There has been dramatic progress recently in atomic physics. It is now routine at hundreds of laboratories worldwide to cool samples of atoms to microkelvin temperatures through a process called laser cooling. Bizarre as it may sound, it is possible to use lasers (which are usually associated with cutting and heating) to slow atoms from the jumbo-jet speed of a room temperature gas to a snail's pace. At these low temperatures exciting new physics is observed, and atoms are treated as waves. An overview of the physics principles that underlie this technique is given, as well as a few examples of recent progress in the emergent fields of atom optics and manipulation of coherent matter waves.
There has been dramatic progress recently in atomic physics. It is now routine at hundreds of laboratories worldwide to cool samples of atoms to microkelvin temperatures through a process called laser cooling. Bizarre as it may sound, it is possible to use lasers (which are usually associated with cutting and heating) to slow atoms from the jumbo-jet speed of a room temperature gas to a snail's pace. At these low temperatures exciting new physics is observed, and atoms are treated as waves. An overview of the physics principles that underlie this technique is given, as well as a few examples of recent progress in the emergent fields of atom optics and manipulation of coherent matter waves.
Introduction This chapter describes remarkable new experiments that have been made possible through recent advances in laser cooling of atoms. Cooling of atoms in a gas by laser light is in itself surprising and this technique is important in a range of applications. In particular the experiments described here take atoms that have been laser cooled and then use other methods to reach even lower temperatures, at which wonderful and fascinating quantum effects occur. As well as revealing new phenomena, these experiments explore the deep analogies between the roles played by light waves in lasers and matter waves. These techniques are widely accessible and enable skilled researchers in laboratories all over the world to explore exciting new physics. The key physics and techniques of laser cooling are described in depth in Chapter 6. What is “temperature”? We use the words “cold” and “hot” every day; for example to refer to the temperature of the air outside. But what do “cold” and “hot” mean for the air’s molecules? Broadly speaking, we can say that atoms in a hot gas move faster than those in a cold gas and that cooling, by taking energy out of the gas, slows the atoms down. In this description of the gas as a collection of atoms behaving like billiard balls, or little hard spheres, the lowest possible temperature occurs when the atoms stop moving but, as we shall see, this picture does not give an accurate description at very low energies, for which quantum effects become important. In this chapter we will look at the fascinating properties of atomic gases cooled to temperatures within one millionth of a degree Kelvin from absolute zero. We shall use the term “ultra-cold” to refer to such extremely low temperatures that have only recently been achieved in experiments on atomic gases. The techniques used to cool liquids and solids have a longer history and have made available a different set of possibilities (see Chapter 8). Note that most of this chapter refers to gases in which the individual particles are atoms, but the general statements about temperature scales apply equally well to gases of molecules, e.g. air, whose major constituents, oxygen and nitrogen, exist as diatomic molecules.
Of the many application of lasers, the one with possibly the most dramatic impact on fundamental physics is laser cooling and trapping: the Nobel Prize for physics was awarded for laser cooling in 1997 [1]. The staggering progress achieved by laser cooling is reflected by considering a thought experiment that begins ‘take an atom. . . ’. Such an experiment was impossible to realize in practice because room temperature atoms move at supersonic speeds and, as atoms are neutral, there was no easy way to grab hold of them. Laser cooling allows one to obtain samples of atoms essentially at rest, with temperatures in the microKelvin region being routine. At such low temperatures, it becomes easy to trap or move atoms with modest electromagnetic forces [2, 3] and a whole new class of experiments previously confined to the realm of fantasy becomes possible. For example, one can throw atoms upwards and use precision spectroscopy to measure time or gravity with unprecedented accuracy. One can place atoms at a desired location on a surface creating a novel lithographic technique or place an atom just above the surface to create a qubit in a quantum information processor. One can take many atoms and cool them further by evaporation to form a Bose-Einstein condensate [4] or degenerate Fermi gas. This last development has propelled atomic and optical physics right into the heart of condensed matter physics and resulted in the award of the Nobel Prize in 2001.
In recent years, there has been significant interest in the laser manipulation of neutral atoms, and specifically in the use of light fields to manipulate not only the internal but also the external coordinates of an atom. Among the most dramatic results of this work have been the production of dense ultra-cold samples of trapped neutral atoms. These samples have been used to expand the horizons of fundamental physics and have made possible significant advancements in metrology. Furthermore, the ability to manipulate atomic motion with light has helped to create entirely new fields of research such as atom optics where matter wave analogues of light wave systems are investigated and optical where light is used to create a periodic array of atoms similar to crystals.
Of the many applications of lasers, the one with possibly the most dramatic impact on fundamental physics is laser cooling and trapping: the Nobel Prize for physics was awarded for laser cooling in 1997. Laser cooling allows one to obtain samples of atoms essentially at rest, with temperatures in the micro-Kelvin region being routine. At such low temperatures, it becomes easy to trap or move atoms with modest electromagnetic forces and a whole new class of experiments previously confined to the realm of fantasy becomes possible. The high brightness of masers and lasers makes it possible to access a regime of atomic excitation for which Einstein’s rate equations are inadequate. Three-dimensional optical molasses using counter-propagating laser beams along three mutually orthogonal directions was first observed in 1985 by Chu et al. The Sisyphus mechanism works well for atoms that are already cold and has a smaller capture velocity than conventional Doppler cooling.
Abstract One of the most remarkable achievements of laser control is the manipulation of atomic particles, their velocities, positions, etc., which is referred to as laser cooling and laser trapping . These two effects are interrelated and usually prove effective when used jointly. That this is so one can see by considering as an example the laser cooling of trapped ions and the laser trapping of laser-cooled neutral atoms (Fig. 5.1). The sequences of processes in these two situations are directly opposite. Ions can easily be trapped in an electromagnetic trap of large depth (Paul 1990) and then be cooled for a long time by laser radiation (sideband cooling) (Wineland and Dehmelt 1975; Dehmelt 1990). Neutral atoms are first slowed down and cooled by laser radiation down to millikelvin temperatures, and only then confined in an optical trap. Trapped atoms can further be cooled to microkelvin temperatures. The present chapter and Chapter 6 consider the cooling and trapping of neutral atoms, the present chapter concentrating on their cooling, and Chapter 6 on trapping. But it would be reasonable first to familiarize the reader with the history of the early ideas in this field that eventually led to the advent of a new domain of physics—the physics of ultracold atoms and molecules (Chapters 7 and 8).
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.