Generating functions for quantum metric, Berry curvature, and quantum Fisher information matrix

TL;DR

This paper demonstrates that fidelity functions serve as generating functions for quantum geometric quantities like the quantum Fisher information, Berry curvature, and classical Fisher information, unifying quantum and classical information geometry.

Contribution

It introduces a formalism linking fidelity to quantum and classical geometric tensors, extending to finite-temperature systems and providing explicit Bloch sphere representations.

Findings

Fidelity acts as a generating function for quantum Fisher information and Christoffel symbols.

For pure states, fidelity and phase generate quantum metric and Berry curvature.

Application to the Su-Schrieffer-Heeger model shows quantum geometry effects at finite temperature.

Abstract

We elaborate that the fidelity between two density matrices is a generating function, through which the quantum Fisher information matrix and Christoffel symbol of the first kind in the parameter space can be obtained through derivatives with respect to the parameters. For pure states, the fidelity and phase of the product between two quantum states are shown to be the generating functions of the quantum metric and Berry curvature, respectively. Further limiting to systems described by real wave functions, our formalism recovers the well-known result that the fidelity between two probability mass functions is the generating function of the classical Fisher information matrix, indicating a hierarchy of quantum to information geometry. The Bloch representation of the generating functions is given explicitly for $2\times 2$ density matrices, and the application to canonical ensemble of Su-Schrieffer-Heeger model suggests the mitigation of quantum geometry at finite temperature.