Advanced microwave control for atomic fountain clocks

Time interval and frequency can be measured with lower uncertainty and greater resolution than any other physical quantity. Using caesium fountain clocks, the SI-second can be realized with uncertainties of several parts in 10(high16). In a fountain clock, microwave fields are used to manipulate the atomic states. These fields are driven by dedicated microwave signals. The generation of microwave signals is a key aspect for the operation of fountain clocks, as it can significantly contribute to the clocks statistical as well as the systematic uncertainty. This thesis discusses the contributions of the microwave signal generation to the uncertainty of a caesium fountain. Several methods aimed at the reduction of the statistical as well as the systematic uncertainty were implemented and assessed. A modular microwave synthesizer has been designed, ensuring high reliability and high availability. By utilizing a high stability local oscillator, the contribution of the microwave signal generation to the statistical uncertainty of the fountain clock could be reduced to an insignificant level. The synthesizer has been augmented with a modulation scheme to implement the method of Rapid Adiabatic Passage for collisional frequency shift measurements. Application of this method in the fountain clock CSF²²2 lead to a significant reduction of the collisional shift uncertainty and enabled fountain operation with high atom numbers. Phase perturbations in the microwave fields during the state manipulation can lead to shifts of the fountain frequency if they are synchronous with the fountain cycle. To facilitate a detailed analysis of cyclic perturbations on the micro-radian level, a dedicated phase transient analyzer was developed. With this system, the effect of cyclic phase perturbations can be evaluated at the low 10 (high−17) level. Uncontrolled interactions between the caesium atoms and resonant microwave fields can also be a source of frequency shifts. A method for the suppression of such shifts has been developed, relying upon a precise control of the field’s frequency detuning. By using this scheme, the uncertainty contributions due to such interactions at CSF could be limited to few parts in 10(high17).