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arXiv:cond-mat/0311475 · Physical Review A

Optimized magneto-optical trap for experiments with ultracold atoms near surfaces

Stephan Wildermuth, P. Krüger, C. Becker, Mihael Brajdic, S. Haupt, A. Kasper, R. Folman, Jörg Schmiedmayer

Published Mar 17, 2004• 97 citations
arXivHTMLPDF

Abstract

We present an integrated wire-based magneto-optical trap for the simplified trapping and cooling of large numbers of neutral atoms near material surfaces. With a modified $\mathrm{U}$-shaped current-carrying $\text{Cu}$ structure we collect more than $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ $^{87}\mathrm{Rb}$ atoms in a mirror magneto-optical trap without using quadrupole coils. These atoms are subsequently loaded to a $\mathrm{Z}$-wire trap where they are evaporatively cooled to a Bose-Einstein condensate close to the surface.

DOI
10.1103/physreva.69.030901

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We present an integrated wire-based magneto-optical trap for the simplified trapping and cooling of large numbers of neutral atoms near material surfaces. With a modified U\mathrm{U}U-shaped current-carrying Cu\text{Cu}Cu structure we collect more than 3\ifmmode×\else\texttimes\fi1083\ifmmode\times\else\texttimes\fi{}{10}^{8}3\ifmmode×\else\texttimes\fi108 87Rb^{87}\mathrm{Rb}87Rb atoms in a mirror magneto-optical trap without using quadrupole coils. These atoms are subsequently loaded to a Z\mathrm{Z}Z-wire trap where they are evaporatively cooled to a Bose-Einstein condensate close to the surface.
A mirror magneto-optical trap (MOT) that exploits a hexapole-compensated magnetic field was developed and used in the experimental surface trapping of neutral atoms. A pair of subsidiary wires, which was placed near the main current-carrying wire, was designed to improve the uniformity of the quadrupole magnetic field and thus increased the effective capture volume of our mirror-MOT. In the experiment, the number of 87Rb^{87}\mathrm{Rb}87Rb atoms captured with our mirror-MOT was approximately twice that captured with a conventional mirror-MOT.
In this paper, we have developed a combined magnetic field system to explore an alternative technical route for the generation of Bose–Einstein condensates containing numerous atoms on an atom chip. The system is characterized by the fact that the quadrupole magnetic field required by the magneto-optic trap is generated by U-shaped current-carrying wires combined with bias magnetic fields, whereas the quadrupole magnetic field adopted for the magnetic trap is generated by anti-Helmholtz coils. By fine-tuning the bias magnetic fields, the collection of atoms in the mirror MOT and the loading of the quadrupole magnetic trap can be optimized. The initial number of [Formula: see text]Rb atoms in the magnetic trap is approximately [Formula: see text], and the lifetime of the atoms is over 30 s. This scheme can facilitate the chip-surface design and is suitable for the precise manipulation of Bose–Einstein condensates near the chip surface.
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 propose two novel double-well magnetic surface traps (i.e., a double-well U-U-shaped wire trap and a double-well U-Z-shaped wire trap) for cold atoms using a straight and U-shaped current-carrying wire or two U-shaped current-carrying wires. These double-well magnetic traps can be continuously changed into a single-well by reducing the current in a straight wire or an inner U-shaped wire from a suitable value to zero, and vice versa. The spatial distributions of the magnetic fields from two current-carrying wire layouts and their field gradients are calculated and analyzed, and the trapped atomic number and its temperature in each magnetic well are estimated. Our study shows that our double-well magnetic surface traps can be used to realize two-species magneto-optical traps (MOTs) above the surface of the wire plane. Under the approximation of low intensity, cold atomic samples with a captured number of ∼10^6 atoms and a temperature of approximately 270 µK can be obtained in each ^85Rb atomic MOT. Since each MOT in two-species MOTs can be prepared independently, our controllable double-well trap can be used to study the cold collisions between two different atomic samples.
The role of U-wire generated quadrupole-like magnetic field profile on loading of cold 87Rb atoms in a magneto-optical trap (MOT) on atom-chip has been investigated. It is shown by simulations that the quadrupole field profile and the centre of the quadrupole field vary depending upon the U-wire current and bias field values. Moreover, the resemblance of generated field profile to the ideal quadrupole field critically depends on the values of current in U-wire and bias magnetic field. The observed number of trapped atoms in the MOT and its position has also been found to depend on the quadrupole field profile. The number of trapped atoms in the MOT reaches maximum when the field profile approaches close to an ideal quadrupole field. These studies are, in general, useful in optimizing the number in the MOT before transfer of these cold atoms to the microtraps of an atom-chip.
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