We evaluate the frequency error from distributed cavity phase in the caesium fountain clock PTB-CSF2 at the Physikalisch-Technische Bundesanstalt with a combination of frequency measurements and ab initio calculations. The associated uncertainty is 1.3 × 10−16, with a frequency bias of 0.4 × 10−16. The agreement between the measurements and calculations explains the previously observed frequency shifts at elevated microwave amplitude. We also evaluate the frequency bias and uncertainty due to the microwave lensing of the atomic wave packets. We report a total PTB-CSF2 systematic uncertainty of 4.1 × 10−16.

Using information from a recent 85 Rb two-color photoassociation experiment, we evaluate the merits of fountain clocks based on 87 Rb and 85 Rb isotopes as alternatives to 133 Cs and find that they offer significant advantages. In the case of 87 Rb the collisionally induced fractional frequency shift is 15 times smaller than for 133 Cs. This small shift is associated with a small difference in the triplet and singlet scattering lengths for 87 Rb. For 85 Rb, the shift produced by the two m f ϭ0 clock states may have opposite signs allowing the shift to be eliminated by controlling the relative populations of these states. We also present collision quantities relevant to atomic fountain clocks containing multiply launched groups of atoms, and for evaporative cooling of 85 Rb atoms. ͓S1050-2947͑97͒50312-X͔

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We summarise the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the developments needed before cold atoms could be operated in space. The cold atom technologies discussed include atomic clocks, quantum gravimeters and accelerometers, and atom interferometers. Prospective applications include metrology, geodesy and measurement of terrestrial mass change due to, e.g., climate change, and fundamental science experiments such as tests of the equivalence principle, searches for dark matter, measurements of gravitational waves and tests of quantum mechanics. We review the current status of cold atom technologies and outline the requirements for their space qualification, including the development paths and the corresponding technical milestones, and identifying possible pathfinder missions to pave the way for missions to exploit the full potential of cold atoms in space. Finally, we present a first draft of a possible road-map for achieving these goals, that we propose for discussion by the interested cold atom, Earth Observation, fundamental physics and other prospective scientific user communities, together with the European Space Agency (ESA) and national space and research funding agencies.