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Bose-Einstein condensation in a robust microtrap -- the combination of wire traps and atom chips

A. Kasper

Published Jan 1, 2004• 1 citation
Publisher

Abstract

In the presented work, we report about the successful creation of a Rubidium Bose-Einstein condensate. We use so called magnetic wire traps, which are especially simple, as they consist out of a wire and a homogeneous bias field. These wire traps are especially suited for Bose-Einstein condensation. Furthermore complex trapping potentials to manipulate a Bose-Einstein condensate can be realized. We combine 'large' and small scale wire traps. The 'large' scale is realized with a massive copper structure, while for the small wire traps we use the so called atom chip. This combination is promising, because it allowed us to develop a condensation process in the copper structure, which is independent of the structures on the atom chip, and thus the exchange of the 'physics' area. First experiments with the Bose-Einstein condensate are presented and discussed in detail.

DOI
10.11588/heidok.00004276

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In the presented work, we report about the successful creation of a Rubidium Bose-Einstein condensate. We use so called magnetic wire traps, which are especially simple, as they consist out of a wire and a homogeneous bias field. These wire traps are especially suited for Bose-Einstein condensation. Furthermore complex trapping potentials to manipulate a Bose-Einstein condensate can be realized. We combine 'large' and small scale wire traps. The 'large' scale is realized with a massive copper structure, while for the small wire traps we use the so called atom chip. This combination is promising, because it allowed us to develop a condensation process in the copper structure, which is independent of the structures on the atom chip, and thus the exchange of the 'physics' area. First experiments with the Bose-Einstein condensate are presented and discussed in detail.
A Bose-Einstein condensate is created in a simple and robust miniature Ioffe-Pritchard trap, the so-called Z trap. This trap results from the mere combination of a Z-shaped current-carrying wire and a homogeneous bias field. The experimental procedure allows condensation of typically $3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 87Rb{}^{87}\mathrm{Rb}87Rb atoms in the ∣F=2,|F=2,∣F=2, mF=2〉{m}_{F}=2〉mF​=2〉 state close to any mirroring surface, irrespective of the surface structure. Thus it is ideally suited as a simple coherent source for miniature surface traps or for cold atom physics near surfaces.
We describe a novel method of fabricating atom chips that are well suited to the production and manipulation of atomic Bose–Einstein condensates. Our chip was created using a silver foil and simple micro-cutting techniques without the need for photolithography. It can sustain larger currents than conventional chips, and is compatible with the patterning of complex trapping potentials. A near pure Bose–Einstein condensate of 4 × 10 4 87 Rb atoms has been created in a magnetic microtrap formed by currents through wires on the chip. We have observed the fragmentation of atom clouds in close proximity to the silver conductors. The fragmentation has different characteristic features to those seen with copper conductors.
Atom chips are devices used to magnetically trap and manipulate ultracold atoms and Bose-Einstein condensates near a surface. In particular, permanent magnetic film atom chips can allow very tight confinement and intricate magnetic field designs while circumventing technical current noise. Research described in this thesis is focused on the development of a magnetic film atom chip, the production of Bose-Einstein condensates near the film surface, the characterisation of the associated magnetic potentials using rf spectroscopy of ultracold atoms and the realisation of a precision sensor based on splitting Bose-Einstein condensates in a double-well potential. The atom chip itself combines the edge of a perpendicularly magnetised GdTbFeCo film with a machined silver wire structure. A mirror magneto-optical trap collects up to 5 x 108 87Rb atoms beneath the chip surface. The current-carrying wires are then used to transfer the cloud of atoms to the magnetic film microtrap and radio frequency evaporative cooling is applied to produce Bose-Einstein condensates consisting of 1 x 105 atoms. We have identified small spatial magnetic field variations near the film surface that fragment the ultracold atom cloud. These variations originate from inhomogeneity in the film magnetisation and are characterised using a novel technique based on spatially resolved radio frequency spectroscopy of the atoms to map the magnetic field landscape over a large area. The observations agree with an analytic model for the spatial decay of random magnetic fields from the film surface. Bose-Einstein condensates in our unique potential landscape have been used as a precision sensor for potential gradients. We transfer the atoms to the central region of the chip which produces a double-well potential. A single BEC is formed far from the surface and is then dynamically split in two by moving the trap closer to the surface. After splitting, the population of atoms in each well is extremely sensitive to the asymmetry of the potential and can be used to sense tiny magnetic field gradients or changes in gravity on a small spatial scale.
We describe the production of BECs on a new type of atom chip based on silver foil. Our atom chip is fabricated with thick wires capable of carrying currents of several amperes without overheating. The silver surface is highly reflective to light resonant with optical transitions used for Rb. The pattern on the chip consists of two parallel Z-trap wires, capable of producing two-wire guide, and two additional endcap wires for varying the axial confinement. Condensates are produced in magnetic microtraps formed within 1 mm of surface of the chip. We have observed the fragmentation of cold atom clouds when brought close to the chip surface. This results from a perturbed trapping potential caused by nanometer deviations of the current path through the wires on the chip. We present results of fragmentation of cold clouds at distances below 100 μm from the wires and investigate the origin of the deviating current. The fragmentation has different characteristics to those seen with copper conductors. The dynamics of atoms in these microtraps is also investigated.