Classical and quantum controllability of a rotating asymmetric molecule

TL;DR

This paper investigates the controllability of classical and quantum rotations of an asymmetric molecule under electric fields, revealing classical controllability universally and quantum controllability under specific dipole orientations.

Contribution

It introduces a perturbative Lie algebraic approach to quantum controllability and establishes conditions for approximate controllability in quantum systems.

Findings

Classical rotational dynamics are controllable for all parameters.

Quantum controllability depends on the dipole orientation.

Quantum system is approximately controllable for most parameters if the dipole is not aligned with principal axes.

Abstract

We study both the classical and quantum rotational dynamics of an asymmetric top molecule, controlled through three orthogonal electric fields that interact with its dipole moment. The main difficulties in studying the controllability of these infinite-dimensional quantum systems are the presence of severe spectral degeneracies in the drift Hamiltonian and the nonsolvability of the stationary free Schr\"odinger equation, which lead us to apply a perturbative Lie algebraic approach. In this paper we show that, while the classical equations given by the Hamiltonian system on ${\rm SO}(3)\times \mathbb{R}^3$ are controllable for all values of the rotational constants and all dipole configurations, the Sch\"odinger equation for the quantum evolution on $L^2({\rm SO}(3))$ is approximately controllable for almost all values of the rotational constants if and only if the dipole is not parallel to any of the principal axes of inertia of the asymmetric rigid body.