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 Ώ(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.