In 1988, before the beginning of the current contract period, our group discovered that sodium atoms could be laser cooled well below the lower limit (the 'Doppler limit') predicted by the then generally accepted theory. Two main consequences of that discovery were a revamping of the theory of laser cooling and a renewed interest in the applications of laser cooling because of the far lower temperatures. Both of these effects are continuing in full force today, and this report is in large part a record of our group's continuing involvement in the new developments in laser cooling during the past three years. The work of the NIST laser cooling group falls functionally into the categories of: investigating laser cooling mechanisms; studies of collisions between laser cooled atoms; development of new trapping techniques for neutral atoms; study of the fluorescent spectrum of laser cooled atoms; manipulation of atoms and atom optics; development of coherent sources of vacuum ultraviolet light for laser cooling and high resolution spectroscopy; application of laser cooling and trapping technology to atomic frequency standards.
In 1988, before the beginning of the current contract period, our group discovered that sodium atoms could be laser cooled well below the lower limit (the 'Doppler limit') predicted by the then generally accepted theory. Two main consequences of that discovery were a revamping of the theory of laser cooling and a renewed interest in the applications of laser cooling because of the far lower temperatures. Both of these effects are continuing in full force today, and this report is in large part a record of our group's continuing involvement in the new developments in laser cooling during the past three years. The work of the NIST laser cooling group falls functionally into the categories of: investigating laser cooling mechanisms; studies of collisions between laser cooled atoms; development of new trapping techniques for neutral atoms; study of the fluorescent spectrum of laser cooled atoms; manipulation of atoms and atom optics; development of coherent sources of vacuum ultraviolet light for laser cooling and high resolution spectroscopy; application of laser cooling and trapping technology to atomic frequency standards.
Abstract : The program is to slow, trap and cool neutral atoms. Laser light is used to slow and cool atoms and magnetic forces are used to trap them. Ultimately we hope to be able to trap atoms for days and cool them to temperatures on the order of .000001 K in addition to the interesting scientific and technical challenges involved in trapping and cooling atoms, we hope to open several new lines of investigation with these cooled atoms. Cold trapped atoms constitute a good system for studying collective phenomena such as Bose condensation, low energy - Na and Na - Na, and coherent optical effects (the deBrogile wavelength can exceed the optical wavelength). They also offer opportunities for performing ultra-high resolution spectroscopy and are extremely promising candidates for a new generation of frequency standards.
A review is presented of some of the principal techniques of laser cooling and trapping that have been developed during the past 20 years. Its approach is primarily experimental, but its quantitative descriptions are consistent in notation with most of the theoretical literature. It begins with a simplified introduction to optical forces on atoms, including both cooling and trapping. Then its three main sections discuss its three selected features, (1) quantization of atomic motion, (2) effects of the multilevel structure of atoms, and (3) the effects of polychromatic light. Each of these features is an expansion in a different direction from the simplest model of a classical, two-level atom moving in a monochromatic laser field.
When an atom or a molecule interacts with a light beam, the light emitted or absorbed carries valuable information about the atomic or molecular structure. This phenomenon underlies the whole field of spectroscopy. But the interaction of a photon with an atom can be used to manipulate the atom as well as to probe its structure. For example, in an approach called optical pumping, invented by Alfred Kastler, one can use the resonant exchange of angular momentum between atoms and polarized photons to align or orient the spins of atoms or to put them in nonequilibrium situations. In his original 1950 paper Kastler also proposed using optical pumping to cool and to heat the internal degrees of freedom, calling the phenomena the “effet luminofrigorique” and the “effet luminocalorique.” Another famous example of the use of photon-atom interaction to control atoms is laser cooling. This technique relies on resonant exchange of linear momentum between photons and atoms to control their external degrees of freedom and thus to reduce their kinetic energy. Laser cooling was suggested independently by Theodor Hänsch and Arthur Schawlow for neutral atoms and by David Wineland and Hans Dehmelt for trapped ions. In an article written three years ago for PHYSICS TODAY (June 1987, page 34), Wineland and Wayne Itano presented the principle of laser cooling and the potential applications of cold atoms to fields of physics such as ultrahigh resolution spectroscopy, atomic clocks, collisions, surface physics and collective quantum effects. At that time laser cooling had brought temperatures down to a few hundred microkelvin, but unexpected improvements during the last three years have dramatically lowered those temperatures to only a few microkelvin. We now feel we understand the new physical mechanisms responsible for these very low temperatures.
Despite the prominent role of laser cooling in modern atomic physics, with applications ranging from quantum sensors to high-precision spectroscopy and ultracold chemistry, laser cooling techniques are still limited to atoms with simple energy level structure and closed transitions that are accessible by currently available continuous wave (cw) laser sources. Two main obstacles prevent the extension of laser cooling techniques to a variety of atomic species and molecules. The first is associated with technical difficulties in creating cw laser light in the vacuum ultraviolet (VUV) spectral range - the absence of those laser sources prevents laser cooling of simple atoms such as hydrogen, deuterium, and antihydrogen. The second obstacle is associated with complex energy level structure of many atoms and almost all molecules - it permits decay of an excited state into a number of lower-lying states that can be widely separated in energy, which makes laser cooling of such species inefficient and experimentally very challenging.
We present a simple review of the basic physical processes allowing one to control, with laser beams, the velocity and the position of neutral atoms. The control of the velocity corresponds to a cooling of atoms, that is, a reduction of the atomic velocity spread around a given value. The control of the position means a trapping of atoms in real space. The best present performances will be given, in terms of the lowest temperatures and the highest densities. The corresponding highest quantum degeneracy will also be estimated. It is imposed by fundamental limits, which will be described briefly. We also give the general trends in this field of research and outline the new directions which look promising for observing quantum statistical effects in laser cooled atomic samples, but which are for the moment restricted by unsolved problems. Introducing the Simple Schemes The radiative forces acting on atoms in a light field can be split into two parts, a reactive one and a dissipative one. The dissipative force (radiation pressure), which basically involves scattering processes, is velocity dependent. We will see that this dependence leads to the Doppler cooling scheme and to the concept of optical molasses, and we will give the corresponding minimal achievable temperature. The dissipative force can be made position dependent, through a gradient of the magnetic field, so that the atoms are also trapped in the so-called magneto-optical trap.
This paper is a short summary of a presentation given at the Fourteenth International Conference on Atomic Physics in Boulder CO, August 1994. It describes several recent advances in the field of laser cooling, trapping and manipulation of neutral atoms: adiabatic transfer of linear momentum from light to atoms; trapping and cooling atoms in optical lattices; evaporative cooling of alkali atoms held in magnetic traps; cyclic cooling of atoms in a magnetic trap; and optical shielding of laser‐cooled and trapped atoms from collisions with other cold atoms. Some of the work described was done in my own laboratory at NIST in Gaithersburg, and some in other laboratories. I have tried to choose topics complementary to those treated by other speakers at the conference. This paper is intended more as a guide to the literature on the subjects treated than a complete treatment in itself.