Past exam of the atomic and Optical Physics course of the University of Cambridge / 2015 / Paper 1 / 1 / ii

With the coupling off, $\mathbf{W}=\delta \hat{z}$. Precession during the dark interval givesThe populations remain $p_1=p_2=1/2$ during the dark interval; only their relative phase changes. For $T=2\pi /\delta$, the vector makes one complete equatorial circuit and returns to $(0,-1,0)$.