Note on the Margolus-Levitin quantum speed limit for arbitrary fidelity

TL;DR

This paper presents a simple, elementary proof for the generalized Margolus-Levitin quantum speed limit applicable to arbitrary fidelity, extending the original limit from the case of zero fidelity.

Contribution

The authors provide a new, straightforward proof of the generalized Margolus-Levitin quantum speed limit using basic differential calculus tools.

Findings

Elementary proof of the generalized Margolus-Levitin limit

Extension of the speed limit to arbitrary fidelity

Clarification of the conjecture's validity with a simple proof

Abstract

For vanishing fidelity between initial and final states two important quantum speed limits, the Mandelstam-Tamm limit (involving energy dispersion) and Margolus-Levitin one (involving excitation energy expectation value) have been derived. While the generalization of the former limit to the case of arbitrary fidelity is straightforward, the relevant generalization of the latter, given in the seminal paper by Giovanetti et al (Phys. Rev. A67 (2003), 052109) was based on the conjectured equality of lower and upper bounds on the right hand side of generalized Margolus-Levitin inequality, verified numerically up to seven digits. Only recently there appear two proofs of the conjecture. We provide below a very elementary new proof, based on the simplest tools from differential calculus. Thus the generalized Margolus-Levitin speed limit can be derived much in the spirit of the original one valid for vanishing fidelity.