A laser based mercury co-magnetometer for the neutron electric dipole moment search

Abstract

This work introduces a laser-based 199Hg co-magnetometer into the experiment searching for a permanent neutron electric dipole moment (nEDM) at the Paul Scherrer Institute, Villigen, Switzerland. The work discusses in detail the advantages of an ultraviolet (UV) laser light source for the 199Hg co-magnetometer in comparison to the 204Hg discharge bulbs used so far. A UV laser system has been commissioned and frequency stabilized with the necessary accuracy. The Dopplerand frequency-modulation-free technique of Sub-Doppler Dichroic Atomic Vapor Laser Lock used to frequency stabilize the UV laser light and the experimental setup are described and discussed in detail. The laser-based 199Hg co-magnetometer showed a more than five times increased signalto-noise-ratio in a direct comparison measurement. The laser-based 199Hg co-magnetometer satisfies the sensitivity requirements for the next generation nEDM experiment (n2EDM). Two models for the signal generation in the 199Hg co-magnetometer are developed to explain the increased magnetometry signal. These models are used to identify possibilities for increasing the magnetometer sensitivity even further. A detailed model of the optical pumping process used to spin polarize the 199Hg atoms in the nEDM setup is presented. It turns out that the achievable spin polarization seems not to be limited by light-induced depolarization of 199Hg atoms. The measurement of paramagnetic rotation as an alternative to light absorption to generate the 199Hg co-magnetometry signal in the nEDM geometry is discussed. Furthermore, possible benefits of using several even Hg isotopes on the magnetometry signal decay time are considered.