Experimental implementation of laser cooling of trapped ions in strongly inhomogeneous magnetic fields

TL;DR

This study demonstrates effective Doppler laser cooling of trapped $^{40}$Ca$^+$ ions in a strong, inhomogeneous magnetic field, enabling new hybrid trapping experiments and quantum science applications involving ions and neutral molecules.

Contribution

It presents the first implementation of laser cooling of ions in strongly inhomogeneous magnetic fields, using a segmented linear Paul trap with permanent magnets.

Findings

Ions achieved secular temperatures similar to those without magnetic fields.

Magnetic field gradients of 800 to 1600 G/mm were successfully used.

A magnetic field map was generated from the Zeeman shifts.

Abstract

We demonstrate the Doppler laser cooling of $^{40}$Ca$^+$ ions confined in a segmented linear Paul trap in the presence of a strong quadrupolar magnetic field generated by two permanent ring magnets. Magnetic field gradients of 800 to 1600 G/mm give rise to a highly position-dependent Zeeman shift on the energy levels of the trapped ions. Efficient laser cooling is demonstrated using two 397 nm cooling laser beams with appropriate wavelengths and polarizations and one 866 nm repumper laser beam. Coulomb crystals of ions are found to exhibit similar secular temperatures to those trapped in absence of the magnetic field. In addition, the position dependency of the Zeeman effect is used to generate a map of the field strength. This work forms the basis for developing hybrid trapping experiments for cold ions and neutral molecules that consist of an ion and a magnetic trap to study cold interactions between these species, and opens up new possibilities for quantum-science experiments that employ trapped ions in inhomogeneous magnetic fields.