Less is more: Experiments with an individual atomic particle at rest in free space

This article surveys experiments with individual closely confined atomic particles since their beginning in 1973. Such experiments have improved information on the size of an elementary particle on the same level as a quark, the electron, by at least three orders of magnitude. Their extension to the optical region promises atomic clocks of a reproducibility improved up to 100 000 times. The most important of the techniques developed for continuously detecting, cooling, and spin-state analyzing a permanently confined individual electron are described in some detail. The electron, trapped in ultrahigh vacuum at liquid helium temperature, is profitably viewed as a man-made atom, geonium.