Alternative route to Bose-Einstein condensation of two-electron atoms

We present a route to Bose-Einstein condensation devised for two-electron atoms, which do not admit practicable cooling techniques based upon narrow intercombination lines. A dipole trap for ${}^{40}$Ca atoms in the singlet ground state is loaded from a moderately cold source of metastable triplet atoms via spatially and energetically selective optical pumping permitting 4 orders of magnitude increase of the phase-space density. Further cooling to quantum degeneracy is achieved by forced evaporation optimized to minimize three-body losses. In a combined loading and evaporation cycle of less than 3 s we are able to condense 3000 atoms.

Figure 1

(a) Electronic level scheme of the calcium atom showing the transitions relevant to this work. Solid lines denote laser transitions, while dashed lines stand for spontaneous decay. (b) Orientation of the singlet- and triplet-MOT beams (blue and orange arrows), HDT beam (red arrow), and TDT beam (green arrow). Gravity acts along the $z$ axis. (c) Expanded view of the overlap between the horizontal dipole trap and depumper beams.

Figure 2

(a) A typical experimental run comprises the following stages: A, loading of the triplet-MOT; B, cooling of the triplet-MOT; C, loading of the HDT; D, spontaneous evaporation in the HDT; E, loading of the CDT; F, forced evaporation in the CDT; G, ballistic expansion of the atomic cloud for variable times after their release from the trap; and H, imaging of the atomic cloud and processing of the image. The shown lines illustrate the powers of the laser beams indicated on the right. (b) Evaporation ramp applied in stage F. The solid black line indicates the trap depths in $\mu$K (left $y$-axis). The dashed red (dotted green) line shows the power in the HDT (TDT) beam (right $y$-axis).

Figure 3

Four series of TOF images taken upon terminating the forced evaporation ramp shown in stage F of Fig. 2a after (a) 375 ms, (b) 695 ms, (c) 1015 ms, and (d) waiting another 100 ms in the final evaporation step. The time of free expansion is indicated below the figure. The trap depths corresponding to each series are 900, 332, 104, and 104 nK, respectively. The anisotropic expansion of the condensate fraction is clearly visible in the last two series, while in the first series an isotropically expanding thermal cloud is observed.

Figure 4

Column density distributions of atoms along the loose trap axis, corresponding to the $x$ axis in the series (a)–(d) of Fig. 3, after 10 ms TOF. The black dots are the data points, each obtained as a running average over three consecutive points in a single measurement. The solid orange line is a bimodal fit using the sum of a Gaussian distribution (dashed green line) and a Thomas-Fermi distribution (dotted blue line).

Figure 5

Thomas-Fermi radii of the condensate [corresponding to Fig. 3d] along the strongly confined (red solid triangles) and weakly confined (blue open circles) trap axes plotted against the TOF. The radii have been obtained through fits of the kind shown in Fig. 4.