Progress towards an electron electric dipole moment measurement using BaF molecules in an inert-gas matrix*

The goal of the EDM³ collaboration is to make an ultraprecise measurement of the electron electric dipole moment (eEDM) using BaF molecules embedded in a cryogenic inert-gas solid. The very large number of molecules, along with the fact that the molecules have their positions and orientations fixed by the matrix, allows for the possibility of large improvements in accuracy for an eEDM measurement. Many of the steps necessary for an EDM³ measurement have been demonstrated, including producing solids with a sufficiently large number of embedded BaF molecules, optical pumping, laser-induced fluorescence detection, and radio-frequency hyperfine transitions. Due to a quantum-mechanical interference effect, the efficiency of optical pumping depends on both the handedness of the circular polarization and the orientation of the molecule, and this effect can be used to preferentially select molecules of a particular orientation, as is required for an eEDM measurement. A tetra-substitution site in an argon crystal, in which a BaF molecule substitutes for four Ar atoms, appears to ideal for making an eEDM measurement.