Two-tone Doppler cooling of radial two-dimensional crystals in a radiofrequency ion trap

TL;DR

This paper demonstrates a two-tone Doppler cooling method for radial 2D Coulomb crystals of barium ions in a radiofrequency trap, enabling cooling of larger ion populations by addressing position-dependent micromotion effects.

Contribution

It introduces a two-tone Doppler cooling technique that effectively cools larger 2D ion crystals with significant micromotion, surpassing previous single-tone limitations.

Findings

Successfully cooled over 50 ions in 4 shells

Characterized micromotion for all ions in the crystal

Located trap center and determined Mathieu parameters

Abstract

We study the Doppler-cooling of radial two-dimensional (2D) Coulomb crystals of trapped barium ions in a radiofrequency trap. Ions in radial 2D crystals experience micromotion of an amplitude that increases linearly with the distance from the trap center, leading to a position-dependent frequency modulation of laser light in each ion's rest frame. We use two tones of Doppler-cooling laser light separated by approximately 100~MHz to efficiently cool distinct regions in the crystals with differing amplitudes of micromotion. This technique allows us to trap and cool more than 50 ions populating 4 shells in a radial two-dimensional crystal, where with a single tone of Doppler cooling light we are limited to 30 ions in 3 shells. We also individually characterize the micromotion of all ions within the crystals, and use this information to locate the center of the trap and to determine the Matthieu parameters $q_{x}$ and $q_{y}$.