Abstract
The second is the only unit of the SI system which is defined in terms of an internal property of an atom. The definition is realized using a thermal beam of caesium atoms. In classical caesium beam frequency standards, the necessary state selection is accomplished by deflecting atoms in magnet state selectors. In devices under development, this state selection is performed through atom-light interaction. The physical bases underlying the caesium beam frequency standards are given. The main design characteristics of caesium beam tubes are described. The limitations to the properties of metrological interest, frequency stability and accuracy are analysed. In particular, the principal sources of frequency offsets are described. The potentialities of classical and of optically pumped caesium beam frequency standards are compared. Manufactured devices designed for field use are considered as well as laboratory primary frequency and time standards.