Bose-Einstein condensation of ${}^{85}$Rb by direct evaporation in an optical dipole trap

We report a simple method for the creation of Bose-Einstein condensates of ${}^{85}$Rb by direct evaporation in a crossed optical dipole trap. The independent control of the trap frequencies and magnetic bias field afforded by the trapping scheme permits full control of the trapped atomic sample, enabling the collision parameters to be easily manipulated to achieve efficient evaporation in the vicinity of the 155 G Feshbach resonance. We produce nearly pure condensates of up to $4\times {10}^4$ atoms and demonstrate the tunable nature of the atomic interactions.

Figure 1

(a) Experimental setup showing the arrangement of coils around the UHV cell and the beam geometry used to create the crossed dipole trap. Trapping: Potentials produced horizontally along one of the beams (black, solid) and vertically (red, dashed) in the hybrid (b) and levitated (c) crossed dipole traps. Insets: False color images of atoms in the respective traps, viewed from above.

Figure 2

Collisional properties. Position of $a=0$ is marked by the red, dashed line: (a) Evaporation efficiency (see text) of a fixed evaporation sequence carried out at different magnetic fields close to the Feshbach resonance. (b) The effect on fitted temperature of the same evaporation ramps. (c) Feshbach resonance in the $F=2,m_F=-2$ state of ${}^{85}$Rb. The scattering length is given in units of the Bohr radius, $a_0\approx 0.529\times {10}^{-10}$ m. (d) Magnetic field dependence of the three-body inelastic loss rate near the Feshbach resonance. Solid lines in (a), (b), and (d) are a guide to the eye only.

Figure 3

Evaporation trajectory to reach BEC. (a) After RF evaporation in the quadrupole trap ($◯$), atoms are loaded into the hybrid dipole trap trap ($\blacksquare$). Following an initial evaporation stage at $\sim$0 G, a 175–185 G bias field is applied. This produces the levitated trap ($\square$) in which further evaporation is carried out. At a PSD of $\sim$0.5 the bias field is ramped to 161.3 G and a final evaporation stage is performed to reach BEC. Inset: Density profiles for (right to left) thermal, bimodal, and condensed atomic samples. (b) Horizontal cross sections of the condensate column density for a thermal (top), bimodal (center), and condensed (bottom) sample as the dipole beam power (top right) is reduced.

Figure 4

Tunable interactions. (a) Change in vertical size of a pure condensate (filled squares) and thermal cloud (open circles) as a function of magnetic field applied during time of flight. (b) Breathing mode oscillation set up by jumping the atomic scattering length from $\sim$$315a_0$ to $\sim$$50a_0$. The hold time shown is that at the new scattering length of $\sim$$50a_0$.