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.
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.
INTRODUCTION A variety of techniques have been developed over the past two decades for influencing the motion of atomic gases using laser light. These techniques, termed collectively 'laser cooling,' produce samples of atoms with temperatures one millionth of a degree above absolute zero, ushering in a new era of atomic physics. The exquisite control offered by these techniques has made possible measurements of fundamental constants, atomic clocks used in the realization of the SI unit of time, new atom-wave interferometers, and the realization of Bose-Einstein condensation. To understand the impact of gravity on such cold atoms, consider that if the atmosphere were an ideal gas cooled to one micro-Kelvin, it would only extend just 5 microns above the Earth's surface.
Forty years ago, twenty years after the discovery of the laser, physicists were developing laser cooling methods for ions trapped in electromagnetic fields.From the 1980s onwards, these techniques were refined and extended to atoms, thanks to the audacity and inventiveness of a generation of pioneering researchers.Actually, it was necessary to succeed simultaneously in trapping and cooling samples of atomic gases in a vacuum at a distance from any wall.Spectacular results followed and extraordinarily low temperatures were quickly reached, very close to absolute zero.The field of "cold atoms" was born, rewarded by successive Nobel prizes, first of which was awarded in 1997 to William D. Phillips, Steven Chu and Claude Cohen-Tannoudji, last in 2022 to Alain Aspect.Gaseous samples of a few thousand to a few billion atoms can be prepared at a few millionths of a degree above absolute zero, which means that the particles move at extremely low speed, of the order of centimetres per second.At these extreme temperatures, the behaviour of matter changes and its properties can only be described using quantum mechanics and the wave properties of particles.New physical phenomena have been discovered and innovations have followed the theoretical and experimental progress of the research.Initially imagined as a wonderful method for perfecting atomic physics, cold atoms have gradually proved to be powerful tools for research in cross-cutting fields of physics, such as condensed matter and even high-energy physics.These atoms are now referred to as 'quantum gases' at such low temperatures that their collective behaviour is modified by the laws of quantum mechanics.
Slowing, cooling, and trapping molecules One of the major achievements in physics during the last two decades of the twentieth century was the ability to cool and trap atoms [1]. Laser cooling can produce previously inconceivable atomic temperatures of less than 1 μK. Atoms with sufficiently low kinetic energy can be trapped in magnetic and optical traps and thus studied for a prolonged time, thereby improving the resolution of spectroscopy by many orders of magnitude as well as reducing Doppler broadening. Furthermore, besides the external degrees of freedom, the internal degrees of freedom of the atoms can also be manipulated, leading to full control of the atoms. One of the holy grails of physics, the achievement of Bose–Einstein condensation in a dilute gas, was achieved mainly because of laser cooling and trapping of atoms. One of the natural extensions of laser cooling and trapping of atoms is the application of the techniques to molecules. Molecules offer a much richer internal structure than atoms, and thus provide extended possibilities to experimentalists. However, atomic laser cooling is based on the repetitive scattering of light and requires a closed, or at least nearly closed, two-level system. Because of the complicated internal structure of molecules, a closed two-level system cannot usually be found because they have many different electronic, vibrational, and rotational states and in general can decay to many different states after absorption of light (see Chap. 16). This requires a complete rethinking of the schemes that are used to cool atoms. Still, because of the extended possibilities cold molecules can offer with respect to cold atoms, there have been a large number of techniques developed to slow, cool, and trap them. These techniques can be divided into two classes, namely direct and indirect cooling. In the first case, molecules at cryogenic or room temperature are directly slowed and cooled. Although many different slowing techniques are used, this chapter focuses on Stark slowing of molecules since it was the first technique developed. Apart from the Stark shift of the molecules, the Zeeman and light shift of molecules can also be used to slow them. The idea behind Stark slowing is to subject the moving molecules to a continuously increasing electric field so that their kinetic energy is converted into potential energy.
The development of techniques to cool/slow and trap atoms and molecules brought about a revolution in atomic and molecular physics, with an impact on physics in large. When slowed/cooled down, atoms and molecules become matter waves whose properties differ vastly from those of thermal matter. In particular, close to absolute zero temperature, the interaction range of atoms or molecules increases far beyond their dimensions, imparting to them highly desirable collective and collisional properties. The latter have been predicted to give rise to a novel ultracold chemistry. The article describes the main techniques of producing cold molecules, both the indirect ones (photoassociation of cold atoms, collisional association of cold atoms controlled via Feshbach resonances) and the direct techniques (buffer-gas cooling, Stark deceleration and others). Ongoing and anticipated future experiments with cold molecules are reviewed, including pioneering ventures aimed at radiative lifetime measurements, reduction of transit time broadening, measurements of ultracold collisional cross-sections, and testing of fundamental symmetries in nature, such as the time-reversal symmetry and parity.
In the 1980s the first successful experiments [312] and theory [98], demonstrating that light could be used to cool and confine atoms to submillikelvin temperatures, opened several exciting new chapters in atomic, molecular, and optical (AMO) physics. Atom interferometry [6, 8], matter–wave holography [294], optical lattices [192], and Bose–Einstein condensation in dilute gases [18, 95] all exemplified significant new physics where collisions between atoms cooled with light play a pivotal role. The nature of these collisions has become the subject of intensive study not only because of their importance to these new areas of AMO physics but also because their investigation has led to new insights into how cold collision spectroscopy can lead to precision measurements of atomic and molecular parameters and how radiation fields can manipulate the outcome of a collision itself. As a general orientation Fig. 1.1 shows how a typical atomic de Broglie wavelength varies with temperature and where various physical phenomena situate along the scale. With de Broglie wavelengths on the order of a few thousandths of a nanometer, conventional gas-phase chemistry can usually be interpreted as the interaction of classical nuclear point particles moving along potential surfaces defined by their associated electronic charge distribution. At one time liquid helium was thought to define a regime of cryogenic physics, but it is clear from Fig. 1.1 that optical and evaporative cooling have created “cryogenic” environments below liquid helium by many orders of magnitude.
Abstract The invention and development of laser methods for cooling and trapping atoms have led to the advent of the physics of ultracold matter. Low-temperature quantum physics has so far dealt with condensed media, wherein quantum phenomena (superfluidity and superconductivity) usually manifest themselves at temperatures of a few kelvin. Laser cooling and trapping of atoms have made it possible to successively attain millikelvin, microkelvin, and nanokelvin temperatures, at which the de Broglie wavelength of the atoms becomes commensurable with the interatomic separation, even in dilute gases. The interaction of atoms in ultracold dilute gases becomes essentially quantum mechanical. The physics of such quantum gases is more akin to the well-developed physics of condensed matter at low temperatures. It is in essence a boundary field between atomic, laser, and condensed-matter physics. Following the first, pioneering experiments on the Bose–Einstein condensation of alkali-metal atoms, this field has continued to develop at a very fast pace, which can be explained by the vast variety of particles (atoms, isotopes, and molecules) that can be laser cooled and trapped under degeneracy conditions. This is much more than physicists had at their disposal previously (3,4He). Moreover, laser radiation and magnetic fields additionally make it possible to control not only the intrinsic quantum state of the interacting ultracold particle species, but also the interaction of the particles. All this makes the research in the field of quantum gases a great breakthrough, both in atomic–molecular physics and in many-body-interaction physics, which heretofore was the prerogative of low-temperature condensed-matter physics.