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Journal of Physics B Atomic Molecular and Optical Physics

Generating a picokelvin ultracold atomic ensemble in microgravity

W. Lu, Peng Zhang, Xuzong Chen, Zhaoyuan Ma

Published Sep 19, 2013• 13 citations
Publisher

Abstract

Applying the direct Monte Carlo simulation (DSMC) method developed for a cold atom system, we study the evaporative cooling process in tilted optical dipole traps with a magnetic field gradient-induced over-levitation or merely a gravitational force. We propose a two-stage decomposed evaporative cooling process in a microgravity environment, and suggest that quantum degeneracy can be obtained at a few picokelvins with several thousand atoms.

DOI
10.1088/0953-4075/46/19/195302

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Applying the direct simulation Monte Carlo (DSMC) method developed for ultracold Bose-Fermi mixture gases research, we study the sympathetic cooling process of 6Li and 133Cs atoms in a crossed optical dipole trap. The obstacles to producing 6Li Fermi degenerate gas via direct sympathetic cooling with 133Cs are also analyzed, by which we find that the side-effect of the gravity is one of the main obstacles. Based on the dynamic nature of 6Li and 133Cs atoms, we suggest a two-stage cooling process with two pairs of crossed beams in microgravity environment. According to our simulations, the temperature of 6Li atoms can be cooled to T = 29.5 pK and T/TF = 0.59 with several thousand atoms, which propose a novel way to get ultracold fermion atoms with quantum degeneracy near pico-Kelvin.

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Abstract Most cold atoms experiments in microgravity platforms or in space are achieved using atom chips, constrained by limited optical access and inhomogeneous magnetic fields. Despite their higher power consumption, optical dipole traps are interesting alternatives because they do not have these drawbacks. Nevertherless, producing samples with a large number of ultracold atoms in a dipole trap, in the absence of gravity, has remained challenging so far. 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 2.3 × 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.

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Applying the direct Monte Carlo simulation (DSMC) method developed for a cold atom system, we study the evaporative cooling process in tilted optical dipole traps with a magnetic field gradient-induced over-levitation or merely a gravitational force. We propose a two-stage decomposed evaporative cooling process in a microgravity environment, and suggest that quantum degeneracy can be obtained at a few picokelvins with several thousand atoms.
Applying the direct simulation Monte Carlo (DSMC) method developed for ultracold Bose-Fermi mixture gases research, we study the sympathetic cooling process of 6Li and 133Cs atoms in a crossed optical dipole trap. The obstacles to producing 6Li Fermi degenerate gas via direct sympathetic cooling with 133Cs are also analyzed, by which we find that the side-effect of the gravity is one of the main obstacles. Based on the dynamic nature of 6Li and 133Cs atoms, we suggest a two-stage cooling process with two pairs of crossed beams in microgravity environment. According to our simulations, the temperature of 6Li atoms can be cooled to T = 29.5 pK and T/TF = 0.59 with several thousand atoms, which propose a novel way to get ultracold fermion atoms with quantum degeneracy near pico-Kelvin.
Applying the direct simulation Monte Carlo (DSMC) method developed for the cold atom system, we explore the possibility of a high-efficiency sympathetic cooling process between 87 Rb and 40 K without gravitational force in an optical trap. The relation between the pre-cooling of Bosons and the sympathetic cooling efficiency is also studied. We find that the absence of gravitational force is beneficial to the process of sympathetic cooling. Furthermore, an inefficient pre-cooling process will in fact hamper the creation of Fermi degenerate gases. This suggests the advantages of sympathetic cooling in microgravity.
Optical interferometers are well known for their high precession in several measurement situations. They are based on a readout of phase differences between optical path length. In contrast, atom interferometers rely on the De Broglie wavelength of atoms and their corresponding phase. A cloud of atoms is split and recombined with light pulses, while in between, the atoms are moving freely. These precise quantum tools will enhance a broad variety of measurements, ranging from large scale phenomena like gravitational wave detection to short scale Casimir-Polder forces and everything in between. The sensitivity of atom interferometers scales with the time squared between light pulses, which is generally limited by gravity and temperature. One technical challenging sollution is the realization of atom interferometers in microgravity. While ultra cold atoms can be prepared in optical and magnetic traps, under weightlessness their preparation is limited to magnetic traps until now. These experiments are based on atom chips with highly asymmetric trapping potentials. The improved symmetry of optical traps can improve the most advanced cooling technique, called delta kick collimation, leading to atomic clouds in the yet unreached femtokelvin range. This thesis is about the first realization of an optical trapping potential in microgravity. It describes the experimental setup, identifies a molasseses technique as the optimal process for dipole trap loading and investigates the further cooling process under the influence and in the absence of gravity. In optical and magnetic traps, the preparation of ultracold atoms is based on evaporation. However, the underlying physics to drive this process are fundamentally different and for optical traps, evaporation is driven by gravity. It is demonstrated that evaporation from an optical potential performs approximately equal with and without gravity, due to a strong mixing of the trapping potentials in all three spatial dimensions. These findings are confirmed with computational simulations, based on the Direct Simulation Monte Carlo method. Atomic ensembles of rubidium-87 with temperatures as low as a 300 nK were generated in the microgravity environment of the drop tower in Bremen with an evaporation time as short as 0.5 s. The final confining trap was too shallow to be reproduced in a laboratory environment. The findings of this thesis will possibly guide the way to improved atom interferometers, with countless applications in precise sensing. Furthermore, optical potentials in microgravity on their own will open up a broad field of fundamental physical experiments. One example is the applicability of magnetic Feshbach resonances. Allowing almost arbitrary tuning of the interatomic scattering length offers new insights in scattering processes or miscibility scenarios.
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 2.5×1042.5\times 10^42.5×104 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.
In this paper, to investigate how atoms evaporate and cool in microgravity environments of the space station, we have developed the dynamic equation for evaporative cooling of trapped atoms and obtained the analytical expression of atomic temperature with respect to trapping laser parameters and gravitational acceleration. In our model, the evaporation of atoms is equivalent to applying a damping force to the trapped atoms, and the evaporative cooling process of trapped atoms is comprehended as the damped oscillation of trapped atoms in the optical dipole traps. By introducing the gravity in our model, we obtained the analytical model of temperature variation with gravity after cooling, and the theoretical results agree well with the evaporation experiment of rubidium-87 atoms on the ground. Our theoretical results show that, compared with the atomic evaporative cooling experiment on the ground, the microgravity environment of the space station can achieve cooler atomic gases when the losses of atoms, such as the one-body loss caused by background-gas collisions and the three-body recombination loss caused by interatomic inelastic collisions, can be ignored.
Abstract Most cold atoms experiments in microgravity platforms or in space are achieved using atom chips, constrained by limited optical access and inhomogeneous magnetic fields. Despite their higher power consumption, optical dipole traps are interesting alternatives because they do not have these drawbacks. Nevertherless, producing samples with a large number of ultracold atoms in a dipole trap, in the absence of gravity, has remained challenging so far. 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 2.3 × 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.