Interaction of twisted light with free twisted atoms

TL;DR

This paper explores how structured twisted light interacts with atoms, enabling transfer of orbital angular momentum, inducing novel electronic transitions, and demonstrating measurable atomic recoil effects, with implications for quantum control.

Contribution

It introduces a detailed quantum wave-packet model of light-atom interactions, revealing new phenomena like the superkick and selfkick effects, and discusses experimental feasibility.

Findings

Vortex photons can transfer OAM efficiently when impact parameter is small.

Wave-packet nature allows electronic transitions violating standard selection rules.

Finite coherence of femtosecond pulses shapes resonant absorption lines.

Abstract

We investigate absorption and scattering of structured light by atoms, treating the photon and the atomic center of mass as spatially localized wave packets. We show that vortex photons can transfer orbital angular momentum (OAM) to the atomic center of mass with near-perfect efficiency in head-on collisions when the impact parameter $b$ is smaller than the atomic transverse coherence length $\sigma$, which ranges from nanometers to sub-micrometer scales. Larger offsets result in a shifted mean OAM and a finite variance, both controlled by the ratio $b/\sigma$. The wave-packet nature of light enables electronic transitions that violate standard selection rules, albeit with a clear hierarchy where the dipole transition dominates. For femtosecond pulses, the finite spatial coherence of the photon leads to measurable shaping of the resonant absorption lines. We demonstrate a transverse recoil of the atom in a vicinity of the photonic vortex, dubbed "the superkick", and its dual effect - "the selfkick" - when an initially twisted atomic packet experiences recoil upon absorbing a gaussian photon. These phenomena are within reach of experimental capabilities using structured light in combination with cold atomic beams and ions in Penning traps, providing a route to the controlled generation and manipulation of non-gaussian atomic packets.