An alternate algorithm for directed quantum search

We present a new algorithm for directed quantum search, which is superior to the Phase-π/3 search algorithm [5] in several aspects. In particular, our algorithm does not require complex phases and achieves the optimal reduction in error probability, from ǫ to ǫ2q+1, for all positive integer values q of oracle queries. (The Phase-π/3 algorithm, requires q = (3n − 1)/2, with n a positive integer.) In our algorithm, oracle queries and diffusion operations are controlled in a special way by two ancilla qubits, and irreversible measurement operations drive the quantum state towards a fixed point. We analyse various features of the algorithm in detail.