Single-query quantum algorithms for symmetric problems

Abstract

Given a unitary representation of a finite group on a finite-dimensional Hilbert space, we show how to find a state whose translates under the group are distinguishable with the highest probability. We apply this to several quantum oracle problems, including the GROUP MULTIPLICATION problem, in which the product of an ordered n-tuple of group elements is to be determined by querying elements of the tuple. For any finite group G, we give an algorithm to find the product of two elements of G with a single quantum query with probability 2/|G|. This generalizes Deutsch’s Algorithm from Z2 to an arbitrary finite group. We further prove that this algorithm is optimal. We also introduce the HIDDEN CONJUGATING ELEMENT PROBLEM, in which the oracle acts by conjugating by an unknown element of the group. We show that for many groups, including dihedral and symmetric groups, the unknown element can be determined with probability 1 using a single quantum query.