Repulsive vacuum-induced forces on a magnetic particle

TL;DR

This paper explores conditions under which a magnetic particle near a surface experiences a repulsive vacuum-induced force, highlighting the potential for magnetic contributions to dominate over electric ones in specific regimes.

Contribution

It introduces a model combining electric and magnetic dipole interactions to demonstrate possible repulsive Casimir-Polder forces, and suggests methods to enhance this effect.

Findings

Magnetic-dipole interactions can surpass electric-dipole attractions under certain conditions.

Longer-range magnetic interactions can lead to repulsive forces.

Enhancement strategies include preparing particles in super-radiant states and designing surface resonances.

Abstract

We study the possibility of obtaining a repulsive vacuum-induced force for a magnetic point particle near a surface. Considering the toy model of a particle with an electric-dipole transition and a large magnetic spin, we analyze the interplay between the repulsive magnetic-dipole and the attractive electric-dipole contributions to the total Casimir-Polder force. Particularly noting that the magnetic-dipole interaction is longer-ranged than the electric-dipole due to the difference in their respective characteristic transition frequencies, we find a regime where the repulsive magnetic contribution to the total force can potentially exceed the attractive electric part in magnitude for a sufficiently large spin. We analyze ways to further enhance the magnitude of the repulsive magnetic Casimir-Polder force for an excited particle, such as by preparing it in a "super-radiant" magnetic sub-level, and designing surface resonances close to the magnetic transition frequency.