A fast quantum mechanical algorithm for database search
STOC '96 Proceedings of the twenty-eighth annual ACM symposium on Theory of computing
Strengths and Weaknesses of Quantum Computing
SIAM Journal on Computing
Logical reversibility of computation
IBM Journal of Research and Development
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Let a classical algorithm be determined by sequential applications of a black box performing one step of this algorithm. If we consider this black box as an oracle which gives a value f(a) for a query a, we can compute T sequential applications of f on a classical computer relative to this oracle in time T. It is proved that if T = O(2n/7), where n is the length of input, then the result of T sequential applications of f can not be computed on quantum computer with oracle for f for all possible f faster than in time Ω(T). This means that there is no general method of quantum speeding up of classical algorithms provided in such a general method a classical algorithm is regarded as iterated applications of a given black box.