The resonant force between atoms and light was first observed in 1933, when Otto Frisch measured the deflection of a sodium beam by a sodium lamp. The invention of lasers opened up new possibilities, leading to the development of the laser-cooling techniques that are the subject of this chapter.
There are two aspects of laser cooling that make it particularly remarkable:
The ability to cool a gas of atoms to very low temperatures has given rise to a whole host of related benefits. Atomic clocks have been made with greater accuracy, and a whole range of new quantum phenomena have been discovered. The most spectacular of these is Bose–Einstein condensation, which was first observed in 1995 and is discussed in Section 10.7.
The description of laser cooling and Bose–Einstein condensation in this chapter focuses on the basic principles. The reader is referred to specialized texts or articles for a more detailed discussion. See, for example, Foot (2004), Metcalf and van der Straten (1999), or Phillips (1998).
In order to understand how laser cooling works, we first need to clarify how the temperature of a gas of atoms is measured. The key point is the link between the thermal motion of the atoms and the temperature. Starting from the Maxwell–Boltzmann distribution (see Eq. [3.39]), it is possible to define a number of different characteristic velocities for the gas.