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

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.