The spins of the neutron and Helium-3 atoms precess in a plane perpendicular to the primary holding field direction (𝒙).

An oscillating magnetic field perpendicular to 𝒙 causes an oscillation in the spin direction relative to this plane and thus modifying the precession frequency (C. Cohen-Tannoudji and S. Haroche, J. Physique 30, 153 (1969)).

With careful choice of the strength and frequency of the oscillating field the neutron and Helium-3 precession frequencies can be made identical, so-called “critical dressing”. Watch an animation of critical dressing. This increases the statistical precision by nearly a factor of two and reduces certain systematic effects.