All-Optical Bose-Einstein Condensates in Microgravity

We report on the all-optical production of Bose-Einstein condensates in microgravity using a combination of grey molasses cooling, light-shift engineering and optical trapping in a painted potential. Forced evaporative cooling in a 3-m high Einstein elevator results in $4\times {10}^4$ condensed atoms every 13.5 s, with a temperature as low as 35 nK. In this system, the atomic cloud can expand in weightlessness for up to 400 ms, paving the way for atom interferometry experiments with extended interrogation times and studies of ultracold matter physics at low energies on ground or in Space.

Figure 1

(a) Schematic of the Einstein elevator. The payload (I), including the science chamber, cooling beam optics, and imaging system, is driven vertically along two air-bearing translation stages. (b) Acceleration profile of the science chamber for sequential parabolas of 400 ms, separated by a dead time of 12 s required to let the motors cool down. (c) Absorption image of the BEC transition after a time of flight of 50 ms in 0 g. The projection of the vertical axis shows the typical double structure of the cloud. The blue (violet) curve is a least-squares fit to the thermal distribution (condensed fraction).

Figure 2

Details of the painted optical trap composed of two crossed beams and the imaging system.

Figure 3

(a) Level structure of the atoms including the light shift due to the painted ODT ($P\simeq 10\mathrm{W}$, $h=100\mu \mathrm{m}$). The cooling and repumping frequencies are shown for the grey (solid blue lines) and red (dashed pink lines) molasses. The cooling and repumping light intensities are respectfully $I_{\mathrm{cool}}\approx 6I_S$ and $I_{\mathrm{rep}}\approx 0.6I_S$. The light shift of the excited (ground) state at the center of the ODT is $\sim 170\mathrm{MHz}$ (2.5 MHz). (b) ODT loading efficiency as a function of the detuning relative to $|F=2⟩\to |F^{\prime }=3⟩$ for the red molasses (pink squares) and grey molasses (blue circles). The atom numbers are estimated from absorption imaging. For each point, five images have been taken, averaged and fitted with a 2D Gaussian profile. The standard deviation of the fit residuals gives an uncertainty of 5% for the loading efficiency. Inset: absorption image of atoms in the ODT using near-resonant light. Atoms at the center of the trap are transparent to the cooling beams.

Figure 4

Adapted ODT parameters during the pull-up, injection, and 0 g phases (blue solid line). The parameters are calculated using the calibration of the ODT power and the measured acceleration. Calculations were validated by measuring the trap frequencies at three different times during the sequence: two in 1g and one in 0 g. (a) ODT optical power. (b) Evolution of the ODT depth. (c) Average ODT frequency taking into account the acceleration sag. For comparison, a sequence using parameters appropriate for 1 g (pink dashed line) shows vanishing trap depth and frequency because of the increased sag during the pull-up phase.

Figure 5

Normalized atom number detected vs the time of flight in 0 g for three different sample temperatures.