Formation of Quantum-Degenerate Sodium Molecules

Ultracold sodium molecules were produced from an atomic Bose-Einstein condensate by ramping an applied magnetic field across a Feshbach resonance. More than ${10}^5$ molecules were generated with a conversion efficiency of $\sim 4\%$. Using laser light resonant with an atomic transition, the remaining atoms could be selectively removed, preventing fast collisional relaxation of the molecules. Time-of-flight analysis of the pure molecular sample yielded an instantaneous phase-space density greater than 20.

Figure 1

(a) Experimental method for producing and detecting ultracold molecules. (i) Bose condensed atoms in an optical dipole trap are exposed to a magnetic field just below a Feshbach resonance. (ii) The field is quickly stepped through the resonance to minimize atom loss. (iii) The field is then swept back through the resonance, creating an atom-molecule mixture. (iv) Unpaired atoms are removed from the trap with resonant light, yielding a pure molecular sample. (v) The trap is switched off, allowing the molecules to expand ballistically. (vi) Finally, the magnetic field is swept back across the resonance to reconvert the molecules to atoms for imaging (vii). (b) Image of the atomic sample after ramping the field to produce molecules; (c) after the resonant light pulse has removed all unpaired atoms; (d) after the molecules ($\sim {10}^5$) have been reconverted to atoms. (b),(c) were taken along the weak axis of the trap after 17 ms ballistic (time-of-flight–TOF) expansion. (e) An image showing both atomic (top) and molecular (bottom) clouds after 14 ms ballistic expansion, spatially separated by a magnetic field gradient. With 4 ms field ramp-down time, some molecules survived even without the blast pulse, but are much more heated. The field of view of each image is $1.8\mathrm{m}\mathrm{m}\times 1.3\mathrm{m}\mathrm{m}$.

Figure 2

Ballistic expansion of a pure molecular sample. Absorption images of molecular clouds (after reconversion to atoms) are shown for increasing expansion time after switching off the optical trap. The small expansion velocity corresponds to a temperature of $\sim 30\mathrm{n}\mathrm{K}$, characteristic of high phase-space density. The images are taken along the weak axis of the trap. The field of view of each image is $3.0\mathrm{m}\mathrm{m}\times 0.7\mathrm{m}\mathrm{m}$.

Figure 3

Molecular phase-space density versus hold time. (a) The phase-space densities of the trapped molecules were observed to decrease significantly after a few milliseconds in the optical trap. The open and solid squares are data from two separate runs on different days. (b),(c) are absorption images of the molecular clouds after (b) 0 ms, (c) 2 ms, (d) 5 ms, (e) 10 ms, (f) 20 ms hold time in the trap. The field of view is $0.8\mathrm{m}\mathrm{m}\times 0.8\mathrm{m}\mathrm{m}$.

Figure 4

Images of (a) atomic and (b) molecular clouds. These absorption images were taken after 7 ms ballistic expansion and show the axial extent of the clouds. Radial excitations in the optical trap resulting from the sudden switching of magnetic fields are manifest as snakelike patterns. Such excitations blur images (c) taken along the long axis of the trap (in 17 ms TOF), leading to an underestimate of the phase-space density. The fields of view are (a),(b) $0.6\mathrm{m}\mathrm{m}\times 3.2\mathrm{m}\mathrm{m}$, (c) $0.6\mathrm{m}\mathrm{m}\times 0.4\mathrm{m}\mathrm{m}$.