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Review of Scientific Instruments

Realization of two-stage crossed beam cooling and the comparison with Delta-kick cooling in experiment

Tian Luan, Yufan Li, Xuesong Zhang, Xuzong Chen

Published Dec 1, 2018• 15 citations
Publisher

Abstract

We report the first experimental realization of the two-stage crossed beam cooling (TSCBC) method that we proposed in 2013 [L. Wang et al., J. Phys. B: At., Mol. Opt. Phys. 46, 195302 (2013)]. With the 87Rb Bose-Einstein condensation apparatus and electromagnet coils providing the magnetic levitation to counteract the gravitation, we simulated the micro-gravity environment and realized the TSCBC with 4 × 104 87Rb atoms. We estimated that the lowest temperature of atoms can be at 3.56 nK with a new method and verified that the cooling process is adiabatic enough with time-of-flight images. According to analysis, we believed that the noise of magnetic field was the main obstacle that hinders the further cooling of the atomic ensemble. Under the same experimental conditions, we carried out the Delta-kick cooling method and got a lowest temperature of 23.3 nK also with 4 × 104 87Rb atoms. According to the results of comparing experiments, we can see that the TSCBC method is more effective.

DOI
10.1063/1.5046815

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We report the first experimental realization of the two-stage crossed beam cooling (TSCBC) method that we proposed in 2013 [L. Wang et al., J. Phys. B: At., Mol. Opt. Phys. 46, 195302 (2013)]. With the 87Rb Bose-Einstein condensation apparatus and electromagnet coils providing the magnetic levitation to counteract the gravitation, we simulated the micro-gravity environment and realized the TSCBC with 4 × 104 87Rb atoms. We estimated that the lowest temperature of atoms can be at 3.56 nK with a new method and verified that the cooling process is adiabatic enough with time-of-flight images. According to analysis, we believed that the noise of magnetic field was the main obstacle that hinders the further cooling of the atomic ensemble. Under the same experimental conditions, we carried out the Delta-kick cooling method and got a lowest temperature of 23.3 nK also with 4 × 104 87Rb atoms. According to the results of comparing experiments, we can see that the TSCBC method is more effective.
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.
The construction of an apparatus capable of producing Bose-Einstein condensates marks a significant milestone in every experimental cold atom laboratory. In this thesis I describe the development of a system to create a Bose-Einstein condensate of 87Rb^{87}Rb87Rb in a Time-Orbiting Potential trap.\n\nI review the optical and magnetic techniques required to trap and cool an atomic sample under vacuum, motivating our decision to build a double MOT system comprised of a high-pressure (10−910^{-9}10−9 torr) chamber to gather atoms and a low-pressure (10−1110^{-11}10−11 torr) chamber to cool atoms to degeneracy.\n\nBy theoretically modeling the atom number and temperature inside the magnetic trap during evaporative cooling I demonstrate a simple approach to determining a cooling path that reaches the transition temperature. By making use of the condensates produced under these non-optimized conditions I determine the heating rate of the condensate in the TOP trap to be 300300300 nK/s. I further use the condensates to make a more precise measurement of the TOP trap bias field.\n\nI improve upon the conventional evaporation path used in TOP trap experiments by introducing and optimizing additional bias field compression stages in between RF evaporation ramps. I demonstrate how, by adding these additional stages, the system is capable of reaching the BEC phase transition with a final atom number of 2times1052\\times 10^{5}2times105. In contrast, RF evaporation after only a single bias field ramp has yielded condensates with only 30times10330\\times 10^330times103 atoms.
A Bose–Einstein condensate with a large atom number is an important experimental platform for quantum simulation and quantum information research. An optical dipole trap is the a conventional way to hold the ultracold atoms, where an atomic cloud is evaporatively cooled down before reaching the Bose–Einstein condensate. A carefully designed trap depth controlling curve is typically required to realize the optimal evaporation cooling. We present and demonstrate a simple way to optimize the evaporation cooling in a crossed optical dipole trap. A polyline shape optical power control profile is easily obtained with our method, by which a pure Bose–Einstein condensate with atom number 1.73 × 105 is produced. Theoretically, we numerically simulate the optimal evaporation cooling using the parameters of our apparatus based on a kinetic theory. Compared to the simulation results, our evaporation cooling shows a good performance. We believe that our simple method can be used to quickly realize evaporation cooling in optical dipole traps.
We present progress towards a planned experiment on atomic tunneling of ultra-cold Rb atoms. As a first step in this experiment we present a realization of an improved form of "delta-kick cooling." By application of a pulsed magnetic field, laser cooled Rb atoms are further cooled by a factor of 10 (in 1-D) over the temperature out of molasses. Temperatures below 700 nK are observed. The technique can be used not only to cool without fundamental limit (but conserving phase-space density), but also to focus atoms, and as a spin-dependent probe.
We report the production of 87Rb Bose–Einstein condensate in an asymmetric crossed optical dipole trap (ACODT) without the need of an additional dimple laser. In our experiment, the ACODT is formed by two laser beams with different radii to achieve efficient capture and rapid evaporation of laser cooled atoms. Compared to the cooling procedure in a magnetic trap, the atoms are firstly laser cooled and then directly loaded into an ACODT without the pre-evaporative cooling process. In order to determine the optimal parameters for evaporation cooling, we optimize the power ratio of the two beams and the evaporation time to maximize the final atom number left in the ACODT. By loading about 6 × 105 laser cooled atoms in the ACODT, we obtain a pure Bose–Einstein condensate with about 1.4 × 104 atoms after 19 s evaporation. Additionally, we demonstrate that the fringe-type noises in optical density distributions can be reduced via principal component analysis, which correspondingly improves the reliability of temperature measurement.