First trap your atom

The paper concerns the trapping of a single ion, or small cloud of ions, for several hours, in an ion trap. The study of such clouds of ions or atoms gives physics what is, in effect, a new state of matter with unusual properties. A description is given of an ion trap, and the way lasers keep ions cool. Uses of ion traps are described including: a) the study of collisions between atoms under closely controlled conditions, b) measuring lifetimes of 'excited' ions, and c) the measurement of masses of ions. How to observe a quantum jump is outlined, along with the use of quantum jumps to provide information from a small sample of ions in a trap operated as a clock. (UK)