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 atoms at a temperature of 15 μK. 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 cm3 s–1 is reached in the spin-polarized regime. Our results constitute a benchmark for the experimental study of ultracold gases of magnetic lanthanide atoms.
Highlights
Open-shell lanthanide atoms bring up new perspectives in the field of ultracold quantum gases, based on their unique physical properties
We present a study of Dy magnetooptical traps (MOTs) operated on the 626 nm optical transition [15]
We presented a detailed experimental study of narrow-line magneto-optical trapping of dysprosium, together with theoretical models supporting our measurements
Summary
Open-shell lanthanide atoms bring up new perspectives in the field of ultracold quantum gases, based on their unique physical properties. The atomic spectrum, which includes narrow optical transitions, further permits the efficient production of artificial gauge fields [10,11,12] This exciting panorama triggered the implementation of laser cooling techniques for magnetic lanthanide atoms, including dysprosium [13,14,15], holmium [16, 17], erbium [16, 18,19,20] and thulium [16, 21]. For large and negative laser detunings (laser frequency on the red of the optical transition), the atomic sample becomes spin-polarized in the absolute ground state ∣J = 8, mJ = -8ñ This spontaneous polarization occurs due to the effect of gravity, which pushes the atoms to a region with a relatively large magnetic field (on the order of 1G), leading to efficient optical pumping [1, 2, 19]. We observe that minimal loss rates are reached in the spin-polarized regime, as predicted by a simple model of atom dynamics in attractive molecular states
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