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Chinese Optics Letters

在光学偶极力阱中87Rb原子玻色-爱因斯坦凝聚的实现

Dezhi Xiong, Pengjun Wang, Zhengkun Fu, Shijie Chai, Jing Zhang

Published Jan 1, 2010• 19 citations
Publisher

Abstract

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.

DOI
10.3788/col20100807.0627

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Acta Physica Sinica•Jan 1, 2017•4 citations
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 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.
The apparatus and procedure required to generate a pure Bose-Einstein condensate (BEC) consisting of about half a million 87Rb atoms at a temperature of <60 nK with a phase density of >54 is described. The atoms are first laser cooled in a vapour cell magneto-optical trap (MOT) and subsequently transferred to an ultra-low pressure MOT. The atoms are loaded into a QUIC trap consisting of a pair of quadrupole coils and a Ioffe coil that generates a small finite magnetic field at the trap energy minimum to suppress Majorana transitions. Evaporation induced by an RF field lowers the temperature permitting the transition to BEC to be observed by monitoring the free expansion of the atoms after the trapping fields have been switched off.PACS Nos.: 03.75.Fi, 05.30.Jp, 32.80.Pj, 64.60.–i
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 have created a Bose-Einstein condensate (BEC) of 87Rb{}^{87}\mathrm{Rb}87Rb atoms directly in an optical trap. We employ a quasielectrostatic dipole force trap formed by two crossed CO2{\mathrm{CO}}_{2}CO2​ laser beams. Loading directly from a sub-Doppler laser-cooled cloud of atoms results in initial phase space densities of $\ensuremath{\sim}1/200$. Evaporatively cooling through the BEC transition is achieved by lowering the power in the trapping beams over $\ensuremath{\sim}2\mathrm{s}$. The resulting condensates are F=1F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1F=1 spinors with $3.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms distributed between the ${m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\ensuremath{-}1,0,1)$ states.
A rapid atomic beam of rubidium (87Rb) is produced by two-dimensional magneto-optical trap (2D MOT), and then trapped by three-dimensional magneto-optical trap (3D MOT) with high vacuum for further cooling. After a process of optical molasses cooling, atoms are reloaded into a magnetic trap, where radio frequency (RF) evaporation cooling is implemented. The precooled atoms in the magnetic trap are then transferred into a far detuning optical dipole trap, where Bose-Einstein condensate (BEC) appears by further evaporation cooling. The 3D MOT is loaded to its maximum within 25 s and then BEC is prepared in 16 s. Due to the linear intensity of magnetic trap, the frequency can be scanned fast in the RF evaporation cooling process. In our experiment, the frequency scans from 39 MHz to 15 MHz in 6 s and then scans to 2 MHz in 5 s. The number of atoms in 3D MOT is about 11010, and there are 5105 atoms in the BEC after a succession of cooling processes. To optimize the performances of 2D MOT, a special light path is constructed. And prisms with high reflectivity are used to reduce the imbalance between opposite propagating cooling +beams. Furthermore, quarter-wave plates are used to keep the polarization state of the cooling beam when reflected by prisms or mirrors. The atoms are cooled to a temperature about 15 K in the magnetic trap by RF evaporation. In such a low temperature, the loss of magnetic trap (Majorana loss) will prevent the atoms from reaching a high density, and the atoms cannot be cooled further. To reduce the loss rate of the magnetic trap, the far blue detuning light (532 nm, 18 W) is added to plug the zero point of the magnetic trap. In the optically plugged magnetic trap, atoms with high density are cooled down enough, which gives a good start for the loading of optical dipole trap.