Magnetic field uniformity in neutron electric dipole moment experiments.

Magnetic field uniformity is of the utmost importance in experiments to measure the electric dipole momentof the neutron. A general parametrization of the magnetic field in terms of harmonic polynomialmodes is proposed, going beyond the linear-gradients approximation. We review the main undesirable effects of non-uniformities: depolarization of ultracold neutrons, and Larmor frequency shifts of neutrons and mercury atoms. The theoretical predictions for these effects were verified by dedicated measurements with the single-chamber nEDM apparatus installed at the Paul Scherrer Institute.