Oracle problems as communication tasks and optimization of quantum algorithms

Abstract

Quantum query complexity studies the number of queries needed to learn some property of a black box. A closely related question is how well an algorithm can succeed with this learning task using only a fixed number of queries. In this work, we propose measuring an algorithm's performance using the mutual information between the output and the actual value. The task of optimizing this mutual information using a single query, is similar to a basic task of quantum communication, where one attempts to maximize the mutual information of the sender and receiver. We make this analogy precise by splitting the algorithm between two agents, obtaining a communication protocol. The oracle's target property plays the role of a message that Alice encodes into a quantum state, which is subsequently sent over to Bob. The first part of the algorithm performs this encoding, and the second part measures the state and aims to deduce the message from the outcome. Moreover, we formally consider the oracle as a separate subsystem, whose state records the unknown oracle identity. Within this construction, Bob's optimal measurement basis minimizes the quantum correlations between the two subsystems. We also find a lower bound on the mutual information, which is related to quantum coherence. These results extend to multiple-query non-adaptive algorithms. As a result, we describe the optimal non-adaptive algorithm that uses at most a fixed number of queries, for any oracle classification problem. Crucially, this mutual-information perspective carries direct practical utility for algorithmic design, providing the theoretical foundation to optimize iterative subroutines in hybrid quantum--classical schemes. Within the present work, we apply this framework to analyze the stage-by-stage information flow and track partial progress in several standard quantum algorithms.