in situ magnetometry and active field cancellation for the ACME electron EDM search

Precision spectroscopy measurements are often affected by uncompensated changes in the magnetic fields. This can generally lead to systematic effects that are difficult to correct. In the search for electron and neutron electric dipole moments (EDM), co-magnetometry has been effective in monitoring and cancelling residual magnetic fields. The ACME electron EDM search, using a cold ThO molecular beam, aims to achieve over one order of magnitude improvement from our previous best limit [Nature, 562, 355 (2018)], |d_e|<1.1x10^-29 e·cm. Known systematic effects associated with magnetic field and its gradients are predicted to be under control once their values reach below 10uG and 1uG/cm. Here, we demonstrate in situ magnetometry with the ThO beam in the ACME experimental configuration. While the H ³Δ₁ state of ThO is used for EDM measurement, the magnetically sensitive Q ³Δ₂ state of the molecule [NJP 22, 023013 (2020)] is now used to probe residual B-fields at the ~200nG absolute accuracy level and <1 cm spatial resolution, over a 20 cm long prototype region. External compensation coils are applied in a feedback loop to cancel the residual field and gradients to below 1uG and 200nG/cm. This is sufficient to suppress systematic errors related to stray magnetic field and gradients in the upcoming ACME III EDM search.