Momentum exchange between light and a single atom: Abraham or Minkowski?

TL;DR

This paper investigates the momentum exchange between light and a single atom, challenging traditional views by identifying Abraham's momentum with kinetic transfer and Minkowski's with canonical momentum, with implications for optical force understanding.

Contribution

It clarifies the distinction between Abraham and Minkowski momentum in light-atom interactions, proposing a new interpretation based on dispersive forces and momentum transfer.

Findings

Atoms are repelled by light due to dispersive forces.

Abraham's momentum corresponds to kinetic momentum transfer.

Minkowski's momentum is associated with canonical momentum.

Abstract

We consider the forces exerted by a pulse of plane-wave light on a single atom. The leading edge of the pulse exerts a dispersive force on the atom, and this modifies the atomic momentum while the atom is enveloped in the light. The standard view of the optical dipole force indicates that red-detuned light should attract the atom towards high intensity. This should increase the average momentum per photon to $\textbf{p}_{0} n$, where $\textbf{p}_{0}$ is the photon momentum in free space and $n$ is the average refractive index due to the presence of the atom in the light. We show, however, that this is the wrong conclusion and that the atom is in fact repelled from the light by the dispersive forces, giving the photons a momentum $\textbf{p}_{0} /n$. This leads us to identify Abraham's optical momentum with the kinetic momentum transfer. The form due to Minkowski is similarly associated with the canonical momentum. We consider the possibility of demonstrating this in the laboratory, and we note an unexpected connection with the Aharonov-Casher effect.