Optimal signal states for quantum detectors

TL;DR

This paper investigates the maximum information a quantum detector can extract, providing solutions for various discrimination tasks and defining a measurement capacity, with analytical results for specific noisy measurements.

Contribution

It introduces a framework to quantify the information extraction capacity of quantum measurements and derives optimal strategies for several key information readout tasks.

Findings

Derived general solutions for Bayesian and unambiguous discrimination.

Established the measurement capacity as a form of mutual information.

Proved optimality for a noisy symmetric informationally complete measurement.

Abstract

Quantum detectors provide information about quantum systems by establishing correlations between certain properties of those systems and a set of macroscopically distinct states of the corresponding measurement devices. A natural question of fundamental significance is how much information a quantum detector can extract from the quantum system it is applied to. In the present paper we address this question within a precise framework: given a quantum detector implementing a specific generalized quantum measurement, what is the optimal performance achievable with it for a concrete information readout task, and what is the optimal way to encode information in the quantum system in order to achieve this performance? We consider some of the most common information transmission tasks - the Bayes cost problem (of which minimal error discrimination is a special case), unambiguous message discrimination, and the maximal mutual information. We provide general solutions to the Bayesian and unambiguous discrimination problems. We also show that the maximal mutual information has an interpretation of a capacity of the measurement, and derive various properties that it satisfies, including its relation to the accessible information of an ensemble of states, and its form in the case of a group-covariant measurement. We illustrate our results with the example of a noisy two-level symmetric informationally complete measurement, for whose capacity we give analytical proofs of optimality. The framework presented here provides a natural way to characterize generalized quantum measurements in terms of their information readout capabilities.