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Physics World

The coolest atoms yet!

Christopher J. Foot, Andrew M. Steane

Published Oct 1, 1990• 0 citations
Publisher

Abstract

New techniques of laser cooling have produced atoms with extremely low temperatures; Christophe Salomon and colleagues at the Ecole Normale Supérieure (ENS), Paris, have just measured caesium atoms with temperatures of only 2.5 ± 0.6 μK, the lowest kinetic temperature yet achieved (1990 Europhys. Lett. 12 683). This impressive experimental achievement is a good example of physics in action since it is the elegant and clear theoretical explanation of the underlying physics of the laser cooling processes, developed at the ENS, which has enabled them to push further than anyone else.

DOI
10.1088/2058-7058/3/10/18

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New techniques of laser cooling have produced atoms with extremely low temperatures; Christophe Salomon and colleagues at the Ecole Normale Supérieure (ENS), Paris, have just measured caesium atoms with temperatures of only 2.5 ± 0.6 μK, the lowest kinetic temperature yet achieved (1990 Europhys. Lett. 12 683). This impressive experimental achievement is a good example of physics in action since it is the elegant and clear theoretical explanation of the underlying physics of the laser cooling processes, developed at the ENS, which has enabled them to push further than anyone else.
A Very ingenious technique for laser cooling has enabled a group at the Ecole Normale Superieure (ENS) in Paris to reduce the spread in transverse momentum in an atomic beam to the equivalent of 2μK. By using a fundamentally new approach, Alain Aspect and colleagues (Phys. Rev. Lert. 61 826) have overcome the 'recoil limit'. Previously laser radiation has been used to produce, via absorption and re-emission, a frictional force on atoms to damp their motion. With such techniques the minimum energy for the atoms is approximately the recoil energy from emitting a photon; however, experimentally this limit had not been reached.
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.
There has been dramatic progress recently in atomic physics. It is now routine at hundreds of laboratories worldwide to cool samples of atoms to microkelvin temperatures through a process called laser cooling. Bizarre as it may sound, it is possible to use lasers (which are usually associated with cutting and heating) to slow atoms from the jumbo-jet speed of a room temperature gas to a snail's pace. At these low temperatures exciting new physics is observed, and atoms are treated as waves. An overview of the physics principles that underlie this technique is given, as well as a few examples of recent progress in the emergent fields of atom optics and manipulation of coherent matter waves.
We have produced a very cold sample of spin-polarized trapped atoms. The technique used dramatically simplifies the production of laser-cooled atoms. In this experiment, 1.8\ifmmode\times\else\texttimes\fi{}107{10}^{7}107 neutral cesium atoms were optically captured directly from a low-pressure vapor in a small glass cell. We then cooled the 1-mm3{\mathrm{mm}}^{3}mm3 cloud of trapped atoms and loaded it into a low-field magnetic trap in the same cell. The magnetically trapped atoms had an effective temperature as low as 1.1\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}K, which is the lowest kinetic temperature ever observed and far colder than any previous sample of trapped atoms.