Ultracold atoms in a dipole trap in microgravity

Abstract

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.

Figures (8)

• Figure 1: Cooling and trapping in a dipole trap in microgravity: (a) Atoms trapped in the two laser beams forming the crossed dipole trap. (b) Fluorescence imaging of the atoms in the dipole trap in microgravity. (c) Thanks to our painting potential, the trap is deformed at the crossing of the two beams and the increase of the trap depth from the modulated trap $U^m_{\hat{X},\hat{Y}}$ (blue) to the compressed trap $U^c_{\hat{X},\hat{Y}}$ (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 $\hat{X}$ and $\hat{Y}$ (purple).
• Figure 2: Production of ultracold atoms in a crossed dipole trap in microgravity: (a) Laser power (blue) and spatial modulation amplitude $h$ (green) during the compression phase (orange zone) and the three phases of evaporative cooling (pink, light blue and yellow respectively). Inset: To improve the evaporative cooling in microgravity, we decrease the final value of the laser power (dashed blue line) at the end of the final ramp. The solid line is the reference ramp in standard gravity. (b) Trap depth in the Z direction (purple) and in the direction of the two crossed beams $\hat{X},\hat{Y}$ (khaki). (c) Trap frequencies along the eigen-axes of the dipole trap ($X,Y,Z$) and (d) phase space density in the dipole trap in microgravity (blue). We measured a gain in the phase space density of 2 orders of magnitude during the compression phase (orange). Reducing the final value of the last ramp (dashed line) leads to a final gain in the phase space density.
• Figure 3: Parameters of the evaporative cooling in the dipole trap in microgravity: Temperature (a) and Phase Space Density (b) versus the atom number in microgravity onboard the 0g aircraft (green and light blue) and in standard gravity (blue and red). The slope allows us to extract the parameters $\alpha_T$ and $\alpha_D$ giving quantitative estimation of the cooling efficiency. (c) Temperature and (d) collision rate versus the trap depth. The data shows $\eta=U_{\hat{X},\hat{Y}}/(k_B T)\approx 6$ during the evaporative cooling (dashed line on (c)).
• Figure 4: Ultracold atoms in microgravity: (a) Spatial expansion of the atom cloud in the Y and Z direction versus the time of flight. The slope gives the equivalent temperature of the atomic sample. (b) Fluorescence detection of the spatial profile of the atom cloud for four durations of free fall from left to right: 20 ms, 40 ms, 80 ms and 100 ms. (c) The atom cloud profile is integrated along the Z direction and fitted along Y by a double Gaussian profile (the experimental data is in blue and the fitted profile in red).
• Figure 5: Time averaged crossed dipole trap in microgravity: schematic of the apparatus including the two crossed beams of the dipole trap, two imaging systems (absorption and fluorescence respectively) and the automatic realignment systems of the dipole trap laser beams. The positions of each beam are measured with a 4 quadrant detector and the feedback is applied to the mirror 1 and 2 respectively using piezo electric actuators. In microgravity, the atoms stay at the center of the vaccuum chamber. AOM: Acousto-optic modulator. $\lambda/2$ :halfwave plate. $\lambda/4$ :quarterwave plate.