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arXiv:2511.05260

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

Wei Chen

quant-phcond-mat.stat-mechSubmitted Nov 7, 2025• Announced Nov 10, 2025• 0 citations
v1 · Nov 7, 2025v2 · Jun 15, 2026v3 · Oct 18, 2026
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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.

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15 pages, 3 figures

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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 2×22\times 22×2 density matrices, and the application to canonical ensemble of Su-Schrieffer-Heeger model suggests the mitigation of quantum geometry at finite temperature.
Abstract The quantum geometric tensor, composed of the quantum metric tensor and Berry curvature, fully encodes the parameter space geometry of a physical system. We first provide a formulation of the quantum geometrical tensor in the path integral formalism that can handle both the ground and excited states, making it useful to characterize excited state quantum phase transitions (ESQPT). In this setting, we also generalize the quantum geometric tensor to incorporate variations of the system parameters and the phase-space coordinates. This gives rise to an alternative approach to the quantum covariance matrix, from which we can get information about the quantum entanglement of Gaussian states through tools such as purity and von Neumann entropy. Second, we demonstrate the equivalence between the formulation of the quantum geometric tensor in the path integral formalism and other existing methods. Furthermore, we explore the geometric properties of the generalized quantum metric tensor in depth by calculating the Ricci tensor and scalar curvature for several quantum systems, providing insight into this geometric information.
The quantum Fisher matrix and geometric tensor are powerful tools for quantifying the nature of quantum systems. Unfortunately, the formalism needed to compute these quantities is opaque and has limited the development of physical intuition for their behavior, even in the ground state. For quantum systems with continuous symmetries, we know the ground state can be understood in a model-independent and intuitive way from the correlation functions of conserved currents and charges. In this paper, we demonstrate an equivalence between the quantum Fisher matrix associated with the ground states of systems with continuous symmetries and the Nambu-Goldstone. bosons produced by broken generators. We will show that the information metric defined by the Fisher matrix is the same as the Zamolodchikov metric that defines the coset space of the system. Next we show that the Uhlmann curvature associated with multi-parameter estimation is the same as the charge density matrix that defines the difference between type-A and type-B Nambu-Goldstone bosons. We demonstrate these connections in several examples and discuss the generalization to finite temperature.
The complete quantum metric of a parametrized quantum system has a real part (usually known as the Provost-Vallee metric) and a symplectic imaginary part (known as the Berry curvature). In this paper, we first investigate the relation between the Riemann curvature tensor of the space described by the metric, and the Berry curvature, by explicit parallel transport of a vector in Hilbert space. Subsequently, we write a generating function from which the complex metric, as well as higher-order geometric tensors (affine connection, Riemann curvature tensor), can be obtained in terms of gauge-invariant cumulants. The generating function explicitly relates the quantities which characterize the geometry of the parameter space to quantum fluctuations. We also show that for a mixed quantum-classical system both real and imaginary parts of the quantum metric contribute to the dynamics, if the mass tensor is Hermitian. A many-operator generalization of the uncertainty principle results from taking the determinant of the complex quantum metric. We also calculate the quantum metric for a number of Lie group coherent states, including several representations of the SU(1,1)\text{SU}(1,1)SU(1,1) group. In our examples nontrivial complex geometry results for generalized coherent states. A pair of oscillator states corresponding to the SU(1,1)\text{SU}(1,1)SU(1,1) group gives a double series for its spectrum. The two minimal uncertainty coherent states show trivial geometry, but, again, for generalized coherent states nontrivial geometry results.
The dynamics of two variants of quantum Fisher information under decoherence is investigated from a geometrical point of view. We first derive the explicit formulas of these two quantities for a single qubit in terms of the Bloch vector. Moreover, we obtain analytical results for them under three different decoherence channels, which are expressed as affine transformation matrices. Using the hierarchy equation method, we numerically study the dynamics of both sets of information in a dissipative model and compare the numerical results with the analytical ones obtained by applying the rotating-wave approximation. We further express the two information quantities in terms of the Bloch vector for a qudit by expanding the density matrix and Hermitian operators in a common set of generators of the Lie algebra su(d)\text{su}(d)su(d). By calculating the dynamical quantum Fisher information, we find that the collisional dephasing significantly diminishes the precision of the phase parameter with the Ramsey interferometry.
Abstract The geometry of the parameter space is encoded by the quantum geometric tensor, which captures fundamental information about quantum states and contains both the quantum metric tensor and the curvature of the Berry connection. We present a formulation of the Berry connection and the quantum geometric tensor in the framework of the phase space or Wigner function formalism. This formulation is obtained through the direct application of the Weyl correspondence to the geometric structure under consideration. In particular, we show that the quantum metric tensor can be computed using only the Wigner functions, which opens an alternative way to experimentally measure the components of this tensor. We also address the non-Abelian generalization and obtain the phase space formulation of the Wilczek–Zee connection and the non-Abelian quantum geometric tensor. In this case, the non-Abelian quantum metric tensor involves only the non-diagonal Wigner functions. Then, we verify our approach with examples and apply it to a system of N coupled harmonic oscillators, showing that the associated Berry connection vanishes and obtaining the analytic expression for the quantum metric tensor. Our results indicate that the developed approach is well adapted to study the parameter space associated with quantum many-body systems.
We develop a unified geometric framework in which the quantum geometric tensor (QGT) simultaneously governs both the metrology of gauge-invariant curvature and the symmetry classification of random-matrix ensembles. The symmetric Fubini–Study sector, generated by the Jordan product of symmetric logarithmic derivatives (SLDs), determines the quantum Fisher information and therefore the information gain associated with closed-loop curvature acquisition. The antisymmetric Berry sector, classified by the de Rham cohomology class of the SLD connection, determines whether the accumulated holonomy phase is linearly observable. We show that time-reversal symmetry acts as a topological gate on the readable Berry channel: the holonomy survives only in the unitary Wigner–Dyson class (GUE), where the cohomology class is nontrivial. This converts the previously phenomenological time-reversal “gate factor” into a cohomological indicator. In the symplectic class (GSE), the non-Abelian obstruction is identified with the Jordan associator curvature and appears operationally as a Holevo incompatibility penalty. The resulting cost-efficiency functional is shown to be genuinely U-shaped across the Dyson classes, with a strict minimum at GUE arising from the non-monotonic topology of the readable holonomy sector. We further establish a bridge between the QGT and thermodynamic fluctuation geometry, derive curvature-statistics scaling laws from Wigner level repulsion, and analyze the deformation of the Kähler structure under decoherence through the Bures metric and mean Uhlmann curvature. The framework unifies Berry geometry, quantum metrology, random-matrix universality, and information geometry into a single cohomological description of curvature acquisition.