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Chinese Physics Letters

Production of 87Rb Bose–Einstein Condensate with a Simple Evaporative Cooling Method*

Rehman Fazal, Jia-Zhen Li, Zhiwen Chen, Yuan Qin, Ya-Yi Lin, Zuan-Xian Zhang, Shanchao Zhang, Wei Huang, Hui Yan, Shi-Liang Zhu

Published Mar 1, 2020• 2 citations
Publisher

Abstract

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.

DOI
10.1088/0256-307x/37/3/036701

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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 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.
To achieve Bose-Einstein condensation, one may implement evaporative cooling by dynamically regulating the power of laser beams forming the optical dipole trap. We propose and experimentally demonstrate a protocol of Bayesian optimization of Bose-Einstein condensation via the evaporative cooling model. Applying this protocol, pure Bose-Einstein condensate of 87Rb with 2.4X10e4 atoms can be produced via evaporative cooling from the initial stage when the number of atoms is 6.0X10e5 at a temperature of 12μK. In comparison with Bayesian optimization via blackbox experiment, our protocol only needs a few experiments required to verify some close-to-optimal curves for optical dipole trap laser powers, therefore it greatly saves experimental resources.
We report a rapid evaporative cooling method using a hybrid trap which is composed of a quadrupole magnetic trap and a one-beam optical dipole trap. It contains two kinds of evaporative coolings to reach the quantum degeneracy: initial radio-frequency (RF) enforced evaporative cooling in the quadrupole magnetic trap and further runaway evaporative cooling in the optical dipole trap. The hybrid trap does not require a very high power laser such as that in the traditional pure optical trap, but still has a deep trap depth and a large trap volume, and has better optical access than the normal magnetic trap like the quadrupole-Ioffe-configuration (QUIC) cloverleaf trap. A high trap frequency can be easily realized in the hybrid trap to enhance the elastic collision rate and shorten the evaporative cooling time. In our experiment, pure Bose—Einstein condensates (BECs) with about 1 × 10 5 atoms can be realized in 6 s evaporative cooling in the optical dipole trap.
Research on degenerate quantum gases of dilute atomic gases has become a well established field in modern physics and revealed many astonishing phenomena. The first experiments on Bose and Fermi gases at low temperatures aimed to prove the predictions of quantum mechanics on a macroscopic scale. Since then we make use of Bose and Fermi gases and mixtures of any combination to model other many body systems. A property of Bose Einstein condensates is superfluidity which we want to investigate further in toroidal potentials with a new apparatus which can produce an all-optical BEC. This means trapping thermal atoms in an optical dipole trap and cooling the sample by evaporation into the quantum degenerate regime. This new technique was first successfully demonstrated by M.D. Barrett et al. in 2001 and promises to be faster and simpler than the well established and common way of evaporation in magnetic traps. A new apparatus was built by the author and we report on the experimental details of the trapping of 87Rb atoms from a hot vapour and cooling them in a magneto-optical trap, and further evaporation to quantum degeneracy in a crossed optical dipole trap operating at a wavelength of l = 1064nm, with accurate control of the power in both beams via a feedback loop. The evaporation of neutral atoms of 87Rb in far red detuned optical dipole traps using linearly polarised laser light is spin independent, and mixed spinor condensates of the F=1 manifold can be formed. As an empirical technique we found by applying a magnetic gradient field during the final evaporation that we can selectively populate mF spin states or prepare mixtures. This intriguing mechanism was found earlier as well by M. S. Chang, but is yet not fully understood and subject of our future research. We can now routinely prepare an almost pure condensate containing up to 7000 atoms in the condensed phase purely in the mF = 0 spin state. This thesis contains the theoretical framework of Bose-Einstein condensation with in mean-field theory, the concepts of cooling of atoms in magneto-optical traps, the trapping and evaporation in optical dipole traps and the experimental realisation. In the future this experiment will be extended by a fast scanning dipole trap to apply arbitrary time averaged potentials of more complex geometries, such as toroidal traps to study the effects of superfluidity in a periodic potential. The production of toroidal traps and cold atoms in such a trap are outlined in this thesis.
We present a combined computational and experimental study to optimize the efficiency of evaporative cooling for atoms in optical dipole traps. By employing a kinetic model of evaporation, we provide a strategy for determining the optimal relation between atom temperature, trap depth, and average trap frequency during evaporation given experimental initial conditions. We then experimentally implement a highly efficient evaporation process in an optical dipole trap, showing excellent agreement between the theory and experiment. This method has allowed the creation of pure Bose-Einstein condensates of 87{}^{87}87Rb with $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms starting from only $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms initially loaded in the optical dipole trap, achieving an evaporation efficiency ${\ensuremath{\gamma}}_{\mathrm{eff}}$ of 4.0 during evaporation.
We present a compact experimental apparatus for Bose-Einstein condensation of 87Rb in the |F = 2, mF = + 2〉 state. A pre-cooled atomic beam of 87Rb is obtained by using an unbalanced magneto-optical trap, allowing controlled transfer of trapped atoms from the first vacuum chamber to the science chamber. Here, atoms are transferred to a hybrid trap, as produced by overlapping a magnetic quadrupole trap with a far-detuned optical trap with crossed beam configuration, where forced radiofrequency evaporation is realized. The final evaporation leading to Bose-Einstein condensation is then performed by exponentially lowering the optical trap depth. Control and stabilization systems of the optical trap beams are discussed in detail. The setup reliably produces a pure condensate in the |F = 2, mF = + 2〉 state in 50 s, which includes 33 s loading of the science magneto-optical trap and 17 s forced evaporation.