Abstract In previous chapters we have seen how laser spectroscopy gives Doppler free spectra and also how other older techniques of radio-frequency and microwave spectroscopy can resolve small splittings, e.g. hyperfine structure. These methods just observe the atoms as they go past, but this chapter describes the experimental techniques that use the force exerted by laser light to slow the atomic motion and manipulate atoms. These techniques have become extremely important in atomic physics and have many applications, e.g. they have greatly improved the stability of the caesium atomic clocks that are used as primary standards of time around the world.
Abstract In previous chapters we have seen how laser spectroscopy gives Doppler free spectra and also how other older techniques of radio-frequency and microwave spectroscopy can resolve small splittings, e.g. hyperfine structure. These methods just observe the atoms as they go past, but this chapter describes the experimental techniques that use the force exerted by laser light to slow the atomic motion and manipulate atoms. These techniques have become extremely important in atomic physics and have many applications, e.g. they have greatly improved the stability of the caesium atomic clocks that are used as primary standards of time around the world.
When the Nobel Prize for Physics was awarded to Claude Cohen-Tannoudji, William D. Philipps and Steven Chu in 1997 for the development of methods for cooling and trapping atoms using lasers, this was the reward for two decades of investigations not only touching upon the fundamental aspects of the light–matter interaction, but also leading to a range of applications. On the theoretical level, the advent of this field of research in the late 1970s stimulated the development of various theoretical methods for describing the radiative forces by which light affects the motion of atoms. On the experimental level, judicious use of the radiative forces exerted by lasers made it possible to obtain a drastic reduction of atomic velocities – in other words, to cool an atomic vapour. These advances were characterized by a remarkable cross-fertilization of theoretical and experimental innovations. It was not long before applications began to appear in the field of high-resolution spectroscopy, since ultracold, i.e. slow, atoms can be observed for longer periods, and this allows greater accuracy when measuring atomic resonance frequencies than could be obtained for atoms at room temperature. Based on such resonances, the most accurate clocks in the world use laser-cooled atoms or ions. Atom interferometry is another application of laser manipulation of atoms, which can be applied to measure inertial effects, due for example to the rotation of the Earth or the motion of a vehicle, with an accuracy that already exceeds traditional methods.
Laser cooling experiments developed in the last decade enable physicists to slow down and spatially confine neutral atoms of a gas, thus lowering the temperature of trapped atoms into the μK range. One of the challenges associated with laser cooling is to stabilize a laser so that its output frequency matches that of a hyperfine structure transition and has an absolute stability of several MHz. In this project, we establish the groundwork for future laser cooling experiments with Rb by performing saturated-absorption Doppler-free spectroscopy studies. We acquire Doppler-free spectra of Rb that resolve individual hyperfine structure transitions, including the transition that the laser must excite in laser cooling experiments. The spectra we obtained can readily be repeated and the experimental apparatus can provide the feedback for an electronic laser stabilization circuit.
The development of laser cooling has an important impact on many aspects of atomic physics and quantum optics. The efforts to understand the various types of force exerted on atoms by laser light have led to some interesting physics and it is now possible for extremely cold clouds of atoms to be produced, which can be confined in atom traps or formed into very monoenergetic atomic beams. There are many new possibilities to explore in this ultra-cold regime where quantum effects are significant, in addition to the potential for great improvements in precision measurements made by r.f. and laser spectroscopy.
This chapter starts with the spectrum of alkali atoms, which have a very similar electronic structure to hydrogen. It describes the hyperfine structure of alkali atoms in the context of the development of microwave atomic clocks that currently provide the definition of the SI second. The simple and accessible electronic transitions of alkali atoms were used for the first demonstration of laser cooling, the revolutionary approach to atomic physics developed in the '80s, which the chapter discusses with a review of its most important configurations (optical molasses, magneto-optical traps, sub-Doppler cooling). The application of laser cooling to the development of very precise atomic clocks and to the field of atom interferometry is also covered.
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 stability and accuracy of atomic microwave frequency standards (atomic clocks) have been improving at a rate exceeding one order of magnitude per decade for the past 30 years. This sustained improvement has been driven mainly by the many and diverse technological applications requiring highly stable and accurate time and frequency standards. There is no reason to suspect that this rate of progress is slowing. Over the last decade, research and development in atomic frequency standards has tended increasingly towards the use of atom and ion trapping technologies to approach the supposedly ideal unperturbed atomic frequency reference consisting of a single atom or ion either motionless or in free-fall in a perfect, field-free vacuum. This work has been facilitated by the relatively recent development of techniques whereby laser light is used to cool, confine and manipulate atoms or ions. This paper reviews the current status of atomic microwave frequency standards based on trapped atoms and trapped ions, and attempts to explain the motivation for their development and the principles of their operation, with particular emphasis on the physics of factors which limit their performance.