.D. Phillips and M. J. Metcalf. Copyright © 1987 by Scientific American, Inc. All rights reserved.)

Within a sodium beam, atoms initially fly at speeds of some 1000 meters per second. Since each encounter with a photon can slow such an atom by three centimeters per second, every atom must absorb and reemit some 30,000 photons to come close to a standstill. Fortunately, this cyclic process repeats so rapidly that the atoms decelerate at rates of some 10⁶ meters per second squared, or 100,000 times greater than the acceleration due to gravity. That suffices to bring them to a stop in as little as a millisecond, along a range of only 50 centimeters.

Similar techniques suffice to cool ions as well as atoms, and ions offer several advantages. Being electrically charged, it is possible to hold them for long periods by applying electric fields. Indeed, Hans Dehmelt, who has pursued his lengthy involvement with Priscilla the Positron, has similarly trapped a barium ion named Astrid. The opportunity to trap ions electrically, in turn, means that one can first trap them and then cool them, which can be easier than the cool-first, trap-later procedure with atoms. A third advantage is that ions that resist laser cooling can nevertheless reach low temperatures by storing them in a trap along with other ions that indeed respond to the laser. This technique, known as sympathetic laser cooling, treats the coolable ions as a type of ice, able to receive heat from the other ions and hence to slow their motions as well.

As an example of the measurements that then become possible, David Weinland and colleagues, at the National Institute of Standards and Technology, have carried out studies on a single positively charged