In standard Grover's algorithm for quantum searching, the probability of finding the marked item is not exactly 1. In this Letter we present a modified version of Grover's algorithm that searches a marked state with full successful rate. The modification is done by replacing the phase inversion by two phase rotation through angle $\phi$. The rotation angle is given analytically to be $\phi=2 \arcsin(\sin{ \pi\over (4J+6)}\over \sin\beta)$, where $\sin\beta={ 1\over \sqrt{ N}}$, $N$ the number of items in the database, and $J$ an integer equal to or greater than the integer part of $({ \pi\over 2}-\beta)/(2\beta)$. Upon measurement at $(J+1)$-th iteration, the marked state is obtained with certainty.
Categories: quant-ph