Most cold atoms experiments in microgravity platforms or in Space are achieved using atom chips, leading to limitations in terms of optical access and inhomogeneous magnetic fields. Optical dipole traps do not have these drawbacks but have difficulties producing atomic samples with a large number of atoms at ultra low temperature in the absence of gravity. Here, we report on an efficient evaporative cooling in two-crossed laser beams during parabolic flights. Time-averaged potentials combine the advantages of large capture volume and trap compression, increasing the initial phase space density and collision rate to favor the evaporative process. With this technique we demonstrate the production of an ultra cold gas of $2.5\times 10^4$ rubidium atoms at a temperature below 100 nK in less than 4 seconds. Our experiment paves the way for the development of quantum sensors applied to fundamental physics and geodesy as well as the study of ultracold atomic physics in Space.
Cold atoms in microgravity is an emerging field which covers a large panel of physical phenomena, including, non exhaustively, few-body physics [1], molecular spectroscopy and ultracold chemistry, exotic topologies and geometries [2,3], and fundamental tests [4,5].
A number of Bose Einstein Condensates experiments have been performed on ground-based microgravity platforms [6,7] or in Space [8,9], demonstrating the maturity and the robustness of the atom chip technology. Despite their promising performances, inherent difficulties due to the proximity of the atom chip are still to be tackled such as stray light, limited optical access, and inhomogeneous magnetic fields. Alternatively, dipole traps have strong advantages to produce ultracold atoms, allowing high optical access, a fast extinction of the trapping potential and facilitating the control of the interactions with an independent control of magnetic fields for Feshbach resonances.
Unfortunately, the inherent decompression of the dipole trap prevents fast and efficient evaporative cooling, making the run-away regime unreachable [10]. Unlike magnetic traps, the trap frequency decreases as the square root of the laser power while reducing the trap depth. On ground, the gravitational potential gradient decreases the potential barrier favoring the evaporation in the vertical direction and become significant during the final stage of the evaporative cooling. In microgravity, the absence of this helping force leads to a new challenge. In return, the laser power (blue) to the compressed trap U c X, Ŷ (purple) leads to an increase in the phase space density before starting the evaporative cooling. (d) In standard gravity (green), the sag decreases the effective trap depth favoring the escape of the atoms in the vertical direction at the end of the evaporative cooling. In microgravity, there is no sag effect (blue) atoms are preferentially evaporated in the weakly confining potential along the directions of the two beams X and Ŷ (purple). can be further decreased compared to standard gravity [11] while maintaining the weak trapping force in the directions of the laser beams. Evaporative cooling [12] was demonstrated at high temperature (> 10µK) and a promising robust set up was tested in the HITec Einstein Elevator in Hannover [13] but no Bose Einstein condensation in microgravity has been demonstrated so far.
In this work, we demonstrate the production of ultracold atoms in a dipole trap in microgravity and reach a phase space density at the threshold of Bose Einstein condensation. Our painted potential created by spatial modulation of a tightly focused laser beam allows for a good trade-off between capture volume and trap depth [7]. Due to the influence of the laser beams, the atoms are trapped in a bimodal potential. The spatial modulation is turned off to change the shape of the trap slowly compared to the internal equilibration time [14], increasing the phase space density. In the meantime the compression of the trap increases the collision rate favoring efficient evaporative cooling. We demonstrate that it is possible to reach high phase space density despite a decrease in collision rate due to the decompression of the trap. Time of flight measurements with a duration of free fall up to 100 ms allow us to evaluate the spatial expansion of the ultracold atomic sample.
The results were obtained onboard the Novespace 0g aircraft performing parabolic flights. A threedimensional magneto-optical trap (MOT) is loaded in the vacuum chamber from a two-dimensional MOT during 1 s, leading to 1.5 × 10 8 trapped rubidium atoms. Then the atom cloud is cooled down to 4.5µK in 9 ms using red optical molasses. After the laser cooling stage, our method [7] combines simultaneously grey molasses cooling and trapping in a time-averaged optical potential. An amplified fibered telecom laser delivers up to 23W of 1550 nm light and the dipole trap is formed by two crossed beams (Fig. 1 (a)) spatially modulated by an acousto-optic modulator (AOM) to increase the capture volume. Approximately 6 × 10 6 atoms are loaded in the dipole trap in 150 ms (Fig. 1 (b)). The variation in the acceleration between the steady flight, the hypergravity and microgravity phase of the parabola leads to a relative misalignment of the two crossed beams of the dipole trap. To mitigate this effect, a real time compensation system realigns the relative position of the two beams at each experimental sequence (see Fig. 5 in Methods).
In the directions X, Ŷ of the two laser beams propagation, the dipole trap potential has a double structure with an additional trapping force due to the focus of the propagating beam. This effect is significant here because the painted potential allows the beam to be focused strongly on the atoms without reducing the capture volume defined by the spatial modulation amplitude. Atoms are thus trapped in a bimodal potential which plays an important role in increasing the init
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