Atom cooling , trapping , and quantum manipulation

Modern atomic, molecular, and optical physics has advanced primarily by using known physics to devise innovative techniques to better isolate and control the atomic system, and then exploiting this nearly ideal system to achieve higher precision and discover new physical phenomena. The striking advances along these lines have been recognized by awards of Nobel Prizes to 21 individuals in this area; most recently, the 1997 Nobel prize was given for laser cooling and trapping of neutral atoms (Phys. Today, 1997). In the first half of the twentieth century, the Stern-Gerlach magnet, and later optical pumping, allowed the preparation and analysis of internal quantum numbers. Resonance techniques allowed the quantum state to be changed controllably, and methods such as Ramsey’s separated oscillatory fields, and spin echos created and exploited coherent superposition of internal quantum states. This control of internal states ultimately led to the invention of the maser and the laser. For a brief discussion of what might be called ‘‘Rabi physics,’’ see the article by Kleppner in this volume. This paper discusses the extension of this pattern of control and study to the external degrees of freedom (position and velocity) that has occurred in the last few decades. The strong forces of electric and magnetic fields on ions allow them to be trapped with high kinetic energy, and once trapped they can be cooled in various ways. The forces available to trap neutral atoms are much weaker. In order to trap them, they must first be cooled below 1 K by radiation pressure that cannot exceed 1 meV/cm for a strong resonant transition. For cold atoms, trapping has been achieved using resonant radiation pressure and/or forces from field gradients acting on either the atoms’ magnetic moments or their induced electric dipole moments. The latter force is produced by the electric field of a near-resonant, tightly focused laser beam. All of these traps have maximum depths, expressed in terms of temperature, on the order of 1 K for practical situations. Traps, together with cooling methods that have achieved kinetic temperatures as low as nanokelvins, have now created the ultimate physical systems thus far