Optical cooling and trapping of highly magnetic atoms: The benefits of a spontaneous spin polarization

TL;DR

This study demonstrates that narrow-line Dysprosium magneto-optical traps naturally polarize atomic spins, simplifying cooling dynamics and enabling high-density, ultracold samples, which benefits research on magnetic quantum gases.

Contribution

The paper provides the first experimental evidence that radiation pressure and gravity induce spontaneous spin polarization in Dysprosium MOTs, enhancing trap performance and simplifying theoretical modeling.

Findings

Achieved large spin polarization at high laser detunings.

Produced ultracold samples with 3×10^8 atoms at 15 μK.

Measured two-body loss rate consistent with Van der Waals forces.

Abstract

From the study of long-range-interacting systems to the simulation of gauge fields, open-shell Lanthanide atoms with their large magnetic moment and narrow optical transitions open novel directions in the field of ultracold quantum gases. As for other atomic species, the magneto-optical trap (MOT) is the working horse of experiments but its operation is challenging, due to the large electronic spin of the atoms. Here we present an experimental study of narrow-line Dysprosium MOTs. We show that the combination of radiation pressure and gravitational forces leads to a spontaneous polarization of the electronic spin. The spin composition is measured using a Stern-Gerlach separation of spin levels, revealing that the gas becomes almost fully spin-polarized for large laser frequency detunings. In this regime, we reach the optimal operation of the MOT, with samples of typically $3\times 10^8$ atoms at a temperature of 15\,$\mu$K. The spin polarization reduces the complexity of the radiative cooling description, which allows for a simple model accounting for our measurements. We also measure the rate of density-dependent atom losses, finding good agreement with a model based on light-induced Van der Waals forces. A minimal two-body loss rate $\beta\sim 2\times10^{-11}\,$cm$^{3}$/s is reached in the spin-polarized regime. Our results constitute a benchmark for the experimental study of ultracold gases of magnetic Lanthanide atoms.