Laser-pumped cesium magnetometers for the PSI-nEDM experiment

This thesis work describes the development of a high resolution magnetic field sensor for a fundamental research project of an international collaboration with participation of the Fribourg Atomic Physics Group. The goal of the collaboration is an experiment searching for an electric dipole moment of the neutron (nEDM), d, which will be performed at Paul Scherrer Institute (PSI). The possible existence and mainly the size of the dipole moment is of great scientific interest concerning a better understanding of the fundamental interactions of elementary particles. If one is able to lower the sensitivity for a nEDM to the level of σ(d) ~ 10-27e • cm, the experimental result will give valuable information regarding "new physics" beyond the Standard Model. Such an improvement in resolution is the purpose of the PSI-nEDM experiment, which has to be performed under ultra-stable conditions. Hereby, a crucial requirement is the control of the fluctuations of a 1μT magnetic field (about 1/50 of the earth magnetic field) on a level of better than 100 fT which corresponds to a relative (in)stability of less than 10-7. The magnetic field stability requirement can directly be understood as a specification for the magnetic field sensors. In order to reach a sub-pT resolution we have developed a specific sensor system using the technique of optically-detected magnetic resonance (ODMR) in cesium vapor. Due to their unpaired valence electron the Cs atom is paramagnetic and has a magnetic moment on the order of one Bohr magneton. This magnetic moment interacts with an external magnetic field resulting in a Larmor precession around the field direction at a frequency proportional to the magnetic field. On the other hand, the orientation of the spin, i.e., of the total angular momentum of the atoms, determines the optical properties of the Cs vapor. The detection of the Cs spin precession is thus possible via their interaction with light. The principles of that technique have been well known for about 50 years and magnetometers are commercially available for particular applications ranging from geology to archeology. Nevertheless, depending on the requirements and specific applications very specialized realizations are needed. In the present thesis work discharge lamps, which usually serve as light sources, were replaced by a frequencystabilized diode-laser system. It could be shown that this leads to an intrinsic sensor resolution by up to a factor of three better than the one obtained with lamp-pumped devices. The active volume of the sensor, given by the size of the cell containing the Cs vapor, was about 180 cm3. After optimizing all parameters a resolution of 14 fT√pHz was obtained, and the magnetic field measurements can be performed with a bandwidth of about 1 kHz. After characterizing and optimizing the sensor in a very controlled environment in Fribourg a multisensor system was built in order to measure magnetic field fluctuations at real experimental sites at PSI and at Institute Laue-Langevin (ILL) in Grenoble. A major step for the realization of such a measurement system was the development of a multi-channel frequency counter based on a commercial sound card, which fulfills – in contrast to commercial frequency counters at hand – the frequency resolution requirement. In parallel to the research and development work for the PSI-nEDM experiment, alternative optical magnetic-field-detection methods were investigated. This work concludes with a newly developed technique using linearly rather than circularly polarized light, a magnetometry technique which is only feasible with laser excitation, and which appears to be very promising for future applications.