Past exam of the laser science and quantum information processing course of the University of Oxford / 2019 / 4 / c / iii

Complete the square: $F=2(p-1/2{)}^2+1/2$. ThereforeEvery equatorial Bloch-sphere state attains the minimum, not just $|+⟩$ and $|-⟩$. Its unread Z measurement gives $I/2$, having removed all information contained in its relative phase.