Systems that saturate the Margolus-Levitin quantum speed limit

• URL: http://arxiv.org/abs/2511.23237v1
• Date: Fri, 28 Nov 2025 14:45:33 GMT
• Title: Systems that saturate the Margolus-Levitin quantum speed limit
• Authors: Ole Sönnerborn,
• Abstract summary: We prove that mixed-state saturation occurs precisely when three structural criteria are fulfilled.<n>We further extend the dual Margolus-Levitin quantum speed limit to mixed states and establish the corresponding saturation conditions.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: We provide a complete characterization of all finite-dimensional quantum states that saturate the Margolus-Levitin quantum speed limit at arbitrary Uhlmann-Jozsa fidelity. Employing a purification-based approach, we prove that mixed-state saturation occurs precisely when three structural criteria are fulfilled: the state's support is confined to the sum of two energy eigenspaces (the ground level and a single excited level); each eigenvector of the state with nonzero weight is a fixed superposition of one ground- and one excited-state energy eigenvector (determined by the minimizer of the objective function identified by Giovannetti et al.) and all such eigenvectors evolve in mutually orthogonal two-dimensional subspaces. These requirements impose a strict rank bound, ruling out saturation by any faithful state. For qubit systems, we derive a purity-resolved and tight Margolus-Levitin bound that reduces to the pure-state result in the limit of unit purity. Through a time-reversal argument, we further extend the dual Margolus-Levitin quantum speed limit to mixed states and establish the corresponding saturation conditions.

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