The past few years have seen remarkable developments in the ability to control atomic motion by optical techniques. Among the key tools that have been developed for this purpose are the techniques of laser cooling, the significance of which was recognized by the award of the 1997 Nobel Prize in Physics to Steven Chu, Claude Cohen-Tannoudji, and William Phillips. These techniques have made it possible to cool atoms to temperatures lower than have been produced in any other physical system - around 10(-8) degrees Kelvin above absolute zero - and to confine those in traps for extended periods of observation.
One of the principle motivations for cooling and trapping atoms has been to attain the state of quietude suitable for the highest-resolution spectroscopy of isolated atoms, which is needed for the improvement of frequency standards and atomic clocks. However, cold-atom systems have been found to provide arenas for the study of fundamental phenomena of many-body physics at unprecedented levels of precision and flexibility.
One such phenomenon is Bose-Einstein condensation (BEC), which was first produced in an atomic gas in 1995. The study of BEC in gases has stimulated intense interest in atomic, condensed-matter, and statistical physics, some of which is reflected in a previous focus issue of this journal, and which is discussed in three papers of the present issue.
Durfee and Ketterle present a review of their recent experiments on BEC, which have provided a wealth of information on condensate dynamics. Their presentation includes a number of animations that enable the reader to view the formation of the condensate in a cooled gas, collective oscillations, collisions between separately-prepared condensates, and the pulsed output of a prototype atom laser.
In early 1997 the generation of a condensate with two components, corresponding to different spin states of the Rb atom, was demonstrated at JILA, and this has stimulated much interest in the range of possibilities for simultaneous BEC of multiple species. The paper of Ejnisman et al. discusses the dynamics of a two-component condensate, consisting of Rb and Na atoms. It shows that there is a wide range of possible dynamical behavior, depending on the details of the interaction between Na and Rb atoms, which are as yet unknown.
The nature of atomic pair interactions is of critical importance for the formation and stability of a condensate. Although condensates have been produced in Rb, Na, and Li, several attempts to generate a Cs condensate have not yet been successful. The paper by Dalibard reports the cooling of Cs gas to 80 nK, about ten times colder than previous attempts. However, attainment of the density required for BEC is frustrated by inelastic atomic collision processes.
The paper by Mitchell et al. deals with another application of laser cooling: the production of very cold ions stored in an electromagnetic trap. Such ions constitute a non-neutral plasma, in which a variety of basic issues of plasma physics can be investigated. In particular, the spectroscopy of modes of plasma oscillation can be performed with high precision even in the nonlinear regime.