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First page of Efficient Quantum Algorithm for Mixed-State Quantum Geometry

Efficient Quantum Algorithm for Mixed-State Quantum Geometry

Chenyang Wang, Qi Zhao, Xiao Yuan, Xiaoxia Cai

quant-ph Oct 17, 2026 · v1
Lean 4 is used to formally verify the bulk–edge deduction behind the asymptotic relation β_K/(4K)→1 for the Gauss–Laguerre LCU coefficient norm; code is available on request.
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.

Mixed-state quantum geometry, meaning the quantum Fisher information matrix (QFIM) and the quantum geometric tensor (QGT), depends on the symmetric logarithmic derivative (SLD). The SLD is defined by an ill-conditioned Lyapunov equation, which makes resource-efficient quantum computation of these quantities difficult.

The SLD is written as an integral of exponential filters on the density-matrix derivative and discretized with exponentially convergent Gauss–Laguerre quadrature. It is implemented via QSVT, block-encoding products and LCU. A Tikhonov-regularized variant handles exponentially small spectral gaps. The asymptotic linear growth of the LCU coefficient norm (β_K/(4K)→1) was checked in Lean 4, conditional on classical Laguerre zero-distribution and uniform bulk/edge asymptotics.

Under a bounded-gap promise, diagonal QFI estimation reaches near-Heisenberg precision scaling without quantum phase estimation, with logarithmic precision dependence and O(log log(1/ε)) ancilla overhead; unified block encodings of the full QFIM and QGT are also constructed. Under a bounded-response promise, the regularized algorithm gives polynomial-resource diagonal estimation.