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  • Print publication year: 2018
  • Online publication date: June 2018

10 - Cold Atoms

from Part II - Applications of Atomic Physics


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:

  • (i) It is highly surprising that the technique works at all. We would normally expect a powerful laser to cause heating rather than cooling. This makes us realize that the technique will only work when special conditions are satisfied.
  • (ii) The very low temperatures achieved by laser cooling are extremely impressive, but this in itself is not the main point, as techniques for achieving very low temperatures have been used for decades by condensed-matter physicists. For example, commercial dilution refrigerators routinely achieve temperatures in the milli-Kelvin range, and as early as the 1950s, Nicholas Kurti and coworkers at Oxford University used adiabatic demagnetisation to achieve nuclear spin temperatures in the micro-Kelvin range. The novelty of laser cooling is that it produces an ultracold gas of atoms, in contrast to the condensed-matter techniques that work on all liquids or solids. These ultracold atoms only interact weakly with each other, which makes it possible to study them with unsurpassed precision.
  • 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).

    Gas Temperatures

    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.

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