Optical Birefringence and Electric fields control for the ACME III Electron Electric Dipole Moment Search*

A measurement of the electron Electric Dipole Moment (eEDM, d_e) can be used as a probe for CP-violating interactions beyond those of the standard model. A non-zero value of d_e could help to explain why there is more matter than antimatter in the universe. The ACME experiment uses a cold beam of ThO molecules to probe for the eEDM.

In ACME III, a pair of ITO-coated transparent electric field plates are used to apply the E field for the experiment. The birefringence of the field plates can cause elliptic imperfections in the laser beams passing through for quantum state manipulation, which is known to cause systematic errors. Stress-free mounting schemes with precise adjustability ensuring birefringence-free for the field plates (induced ellipticity on linearly polarized light, or S/I, <0.01%/mm) have been developed and demonstrated. Electric field imperfections are another potential source for systematic errors; shimming plates designs are developed to minimize edge effect from finite dimensions and non-reversing effect of E field from dielectric materials. Parallelism of the plates is ensured with white light interferometry after installation. 

We will also go through some other technical upgrades of the experiment, including laser system with PDH locking to ultra-low expansion (ULE) cavities and phase-locking to GPS-steered comb for improved robustness and stability.