Quantum Query as a state decomposition

Abstract

The Quantum Query Model is a framework that allows us to express most known quantum algorithms. Algorithms represented by this model consist of a set of unitary operators acting over a finite Hilbert space, and a final measurement step consisting of a set of projectors. In this work, we prove that the application of these unitary operators before the measurement step is equivalent to decomposing a unit vector into a sum of vectors and then inverting some of their relative phases. We also prove that the vectors of that sum must fulfill a list of properties and we call such vectors a Block Set. If we define the measurement step for the Block Set Formulation similarly to the Quantum Query Model, then we prove that both formulations give the same Gram matrix of output states, although the Block Set Formulation allows a much more explicit form. Therefore, the Block Set reformulation of the Quantum Query Model gives us an alternative interpretation of how quantum algorithms work. Finally, we apply our approach to the analysis and complexity of quantum exact algorithms.