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Physical Review A

Three-wire magnetic trap for direct forced evaporative cooling

Shengwang Du, Eun Oh

Published Jan 8, 2009• 6 citations
Publisher

Abstract

We propose a simple three-wire-based magnetic trap potential for direct forced evaporative cooling of neutral atoms without using induced spin-flip technologies. We have devised a method for controlling the trap depth without sacrificing its frequencies by only varying wire currents and external magnetic fields. By having multiples of these wires on different levels integrated into an atom chip, it is possible to attain Bose-Einstein condensation without the conventional forced evaporation technique.

DOI
10.1103/physreva.79.013407

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•Jan 1, 2015•4 citations
We propose a simple three-wire-based magnetic trap potential for direct forced evaporative cooling of neutral atoms without using induced spin-flip technologies. We have devised a method for controlling the trap depth without sacrificing its frequencies by only varying wire currents and external magnetic fields. By having multiples of these wires on different levels integrated into an atom chip, it is possible to attain Bose-Einstein condensation without the conventional forced evaporation technique.
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We demonstrate a new magneto-optical trap (MOT) configuration using a simple pair of crossed wires rotated at 45 deg and an appropriate bias field to generate a MOT of >10^8 atoms. The same pair of wires, with slightly adjusted control parameters, is then used to magnetically trap the atoms and cool them via forced evaporative cooling into a Bose-Einstein condensate (BEC) with >10^4 atoms. We present the theoretical framework for generating a quadrupole field using a pair of crossed wires with arbitrary rotation angle, along with the atom chip design and fabrication. Finally, we describe the experimental protocols required for BEC production using only a single crossed-wire atom chip.
We demonstrate a simple scheme to achieve fast, accelerating (runaway) evaporative cooling of optically trapped atoms by tilting the optical potential with a magnetic field gradient. Runaway evaporation is possible in this trap geometry due to the weak dependence of vibration frequencies on trap depth, which preserves atomic density during the evaporation process. Using this scheme, we show that Bose-Einstein condensation with $\ensuremath{\sim}{10}^{5}$ cesium atoms can be realized in 2−−4s2--4\phantom{\rule{0.3em}{0ex}}\mathrm{s}2−−4s of forced evaporation. The evaporation speed and energetics are consistent with the three-dimensional evaporation picture, despite the fact that atoms can only leave the trap in the direction of tilt.
We have evaporatively cooled caesium atoms in a magnetic trap to temperatures as low as 8 nK and produced a final phase space density within a factor of four of that required for the onset of Bose–Einstein condensation. At the end of the forced radio-frequency evaporation, 1500 atoms in the F = 3, m F = −3 state remain in the magnetic trap. We observe a decrease in the one-dimensional evaporative cooling efficiency at very low temperatures as the trapped sample enters the collisionally thick (hydrodynamic) regime. To alleviate this problem we propose a modified trapping scheme where three-dimensional evaporation is possible. In addition, we report measurements of the two-body inelastic collision rates for caesium atoms as a function of magnetic field. We confirm the positions, with reduced uncertainties, of three previously identified resonances at magnetic fields of 108.87(6), 118.46(3) and 133.52(3) G.
We demonstrate a simple scheme to reach Bose–Einstein condensation (BEC) of metastable triplet helium atoms using a single-beam optical dipole trap with moderate power of less than 3 W. Our scheme is based on RF-induced evaporative cooling in a quadrupole magnetic trap and transfer to a single-beam optical dipole trap that is located below the magnetic trap center. We transfer 1×1061\times 10^61×106 atoms into the optical dipole trap, with an initial temperature of $$14\,\upmu\hbox{K}$$ , and observe efficient forced evaporative cooling both in a hybrid trap, in which the quadrupole magnetic trap operates just below the levitation gradient, and in the pure optical dipole trap, reaching the onset of BEC with 2×1052\times 10^52×105 atoms and a pure BEC of 5×1045\times 10^45×104 atoms. Our work shows that a single-beam hybrid trap can be applied for a light atom, for which evaporative cooling in the quadrupole magnetic trap is strongly limited by Majorana spin-flips, and the very small levitation gradient limits the axial confinement in the hybrid trap.