Quantum speed limits for time evolution of a system subspace

TL;DR

This paper generalizes quantum speed limits from individual states to entire subspaces, providing optimal bounds on how fast a quantum system's subspace can evolve over time.

Contribution

It introduces a new bound on the evolution speed of quantum subspaces, extending existing state-based limits like Mandelstam-Tamm and Fleming bounds.

Findings

Derived an optimal estimate for subspace evolution speed.

Generalized Fleming bound to subspaces.

Applicable to infinite-dimensional quantum systems.

Abstract

One of the fundamental physical limits on the speed of time evolution of a quantum state is known in the form of the celebrated Mandelstam-Tamm inequality. This inequality gives an answer to the question on how fast an isolated quantum system can evolve from its initial state to an orthogonal one. In its turn, the Fleming bound is an extension of the Mandelstam-Tamm inequality that gives an optimal speed bound for the evolution between non-orthogonal initial and final states. In the present work, we are concerned not with a single state but with a whole (possibly infinite-dimensional) subspace of the system states that are subject to the Schroedinger evolution. By using the concept of maximal angle between subspaces we derive an optimal estimate on the speed of such a subspace evolution that may be viewed as a natural generalization of the Fleming bound.