Observation of Space-Dependent Rotational Doppler Shifts with a Single Ion Probe

TL;DR

This study demonstrates the rotational Doppler effect at the single-ion level using vortex laser beams, revealing how the effect varies with position and optical angular momentum, with implications for atomic motion control.

Contribution

It introduces a novel experimental approach to observe space-dependent rotational Doppler shifts with a single ion, combining precise positioning and theoretical modeling.

Findings

Doppler shift increases near the beam center

Shift depends on the difference in optical orbital angular momentum

Effect is independent of the beam waist

Abstract

We present an experiment investigating the rotational Doppler effect using a single trapped ion excited by two copropagating vortex laser beams. The setup isolates the azimuthal gradients of the fields, eliminating longitudinal and curvature effects. We provide a detailed characterization of the phenomenon by deterministically positioning a single ion across the beams, achieving a signal which depends on the angular velocity of the ion and the difference of optical orbital angular momentum between the two beams. The interpretation of the measurements is supported by numerical simulations and by a simplified analytical model. Our results reveal key properties of the rotational Doppler effect, showing that it increases approaching the center of the beam and that it is independent of the waist of the beam. This offers insights into the feasibility of super-kicks or super-Doppler shifts for sensing and manipulating atomic motion transverse to the beams' propagation direction.