Past exam of the mathematics course of the University of Cambridge / 2008 / iii / Paper 57 / 4 / b / Solution

Set $L=2^N$. Applying the inverse quantum Fourier transform to the register in part (a) gives probability amplitude at label $m$ equal toChoose $m$ nearest to $\eta =L{\varphi }_j/(2\pi )$, with $m$ reduced modulo $L$ at the phase boundary. Then the difference can be represented with $|\delta |\le \pi /L$. For $\delta \ne 0$, sum the finite geometric series to obtainLet $z=L|\delta |/2\le \pi /2$. On $[0,\pi /2]$, concavity of sine gives $\sin z\ge 2z/\pi$, the line through its endpoint values. Also $|\sin (\delta /2)|\le |\delta |/2$. Thereforeand the Born rule proves the nearest-integer success bound for quantum phase estimation:If $\delta =0$, every summand is one and the probability is exactly one. If the scaled phase is halfway between two integers, either nearest label separately satisfies the displayed bound. Interpreting the measured integer as $m/L$ gives the closest available $N$-bit phase estimate modulo one.