Electron Electric Dipole Moment Experiment with Laser-Cooled Atoms

Harvey Gould, Jason Amini

M. A. Trigatzis

That the universe contains a disproportionate amount of matter compared to antimatter cannot be explained by the standard model of particle physics; this is because it contains too little charge-parity (CP) symmetry violation. Low energy experiments with atoms and molecules can be extremely sensitive probes for many types of new physics. An active area of research is the search for electric dipole moments (EDMs) of fundamental particles or nuclei; a nonzero EDM would be evidence for new sources of CP violation. Ytterbium monofluoride (YbF) molecules in a supersonic beam are currently being used to measure the electric dipole moment of the electron (eEDM). This thesis presents progress towards a new, more sensitive eEDM experiment that will use a slower YbF beam source and transverse laser cooling to collimate this beam. We have demonstrated laser cooling of YbF in one transverse dimension to less than 100 μK, which is below the Doppler cooling limit. This was done using a Sisyphus-type mechanism called magnetic field induced laser cooling. We also demonstrate polarisation gradient cooling of YbF. After rebuilding and upgrading much of the experiment, we cooled the beam in both transverse dimensions. By scanning experimental parameters, we have gained insight into the cooling mechanisms, and the optimum conditions for laser cooling. We placed an upper limit of a few mK on the transverse temperature of the molecules, although I argue that the true temperature must be very much lower. The number of ultracold molecules was carefully quantified and found to be 2.0(4)x10^5 per pulse. These experiments lay the foundations for an eEDM sensitivity improvement by up to a factor of 100.

Harvey Gould

J. Almond

Cold molecules are attractive for a wide range of scientific applications, including quantum computation, the study of chemical reactions, and tests of fundamental physics. Laser cooling has proved to be an invaluable technique in the cooling of atoms. This technique was once thought to be infeasible for molecules, because it is difficult to find a closed cycling transition due to their vibrational structure. Recently, laser cooling of several diatomic species has been demonstrated. These molecules possess electronic transitions with highly diagonal Franck-Condon matrices, which make it possible to drive a quasi-closed cycling transition. Ytterbium fluoride (YbF) molecules are amenable to laser cooling and are especially interesting because they are used to measure the electron's electric dipole moment (eEDM). Measurements of the eEDM test the prediction of theories that extend the Standard Model of particle physics. The sensitivity of an eEDM experiment could be greatly increased by using ultracold molecules produced by direct laser cooling. This thesis presents work done towards producing a laser-cooled beam of YbF for an eEDM experiment. This work includes the construction of the cooling experiment, a novel method for efficiently combining laser beams of very similar frequencies, results of spectroscopic measurements to find the required transitions for laser cooling, the results of initial optical cycling experiments, and the first laser cooling results of YbF. Using a one-dimensional optical molasses, a beam of molecules is Doppler cooled in one transverse direction to a temperature of approximately 70 mK. Preliminary evidence of cooling to lower temperatures through a Sisyphus mechanism is also presented. Finally, paths towards improving the laser cooling are suggested. The work opens the door to improved measurements of the eEDM using ultracold YbF molecules.

Jongseok Lim, J. Almond, M. A. Trigatzis +5 more

We demonstrate one-dimensional sub-Doppler laser cooling of a beam of YbF molecules to 100 μK. This is a key step towards a measurement of the electron's electric dipole moment using ultracold molecules. We compare the effectiveness of magnetically assisted and polarization-gradient sub-Doppler cooling mechanisms. We model the experiment and find good agreement with our data.

Edward A. Hinds