Efficient production of large ${}^{39}\mathrm{K}$ Bose-Einstein condensates

We describe an experimental setup and the cooling procedure for producing ${}^{39}\mathrm{K}$ Bose-Einstein condensates of over $4\times {10}^5$ atoms. Condensation is achieved via a combination of sympathetic cooling with ${}^{87}\mathrm{Rb}$ in a quadrupole-Ioffe-configuration (QUIC) magnetic trap and direct evaporation in a large-volume crossed optical dipole trap, where we exploit the broad Feshbach resonance at 402 G to tune the ${}^{39}\mathrm{K}$ interactions from weak and attractive to strong and repulsive. In the same apparatus we create quasipure ${}^{87}\mathrm{Rb}$ condensates of over $8\times {10}^5$ atoms.

Figure 1

Outline of the experimental system. (a) Horizontal cross section through the vacuum chamber. Atoms are transported from the MOT cell to the science cell by the transport coils, mounted on a mechanical translation stage. (b) Region around the science cell as viewed down the transportation axis, showing the geometry of the coils used to create the QUIC trap and to access Feshbach resonances.

Figure 2

Evaporative and sympathetic cooling in the QUIC trap. (a) Cooling trajectories in single- (squares) and two-species (circles) experiments. Open and solid markers represent ${}^{87}\mathrm{Rb}$ and ${}^{39}\mathrm{K}$, respectively. The two linear fits show the difference between the steep trajectory for sympathetic cooling (solid line, ${\gamma }_{\mathrm{sym}}=20$) and that for efficient evaporative cooling (dashed line, $\gamma =3.6$). Vertical error bars lie within the markers. (b) Final temperature reached by sympathetic cooling for different sizes of the ${}^{39}\mathrm{K}$ load. The dashed line shows the temperature reached by ${}^{87}\mathrm{Rb}$ in the absence of ${}^{39}\mathrm{K}$.

Figure 3

Transfer efficiency from QUIC to CDT. The numbers at each point are the temperatures in the CDT (QUIC).

Figure 4

CDT depth versus laser power for both ${}^{87}\mathrm{Rb}$ (dashed line) and ${}^{39}\mathrm{K}$ (solid line), for a beam waist of 140 $\mu$m. There are two regimes where different mechanisms limit the trap depth. At high power, the atoms are least confined along the beams whereas, at lower power, they are less confined vertically due to the effect of gravity. There is a clear kink at the transition between the two regimes. The marked zones illustrate the possibility of sympathetically cooling ${}^{39}\mathrm{K}$ with ${}^{87}\mathrm{Rb}$ for low trap depths and the necessity to abandon this approach for deeper traps.

Figure 5

Condensation of ${}^{39}\mathrm{K}$ atoms. From left to right the number of atoms in the BEC varies from $1.5\times {10}^5$ to $4.2\times {10}^5$, while the condensate fraction grows from 20$\%$ to 80$\%$ (the sidebar shows the optical-density scale). The bottom panels show the line densities of the clouds, obtained by integrating the images along the vertical direction. Images are taken after 20 ms time of flight. The changing aspect ratio of the condensate reflects the increasing importance of the gravitational potential at low CDT powers.