Field Plates for ACME III Electron EDM Search

The measurement of electron Electric Dipole Moment (eEDM), d_e, is a powerful probe for physics beyond Standard Model. The current most stringent limit of |d_e|<1.1×10^-29 e·cm was reported by the ACME II experiment (Nature, 562(2018), 355). A primary source of systematic error in the first two generations of ACME was the polarization imperfection of the state preparation and readout lasers used to detect electron spin precession in the H ³△₁ state of ThO molecules. Most of this imperfection comes from stress-induced birefringence of the optical components along the laser path, including vacuum chamber windows and field plates. For ACME III we aim to reduce birefringence and here report on progress towards new field plates and window designs. In order to take advantage of longer precession time in our molecular beam, we are working to develop methods for fabricating larger field plates. Use of a specialized glass material with ultra-low stress-optics coefficient has been examined, and a technique for joining multiple pieces of glass with smooth surfaces at the interface has been developed. A self-calibrating polarimeter was also developed to measure small birefringence in field plate components (arXiv:1703.00963).