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Physical Review A

All-optical production of a large Bose-Einstein condensate in a double compressible crossed dipole trap

Kazuya Yamashita, Kouhei Hanasaki, Akihiro Ando, Masahiro Takahama, Toshiya Kinoshita

Published Jan 11, 2017• 15 citations
Publisher

Abstract

We report on an all-optical production of a $^{87}\mathrm{Rb}$ Bose-Einstein condensate (BEC) of ${10}^{6}$ 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.

DOI
10.1103/physreva.95.013609

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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\ensuremathμm1.06\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}1.06\ensuremathμm. A very cold dense gas is first cooled by polarization gradient cooling in a three-dimensional optical lattice. More than 2\ifmmode×\else\texttimes\fi1072\ifmmode\times\else\texttimes\fi{}{10}^{7}2\ifmmode×\else\texttimes\fi107 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×\else\texttimes\fi1065\ifmmode\times\else\texttimes\fi{}{10}^{6}5\ifmmode×\else\texttimes\fi106 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×\else\texttimes\fi1061.2\ifmmode\times\else\texttimes\fi{}{10}^{6}1.2\ifmmode×\else\texttimes\fi106 atoms in the ∣FmF\ensuremath⟩=∣11\ensuremath⟩|F{m}_{F}\ensuremath{\rangle}=|11\ensuremath{\rangle}∣FmF​\ensuremath⟩=∣11\ensuremath⟩ 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 develop an all-optical method to rapidly produce a large 85Rb^{85}\mathrm{Rb}85Rb Bose-Einstein condensate. 85Rb^{85}\mathrm{Rb}85Rb has a reputation as a difficult species to cool to quantum degeneracy due to its collisional properties. We solve these intrinsic difficulties by a combination of polarization gradient cooling in a 3D optical lattice and a compressible double-crossed dipole trap (DCDT). This lattice cooling circumvents the low rethermalization rate and loads more than 107{10}^{7}107 atoms with a phase-space density larger than ${10}^{\ensuremath{-}3}$ into the enlarged DCDT. The subsequent compressions and evaporations in the size-adjustable DCDT can maximize the cooling efficiency while reducing unnecessary inelastic losses. We fix the bias magnetic field to 161 G where the sss-wave scattering length is positive and large. The forced evaporations function well against inelastic collisions at an average density of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. A nearly pure condensate of $2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms is produced in 3.2 s of evaporation time.
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 describe an all-optical method of making a 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate (BEC). Atoms are first cooled in an optical lattice and loaded into a crossed dipole trap with $1.06\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ light. Then the density is increased by dynamically reducing the trap size, while the atoms are evaporatively cooled by reducing the light intensity. Every 3.3s3.3\phantom{\rule{0.3em}{0ex}}\mathrm{s}3.3s, we obtain a nearly pure BEC with $3.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms in their lowest internal energy state.
We report on an efficient production scheme for a large quantum degenerate sample of fermionic lithium. The approach is based on our previous work on narrow-line $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ laser cooling resulting in a high phase-space density of up to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This allows utilizing a large-volume crossed optical dipole trap with a total power of 45W45\phantom{\rule{0.16em}{0ex}}\mathrm{W}45W, leading to high loading efficiency and $8\ifmmode\times\else\texttimes\fi{}{10}^{6}$ trapped atoms. The same optical trapping configuration is used for rapid adiabatic transport over a distance of 25cm25\phantom{\rule{0.16em}{0ex}}\mathrm{cm}25cm in 0.9s0.9\phantom{\rule{0.16em}{0ex}}\mathrm{s}0.9s, and subsequent evaporative cooling. With optimized evaporation we achieve a degenerate Fermi gas with $1.7\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms at a temperature of 60nK60\phantom{\rule{0.28em}{0ex}}\mathrm{nK}60nK, corresponding to T/TF=0.16(2)T/{T}_{\text{F}}=0.16\left(2\right)T/TF​=0.16(2). Furthermore, the performance is demonstrated by evaporation near a broad Feshbach resonance creating a molecular Bose-Einstein condensate of $3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ lithium dimers.
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.