Past exam of the physics A course of the University of Cambridge / 2011 / Paper 2 / Section B / B8 / a

Put $c=\cos \omega t$, $s=\sin \omega t$. The real part of the off-diagonal coherence vanishes:This also follows from $[H,S_x]=0$ and its zero initial expectation. Individual measurements still give $\pm \hslash /2$, each with probability $1/2$.