All-optical production of chromium Bose-Einstein condensates

We report on the production of ${}^{52}\mathrm{Cr}$ Bose-Einstein condensates with an all-optical method. We first load $5\times {10}^6$ metastable chromium atoms in a one-dimensional (1D) far-off-resonance optical trap (FORT) from a magneto-optical trap, by combining the use of radio-frequency sweeps and depumping toward the ${}^{5}S_2$ state. The atoms are then pumped to the absolute ground state, and transferred into a crossed FORT in which they are evaporated. The fast loading of the 1D FORT ($35\mathrm{ms}$ $1∕e$ time) and the use of relatively fast evaporative ramps allow us to obtain in $20\mathrm{s}$ about $15000$ atoms in an almost pure condensate.

Figure 1

(Color online) Loading of the 1D horizontal FORT (right). We plot the number of metastable atoms after a given accumulation duration. The presence of rf frequency ramps during the accumulation almost doubles the final number of atoms (top curve versus bottom). The solid lines are exponential fits. The error bar gives (as in the other figures) the typical statistical uncertainty. We show (left) the atomic levels and transitions of interest.

Figure 2

(Color online) Top: Experimental evidence for the importance of the relative polarizations of the different ir beams creating the crossed FORT. After a dimple has been formed by transferring half of the ir power to the vertical beam, the number of atoms in the dimple is plotted versus time. When the three ir beams (the two horizontal ones and the vertical) have a linear polarization along three orthogonal axes (triangles), the loading from the horizontal FORT is more efficient than when two have parallel polarizations (circles). Besides, the losses are reduced. Solid lines are guides for the eye. Bottom: Intensity noise spectrum of the ir laser observed with a $1\mathrm{GHz}$ bandwidth photodetector (top curve); the noise spectrum obtained with no light is shown below.

Figure 3

(Color online) Calculated potential depths created by the horizontal and vertical (bell shape) FORT beams (log scale) during the transfer in the crossed FORT and the evaporation. We show as well the classical phase-space density (dots) in the initial 1D FORT (at $t=0$), and after all the atoms are transferred into the dimple. At the very end of the ramp, the cloud reaches degeneracy, and the classical phase-space density (exceeding 1) is therefore not shown.

Figure 4

(Color online) Analysis of the experimental free expansion of an almost pure BEC. The Thomas-Fermi radii along $y$ (diamonds) and $z$ (triangles) are deduced from absorption pictures. The solid lines are results of numerical simulations using the measured trap frequencies, with the in situ radii giving the best agreements with the experimental data. Insets: integrated optical density of the atomic cloud after $4\mathrm{ms}$ of free expansion, when $T_c$ is reached (left), and well below $T_c$ (right); one pixel corresponds to $2.2\mathrm{\mu }\mathrm{m}$.