Spiraling light: from donut modes to a Magnus effect analogy

TL;DR

This paper explores the coupling of orbital and spin angular momentum in focused laser beams, revealing effects like atom deflection akin to the Magnus effect, with implications for quantum control and optical trapping.

Contribution

It introduces the concept of spin-orbit coupling in tightly focused laser beams and demonstrates how this leads to controllable atomic displacements in optical tweezers.

Findings

Atoms can be displaced up to λ/2π from the optical axis depending on spin state.

Coupling of orbital and spin angular momentum produces a Magnus effect analogy.

Potential applications in quantum gates and interferometry.

Abstract

The insight that optical vortex beams carry orbital angular momentum (OAM), which emerged in Leiden about 30 years ago, has since led to an ever expanding range of applications and follow-up studies. This paper starts with a short personal account of how these concepts arose. This is followed by a description of some recent ideas where the coupling of transverse orbital and spin angular momentum (SAM) in tightly focused laser beams produces interesting new effects. The deflection of a focused light beam by an atom in the focus is reminiscent of the Magnus effect known from aerodynamics. Momentum conservation dictates an accompanying light force on the atom, transverse to the optical axis. As a consequence, an atom held in an optical tweezer will be trapped at a small distance of up to $\lambda/2\pi$ away from the optical axis, which depends on the spin state of the atom and the magnetic field direction. This opens up new avenues to control the state of motion of atoms in optical tweezers as well as potential applications in quantum gates and interferometry.