Continuous beams of cold atoms for space applications

The precision of atomic state measurements should ideally be limited by irreducible fluctuations associated with the number of atoms available, e.g. shot noise or quantum projection noise. In practice, other noise sources can limit the precision achievable; a well-known effect is the intermodulation effect, or Dick effect, which degrades the stability of atomic fountain clocks using pulsed sources. One way to beat this source of instability consists of interrogating the atoms with a microwave signal derived from exceptionally stable local oscillators, such as cryogenic oscillators, which are however bulky and not ideally suited to the constraints of space. Another way of reaching atomic-noise limited instability is to use continuous sources of cold atoms. Experimental results obtained both on a fountain standard and on an experimental Cs fountain illustrate the potential of continuous cold atomic beams for improving signal-to-noise ratio and precision of measurement. Current developments towards higher atomic beam flux are also described.