Fidelity and Fisher information on quantum channels

TL;DR

This paper introduces a fidelity function for quantum channels that unifies measures of channel distance, quantum discrimination, and metrology, providing new insights into the ultimate precision limits and resource requirements.

Contribution

It proposes a fidelity measure for quantum channels, linking channel distances to Fisher information and unifying quantum discrimination and metrology frameworks.

Findings

Quantum channel fidelity extends state fidelity concepts.

Distances between channels relate to quantum Fisher information.

Minimum uses for perfect discrimination match precision limits.

Abstract

The fidelity function of quantum states have been widely used in quantum information science and frequently arises in the quantification of optimal performances for the estimation and distinguish of quantum states. A fidelity function on quantum channel is expected to have same wide applications in quantum information science. In this paper we propose a fidelity function on quantum channels and show that various distance measures on quantum channels can be obtained from this fidelity function, for example the Bures angle and the Bures distance can be extended to quantum channels via this fidelity function. We then show that the distances between quantum channels lead naturally to a new Fisher information which quantifies the ultimate precision limit in quantum metrology, the ultimate precision limit can thus be seen as a manifestation of the distances between quantum channels. We also show that the fidelity on quantum channels provides a unified framework for perfect quantum channel discrimination and quantum metrology, in particular we show that the minimum number of uses needed for perfect channel discrimination is exactly the counterpart of the precision limit in quantum metrology, and various useful lower bounds for the minimum number of uses needed for perfect channel discrimination can be obtained via this connection.