Past exam of the mathematics course of the University of Cambridge / 2020 / ii / Paper 3 / 10C / iv / Solution

Choose $U$ to prepare the uniform superposition, so the marked-state probability is $p=1/4$. Apply the marked-state phase oracle $R_g^{\pi }$, followed by the reflection $U{R}_{00}^{\pi }{U}^{†}$ about the prepared state. Part (iii) shows that this single Grover iteration produces $-|g⟩$. A projective measurement in the four-item basis therefore returns the marked element with probability one. This is exact Grover search on four entries.