Systems that saturate the Margolus-Levitin quantum speed limit

TL;DR

This paper characterizes all finite-dimensional quantum systems that saturate the Margolus-Levitin quantum speed limit, providing structural criteria for mixed states and extending the limit to mixed states via a time-reversal argument.

Contribution

It offers a complete structural characterization of mixed states saturating the Margolus-Levitin bound and extends the quantum speed limit to mixed states using a novel approach.

Findings

Mixed-state saturation occurs under specific energy support and eigenvector superposition conditions.

Derived a purity-resolved, tight Margolus-Levitin bound for qubits.

Extended the quantum speed limit to mixed states through a time-reversal argument.

Abstract

We provide a complete characterization of all finite-dimensional quantum systems 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 subspaces. These requirements impose a strict rank bound, ruling out saturation by any faithful state. For quantum bits, 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.