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Journal of the Physical Society of Japan

Simple and Fast Production of Bose–Einstein Condensate in a 1 µm Cross-Beam Dipole Trap

Sanjay Kumar, Shuuichi Hirai, Y. Suzuki, Masahide Kachi, Mark Sadgrove, Ken’ichi Nakagawa

Published Jul 27, 2012• 9 citations
Publisher

Abstract

We demonstrate a fast and efficient all optical realization of an 87 Rb Bose–Einstein condensation in a crossed dipole trap formed by a fiber laser with wavelength 1060 nm. Laser cooled Rb atoms were loaded into the cross trap with a relatively high initial phase space density of 7×10 -4 and an efficient evaporation cooling path was used allowing Bose–Einstein condensation to be achieved in about 3 s.

DOI
10.1143/jpsj.81.084004

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We demonstrate a fast and efficient all optical realization of an 87 Rb Bose–Einstein condensation in a crossed dipole trap formed by a fiber laser with wavelength 1060 nm. Laser cooled Rb atoms were loaded into the cross trap with a relatively high initial phase space density of 7×10 -4 and an efficient evaporation cooling path was used allowing Bose–Einstein condensation to be achieved in about 3 s.
Summary form only given. Recently, we have created a Bose condensate of /sup 87/Rb atoms directly in an optical trap formed by tightly focused laser beams. Following initial loading from a laser cooled gas, evaporative cooling through the BEC transition is achieved by simply lowering the depth of the optical trap. Our success is due in part to a high initial phase space density realized in the loading of our optical dipole trap and in part to the tight confinement of the atoms that permits rapid evaporative cooling to the BEC transition in 2 s.
We present an optimized strategy for the production of tightly confined Bose-Einstein condensates (BEC) of 87{}^{87}87Rb in a crossed dipole trap with direct loading from a magneto-optical trap. The dipole trap is created with light of a multifrequency fiber laser with a center wavelength of 1070 nm. Evaporative cooling is performed by ramping down the laser power only. A comparison of the resulting atom number in an almost pure BEC to the initial atom number and the value for the gain in phase space density per atom lost confirm that this straightforward strategy is very efficient. We observe that the temporal characteristics of evaporation sequence are strongly influenced by power-dependent two-body losses resulting from enhanced optical pumping to the higher-energy hyperfine state. We characterize these losses and compare them to results obtained with a single-frequency laser at 1030 nm.
We report on an all-optical production of a 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate (BEC) of 106{10}^{6}106 atoms. We construct a double compressible crossed dipole trap (DCDT) formed by a high-power multimode fiber laser (MCDT) and a single-mode fiber amplifier (SCDT), which are both operated at $1.06\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. A very cold dense gas is first cooled by polarization gradient cooling in a three-dimensional optical lattice. More than $2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms are loaded into the enlarged DCDT. Both CDTs are then simultaneously compressed to significantly different sizes followed by evaporation, which is performed by lowering only the MCDT power. The tighter SCDT produces an extremely high collision rate and maintains the trap stiffness, which leads to rapid and efficient evaporation. After 0.4 s, a gas of $5\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms with a phase-space density of 0.2 is confined within the SCDT alone. Further evaporation in 2.8 s yields a nearly pure BEC of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms in the $|F{m}_{F}\ensuremath{\rangle}=|11\ensuremath{\rangle}$ state. This number is the largest generated among all-optical methods. Our approach significantly improves the atom number of a condensate and circumvents the severe atom loss previously reported for multimode fiber lasers.
We create a Bose-Einstein condensate (BEC) of 87 Rb atoms by runaway evaporative cooling in an optical trap.Two crossed infrared laser beams with a wavelength of 1064 nm are used to form an optical dipole trap.After precooling the atom samples in a quadrupole-Ioffe configuration (QUIC) trap under 1.5 µK by radio-frequency (RF) evaporative cooling, the samples are transferred into the center of the glass cell, then loaded into the optical dipole trap with 800 ms.The pure condensate with up to 1.5×10 5 atoms is obtained over 1.17 s by lowering the power of the trap beams.
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.
We report on the realization of a sodium Bose–Einstein condensate (BEC) in a combined red-detuned optical dipole trap formed by two beams crossing in a horizontal plane and a third, tightly focused dimple trap (dT) propagating vertically. We produce a BEC in three main steps: loading of the crossed dipole trap from laser-cooled atoms, an intermediate evaporative cooling stage that results in efficient loading of the auxiliary dT, and a final evaporative cooling stage in the dT. Our protocol is implemented in a compact setup and allows us to reach quantum degeneracy even with relatively modest initial atom numbers and available laser power.