Past exam of the mathematics course of the University of Cambridge / 2012 / iii / Paper 67 / 4 / c

Let $\lambda =e^{2\pi i\varphi }$. In terms of the Pauli X gate, the given unitary operator is $U_{\varphi }=aI+bX$, with $a=(1+\lambda )/2$ and $b=(1-\lambda )/2$. Since $X|+⟩=|+⟩$ and $X|-⟩=-|-⟩$,Thus the normalized eigenvectors are $|\pm ⟩=(|0⟩\pm |1⟩)/\sqrt{2}$, with eigenvalues $1$ and $e^{2\pi i\varphi }$ respectively. At $\varphi =0$ they coincide as eigenvalues, and the whole two-dimensional Hilbert space is the eigenspace; the same chosen eigenbasis remains valid.

Prepare $|-⟩$ independently of $\varphi$ and use the exact quantum phase estimation circuit of the preceding parts on the controlled-$U_{\varphi }$ black box. It returns $\varphi$ exactly usingThe chosen eigenstate contains all the unknown phase information, unlike $|+⟩$, whose eigenvalue is independent of $\varphi$.