Affiliated operators for classical and quantum control

TL;DR

This paper introduces a von Neumann algebra-based framework for analyzing controllability in classical and quantum infinite-dimensional systems, establishing conditions for optimal and approximate control.

Contribution

It develops a novel approach using affiliated operators to study controllability, including unbounded operators, in both classical and quantum control systems.

Findings

Proves existence of time-optimal controls under norm bounds.

Shows the dynamical Lie algebra can determine approximate controllability.

Applies the framework to classical systems via Koopman operators.

Abstract

Using techniques from the theory of von Neumann algebras, we propose a framework for addressing questions of controllability of bilinear systems on infinite dimensional Hilbert spaces. In the setup, we assume only that the drift and control terms arising in a bilinear control system are affiliated with a von Neumann algebra of finite type acting on the same Hilbert space. When the control terms satisfy basic norm bound conditions, we prove existence of time-optimal controls. In the more general setting where all operators may be unbounded, we show how the dynamical Lie algebra for the system is still well-defined and may be used to check approximate controllability of the system in question. We discuss how this approach can be applied to classical dynamical systems through the Koopman operator formalism, and investigate potential candidates for the von Neumann algebra which may guide the choice of controls. We illustrate how an affiliation relation naturally arises in both classical and quantum control systems with a few examples.