Interrogation of cold atoms in a primary frequency standard

We have studied the microwave field phase inside several cavities in the scope of optimizing the cold atoms interrogation scheme in a space clock. Numerical calculations are performed by a 3D finite elements method. This leads to a precise map of the field phase and amplitude, and to its sensitivity to geometrical defects. Experimentally, we have used our space clock prototype to assess the numerical results. Thanks to cold atoms, it is possible to obtain local information on the field phase by pulsing the microwave field. We have tested a cylindrical TE₀₁₃ cavity. It exhibits large phase inhomogeneities and is not compatible with a 10^-16 relative frequency accuracy. To refine the measurements, we have added a second detection area in our prototype. Then, we can operate the clock in two different modes: one pass through the cavity as in a space geometry or two pass through the cavity in a fountain geometry. We can also slice the atomic cloud to improve the spatial resolution of the field phase measurements. We plan to use this configuration to study the phase distribution of the micro-wave field inside a TE₀₁₁ cavity. In a fountain geometry operation, the measured frequency stability is 1.4×10^-13τ^-1/2 and a preliminary frequency comparison with the BNM-LPTF FO1 indicates an absolute frequency agreement better than 2×10^-15