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Chinese Physics B

Double Wilczek–Zee connection and mixed-state quantum geometric tensor

Xiaoguang Wang, Xiao-Ming Lu, Jing Liu, Wenkui Ding, Libin Fu

Published Jul 28, 2025• 1 citation
Publisher

Abstract

Abstract The Wilczek–Zee connection (WZC) is a key concept in the study of topology of quantum systems. Here, we introduce the double Wilczek–Zee connection (DWZC) which naturally appears in the pure-state quantum geometric tensor (QGT), another important concept in the field of quantum geometry. The DWZC is Hermitian with respect to the two integer indices, just like the original Hermitian WZC. Based on the symmetric logarithmic derivative operator, we propose a mixed-state quantum geometric tensor. Using the symmetric properties of the DWZC, we find that the real part of the QGT is connected to the real part of the DWZC and the square of eigenvalue differences of the density matrix, whereas the imaginary part can be given in terms of the imaginary part of the DWZC and the cube of the eigenvalue differences. For density matrices with full rank or no full rank, the QGT can be given in terms of real and imaginary parts of the DWZC.

DOI
10.1088/1674-1056/adf4b0

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Abstract The Wilczek–Zee connection (WZC) is a key concept in the study of topology of quantum systems. Here, we introduce the double Wilczek–Zee connection (DWZC) which naturally appears in the pure-state quantum geometric tensor (QGT), another important concept in the field of quantum geometry. The DWZC is Hermitian with respect to the two integer indices, just like the original Hermitian WZC. Based on the symmetric logarithmic derivative operator, we propose a mixed-state quantum geometric tensor. Using the symmetric properties of the DWZC, we find that the real part of the QGT is connected to the real part of the DWZC and the square of eigenvalue differences of the density matrix, whereas the imaginary part can be given in terms of the imaginary part of the DWZC and the cube of the eigenvalue differences. For density matrices with full rank or no full rank, the QGT can be given in terms of real and imaginary parts of the DWZC.
Abstract The quantum geometric tensor (QGT) is a fundamental quantity for characterizing the geometric properties of quantum states and plays an essential role in elucidating various physical phenomena. The traditional QGT, defined only for pure states, has limited applicability in realistic scenarios where mixed states are common. To address this limitation, we generalize the definition of the QGT to mixed states using the purification bundle and the covariant derivative. Notably, our proposed definition reduces to the traditional QGT when mixed states approach pure states. In our framework, the real and imaginary parts of this generalized QGT correspond to the Bures metric and the mean gauge curvature, respectively, endowing it with a broad range of potential applications. Additionally, using our proposed mixed-state QGT, we derive the geodesic equation applicable to mixed states. This work establishes a unified framework for the geometric analysis of both pure and mixed states, thereby deepening our understanding of the geometric properties of quantum states.
The quantum geometric tensor (QGT) reveals local geometric properties and associated topological information of quantum states. Here a generalization of the QGT to mixed quantum states at finite temperatures based on the Sj\"oqvist distance is developed. The resulting Sj\"oqvist QGT is invariant under gauge transformations of individual spectrum levels of the density matrix. A Pythagorean-like relation connects the distances and gauge transformations, which clarifies the role of the parallel-transport condition. The real part of the QGT naturally decomposes into a sum of the Fisher-Rao metric and Fubini-Study metric, allowing a distinction between different contributions to the quantum distance. The imaginary part of the QGT is proportional to a weighted summation of the Berry curvatures, which leads to a geometric phase for mixed states under certain conditions. We present three examples of different dimensions to illustrate the temperature dependence of the QGT and a discussion on possible implications.
The Uhlmann phase extends geometric phases to mixed quantum states via a parallel-transport condition on purification amplitudes, yet its direct implementation under standard Hamiltonian dynamics is obstructed by the non-Hermitian nature of the purification. We establish that for any smooth one-dimensional closed loop of full-rank qubit density matrices, there exists a four-level Hermitian parent Hamiltonian whose doubly degenerate ground-state subspace carries a Wilczek--Zee connection exactly equal to the Uhlmann connection. Consequently, the Uhlmann holonomy is faithfully reproduced by adiabatic evolution in the enlarged system. We further prove that this auxiliary-field construction is obstructed in generic two-dimensional parameter spaces by a Frobenius integrability condition, which we derive explicitly. The one-dimensional Uhlmann phase is thus placed on the same footing as the non-Abelian Berry phase, offering a purely Hermitian, Hamiltonian-based route to simulating mixed-state geometric phases. Numerical integration of the adiabatic dynamics confirms the exact correspondence and validates the convergence to the Uhlmann holonomy in the large-gap limit.
Geometrical concepts in quantum mechanics, such as the Berry phase or quantum metric, have been studied extensively in the case of pure quantum states, but their generalisation to density operators is still undergoing active research. It has become clear that more than one valid approach exists and different definitions of the quantum geometric tensor (QGT) may give inequivalent results. In this report, we cover some of the foundations of quantum geometry for pure states, motivate the need for mixed-state generalisations, and examine some approaches in that direction. The final section analyses more closely a recent proposal, deemed the “Sjöqvist QGT” and argues that its imaginary part does not produce a generalisation of the Berry curvature.
We examine the geometric interpretation of the quantum geometric tensor proposed in [Phys. Rev. B {\bf 110}, 035404 (2024)] for mixed quantum states, focusing on its imaginary part, which is proportional to a weighted sum of the Berry curvatures of the eigenstates of the density operator. While the real part naturally decomposes into Fisher--Rao and weighted Fubini--Study contributions, we show that the imaginary part does not, in general, coincide with the curvature of a connection on a globally defined U(1) line bundle whose holonomy yields a mixed-state geometric phase. Using a two-level system as an explicit example, we demonstrate that its surface integral depends on the choice of surface bounded by the same closed path, with ambiguities that are not integer multiples of 2π2π2π.
Mixed-state quantum geometry plays a fundamental role in quantum metrology, multiparameter estimation, and the characterization of quantum many-body systems. Yet, unlike its pure-state counterpart, accessing mixed-state geometry requires solving an ill-conditioned Lyapunov equation for the symmetric logarithmic derivative (SLD), posing a major challenge for resource-efficient quantum computation. Here we address the challenge and develop a novel quantum algorithmic framework for efficiently accessing the quantum Fisher information matrix (QFIM) and quantum geometric tensor (QGT) of mixed states. Our key insight is to uncover a previously unexplored connection between the positive semidefinite structure of density matrices and exponentially convergent Gauss-Laguerre quadrature, enabling an efficient integral-based construction of the SLD. A rigorous complexity analysis further establishes the resource efficiency of our algorithm, demonstrating logarithmic precision dependence and double-logarithmic ancillary-qubit overhead at fixed conditioning. Building on this construction, we achieve near-Heisenberg precision scaling for diagonal QFI estimation without quantum phase estimation and construct unified block encodings of the full QFIM and QGT under a bounded spectral-gap assumption. A complementary regularized algorithm extends efficient diagonal estimation to exponentially small spectral gaps under a bounded-response condition. Our results establish a resource-efficient route to coherent mixed-state quantum geometry, enabling its direct integration into quantum linear-algebra algorithms without classical reconstruction and opening new possibilities for quantum-enhanced multiparameter metrology.