Bose-Einstein Condensation of Strontium

We report on the attainment of Bose-Einstein condensation with ultracold strontium atoms. We use the ${}^{84}\mathrm{Sr}$ isotope, which has a low natural abundance but offers excellent scattering properties for evaporative cooling. Accumulation in a metastable state using a magnetic-trap, narrowline cooling, and straightforward evaporative cooling in an optical trap lead to pure condensates containing $1.5\times {10}^5\mathrm{\text{atoms}}$. This puts ${}^{84}\mathrm{Sr}$ in a prime position for future experiments on quantum-degenerate gases involving atomic two-electron systems.

Figure 1

Schematic illustration of the energy levels and transitions used for cooling and trapping of Sr atoms. The blue MOT is operated on the strong ${}^{1}S_0\mathrm{\text{−}}{}^{1}P_1$ transition. The loading of the magnetic trap proceeds via the weak leak of the excited state (branching ratio $1:50000$) into the ${}^{1}D_2$ state, which itself decays with a $1:3$ probability into the metastable ${}^{3}P_2$ state. Here the atoms can be magnetically trapped and accumulated for a long time. The transition ${}^{3}P_2\mathrm{\text{−}}{}^{3}D_2$ allows us to depopulate the metastable state by transferring the atoms into the ${}^{3}P_1$ state. The latter represents the excited state of the ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_1$ intercombination line, used for narrowline cooling in the red MOT.

Figure 2

Absorption images and integrated density profiles showing the BEC phase transition for different times $t_{\mathrm{ev}}$ of the evaporative cooling ramp. The images are along the vertical direction 25 ms after release from the trap. The solid line represents a fit with a bimodal distribution, while the dashed line shows the Gaussian-shaped thermal part, from which the given temperature values are derived.

Figure 3

Inversion of the aspect ratio during the expansion of a pure BEC. The images (field of view $250\mu \mathrm{m}\times 250\mu \mathrm{m}$) are taken along the vertical direction. The first image is an in situ image recorded at the time of release. The further images are taken 5, 10, 15, and 20 ms after release.