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Chinese Physics Letters

The Mode Matching of Hybrid Trap by Frequency Calibration

Zhong-Kai Wang, Dong Hu, Linxiao Niu, Jiahua Zhang, Xuzong Chen, Xiaoji Zhou

Published May 1, 2015• 0 citations
Publisher

Abstract

We demonstrate a method to precisely calibrate the relative position between an optical dipole trap and a magnetic quadrupole potential in making a Bose-Einstein condensate via the hybrid trap. We measure the displacement by the dipole oscillation in the gravity direction, where the oscillation frequency can be adjusted through an additional bias magnetic field. One frequency corresponds to the case of the mode matching while two frequencies are not, and the great consistency between theoretical simulation and experimental results indicates its usefulness.

DOI
10.1088/0256-307x/32/5/054208

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We describe an experiment to produce 87{}^{87}87Rb Bose-Einstein condensates in an optically plugged magnetic quadrupole trap, using a blue-detuned laser. Due to the large detuning of the plug laser with respect to the atomic transition, the evaporation has to be carefully optimized in order to efficiently overcome the Majorana losses. We provide a complete theoretical and experimental study of the trapping potential at low temperatures and show that this simple model describes our data well. In particular, we demonstrate methods to reliably measure the trap oscillation frequencies and the bottom frequency, based on periodic excitation of the trapping potential and on radio-frequency spectroscopy, respectively. We show that this hybrid trap can be operated in a well-controlled regime that allows a reliable production of degenerate gases.
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We describe an apparatus for quickly and simply producing 87Rb^{87}\text{R}\text{b}87Rb Bose-Einstein condensates. It is based on a magnetic quadrupole trap and a red-detuned optical dipole trap. We collect atoms in a magneto-optical trap (MOT) and then capture the atoms in a magnetic quadrupole trap and force rf evaporation. We then transfer the resulting cold dense cloud into a spatially mode-matched optical dipole trap by lowering the quadrupole field gradient to below gravity. This technique combines the efficient capture of atoms from a MOT into a magnetic trap with the rapid evaporation of optical dipole traps; the approach is insensitive to the peak quadrupole gradient and the precise trapping beam waist. Our system reliably produces a condensate with $N\ensuremath{\approx}2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms every 16 s.
We have realized Bose-Einstein condensation (BEC) of 87Rb in the F = 2, m F = 2 hyperfine substate in a hybrid trap, consisting of a quadrupole magnetic field and a single optical dipole beam. The symmetry axis of the quadrupole magnetic trap coincides with the optical beam axis, which gives stronger axial confinement than previous hybrid traps. After loading 2 × 106 atoms at 14 μK from a quadrupole magnetic trap into the hybrid trap, we perform efficient forced evaporation and reach the onset of BEC at a temperature of 0.5 μK and with 4 × 105 atoms. We also obtain thermal clouds of 1 × 106 atoms below 1 μK in a pure single beam optical dipole trap, by ramping down the magnetic field gradient after evaporative cooling in the hybrid trap.