Gradients in the magnetic field will result in poor polarization lifetime and systematic errors (most notably the “geometric phase”).
Fluctuations in the magnetic field (even fluctuations well below what we can eliminate with our careful design) will cause changes in the precession frequency that would completely mask the changes resulting from a non-zero nEDM at the level we aim to see.
Both these issues are addressed by introducing a co-magnetometer – a species with a finite magnetic moment and negligible EDM – that samples the same magnetic field as the neutron and whose precession frequency can be directly detected.
Helium-3, used for spin analysis, does double duty as our co-magnetometer.
Oak Ridge National Laboratory is managed by UT-Battelle LLC for the US Department of Energy