New laser cooling mechanisms, more efficient than Doppler cooling, allow one to reach new regimes where the quantum nature of the translational degrees of freedom of an atom can no longer be ignored. This paper reviews a few examples of quantum phenomena observable on laser cooled atoms: quantization of atomic motion in an optical potential, de Broglie wavelengths larger than the wavelengths of the laser used to cool the atoms. New theoretical approaches to these problems will be also briefly mentioned.
New laser cooling mechanisms, more efficient than Doppler cooling, allow one to reach new regimes where the quantum nature of the translational degrees of freedom of an atom can no longer be ignored. This paper reviews a few examples of quantum phenomena observable on laser cooled atoms: quantization of atomic motion in an optical potential, de Broglie wavelengths larger than the wavelengths of the laser used to cool the atoms. New theoretical approaches to these problems will be also briefly mentioned.
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.
Laser cooling has advanced to the point where atomic motion can no longer be described in terms of discrete particles moving on classical trajectories. Instead, de Broglie wave fields occupy quantum states of a potential created by the interaction between atoms and the light field. Atomic transitions between these quantum states are mediated by optical pumping, in much the same way as transitions were made between internal angular momentum states in Prof. Series' time. Some examples are studied here, with special emphasis on dark states.
The techniques of laser cooling have now become sufficiently developed that the focus has shifted toward interesting applications such as the quantum domain of atomic motion.This topic is characterized by the failure of the classical description in which atoms move as point particles whose trajectories can be known: instead, atomic motion must be described as the optics of de Broglie waves.For example, when the de Broglie wavelength λ d B exceeds λορtical, then a classical description is insufficient (Bose condensation is done in the dark, and the quantum condition becomes λdB > nearest.neighbor distance).One of the most fascinating topics of quantized atomic motion in a laser field derives from optical dark states that can even occur in the simplest (two-level) atoms, where there are no magnetic sublevels and the polarization is irrelevant.In spite of the simplicity of this two-level atom case however, the more interesting cases occur in multilevel atoms where the internal magnetic states and external quantum states of atomic motion become truly entangled.Schrödinger called such states "the heart of quantum mechanics" because they led to puzzles such as his famous "cat" and the EPR paradox.
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.
By using quasi-resonant exchanges of energy, linear and angular momentum between atoms and photons, it is possible to polarize atoms, to displace their energy levels and to control their position and their velocity. A few physical mechanisms allowing one to trap atoms and to cool them in the microkelvin, and even in the nanokelvin range, are briefly described and classified with the help of a few simple guidelines. Various possible applications of such ultracold atoms are also reviewed. They take advantage of the long interaction times and long de Broglie wavelengths which are now available with laser cooling and trapping techniques. New quantum situations can also be achieved in these experiments, calling for new theoretical approaches. The last part of this paper is devoted to a brief discussion of these problems.
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.