Interest of a multipole trap to improve the frequency stability in microwave clocks based on confined ions
Discipline: Physics and Matter Sciences & Speciality: Optics, Photonics, and Image processing
- Luca GUIDONI – University of Paris – Referee
- Roland WESTER – University of Innsbruck – Referee
- Michael DREWSEN – Aarhus University – Examiner
- Tanja MEHLSTAUBLER – Leibniz University – Examiner
- Richard THOMPSON – Imperial College London – Examiner
- Frédéric ZOLLA – Aix-Marseille University – Examiner
- Caroline CHAMPENOIS – Aix-Marseille University – Supervisor
The stability properties of radio-frequency traps make them a technology of choice for the design of embedded microwave ion clocks for deep space navigation applications. The main effect limiting the frequency stability of such clocks is the frequency shift induced by the second order Doppler effect which fluctuates with the number of trapped ions. The use of radio-frequency multipole traps in the design of microwave clocks is motivated by a built-in reduction of the radio-frequency driven motion of the ions with the increase of the order of the multipole field. Experimental realizations by NASA-JPL of ion clocks involving higher order multipole traps have demonstrated a stability gain, but no direct measurement has yet been undertaken to clarify wherever this gain is a direct consequence of the number of electrodes in the trap or from a global optimization of the whole setup. One of the objectives of the TADOTI experiment is to conduct a comparative measurement of the velocity distribution over a laser cooled Ca+ ion cloud trapped in a quadrupole and an octupole trap. Observations of cold samples have shown an inhomogeneous spatial distribution of the trapped particles with clustering of the ions in local potential wells. Simulations attribute this organization to a symmetry breaking in the trap, induced by realistic mis-positioning of the electrodes. A prerequisite for the velocity distribution characterization is to restore to symmetry of the potential of the octupole trap by tuning the RF voltage applied to each electrode. This thesis focuses on the analytical description of the potential perturbations induced by the structural deformation of the trap, and proposes a characterization and compensation protocol of the perturbations, based on the localization of the laser-cooled ion in the trap. These tools can be used to create any configuration of three parallel ion clouds in the octupole trap.
Keywords: Trapped ions, Frequency metrology, Laser cooling, Doppler effect, Radio-frequency trap, Octupole trap.