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arXiv:physics/0403020 · Physical Review A

Role of wire imperfections in micromagnetic traps for atoms

Jérôme Estève, Christine Aussibal, Thorsten Schumm, C. Figl, D. Mailly, Isabelle Bouchoule, Christoph Ira Westbrook, Alain Aspect

Published Oct 29, 2004• 124 citations
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Abstract

We present a quantitative study of roughness in the magnitude of the magnetic field produced by a current carrying microwire, i.e., in the trapping potential for paramagnetic atoms. We show that this potential roughness arises from deviations in the wire current flow due to geometric fluctuations of the edges of the wire: a measurement of the potential using cold trapped atoms agrees with the potential computed from the measurement of the wire edge roughness by a scanning electron microscope.

DOI
10.1103/physreva.70.043629

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We present a quantitative study of roughness in the magnitude of the magnetic field produced by a current carrying microwire, i.e., in the trapping potential for paramagnetic atoms. We show that this potential roughness arises from deviations in the wire current flow due to geometric fluctuations of the edges of the wire: a measurement of the potential using cold trapped atoms agrees with the potential computed from the measurement of the wire edge roughness by a scanning electron microscope.
We analyze the effects of roughness in the magnitude of the magnetic field produced by a current carrying microwire, which is caused by the geometric fluctuation of the edge of the wire. The relation between the fluctuation of the trapping potential and the height the atom trap lies above the wire is very well consistent with the experimental data when the colored noise with a finite correlation length is considered. On this basis, we generate the random potential and get the density distribution of the Bose-Einstein condensate (BEC) atoms by solving the Gross-Pitaevskii equation, which coincides well with the experimental image, especially in the number of fragmentations. The results help us further understand the nature of the fluctuation and predict its possible application in the precise measurement.
Atom chips use current flowing in lithographically patterned wires to produce microscopic magnetic traps for atoms. The density distribution of a trapped cold atom cloud reveals disorder in the trapping potential, which results from meandering current flow in the wire. Roughness in the edges of the wire is usually the main cause of this behaviour. Here, we point out that the edges of microfabricated wires normally exhibit self-affine roughness. We investigate the consequences of this for disorder in atom traps. In particular, we consider how closely the trap can approach the wire when there is a maximum allowable strength of the disorder. We comment on the role of roughness in future atom--surface interaction experiments.
Atom chips use current flowing in lithographically patterned wires to produce microscopic magnetic traps for atoms. The density distribution of a trapped cold atom cloud reveals disorder in the trapping potential, which results from meandering current flow in the wire. Roughness in the edges of the wire is usually the main cause of this behaviour. Here, we point out that the edges of microfabricated wires normally exhibit self-affine roughness. We investigate the consequences of this for disorder in atom traps. In particular, we consider how closely the trap can approach the wire when there is a maximum allowable strength of the disorder. We comment on the role of roughness in future atom–surface interaction experiments.
Microscopic atom optical devices integrated on atom chips allow to precisely control and manipulate ultra-cold ( T < 1 µK) neutral atoms and Bose-Einstein condensates (BECs) close to surfaces. The relevant energy scale of a BEC is extremely small (down to < 10 −11 eV). Consequently, BECs can be utilized as a sensor for variations of the potential energy of the atoms close to the surface. Here we describe how to use trapped atoms as a measurement device and analyze the performance and flexibility of the field sensor. We demonstrate microscopic magnetic imaging with simultaneous high spatial resolution (3 µm) and high field sensitivity (4 nT). With one dimensional BECs, we probe the magnetic field variations close to the surface at distances down to a few microns. Measurements of the magnetic field of a 100 µm wide current carrying wire imply that the magnetic field variations stem from residual variations of the current flow direction, resulting from local properties of the wire. These disorder potentials found near lithographically fabricated wires are two orders of magnitude smaller than those measured close to electroplated conductors.
Surface based geometries of microfabricated wires or patterned magnetic films can be used to magnetically trap and manipulate ultracold neutral atoms or Bose-Einstein condensates. We investigate the magnetic properties of such atom chips using a scanning magnetoresistive (MR) microscope with high spatial resolution and high field sensitivity. By comparing MR scans of a permanent magnetic atom chip to field profiles obtained using ultracold atoms, we show that MR sensors are ideally suited to observe small variations of the magnetic field caused by imperfections in the wires or magnetic materials which ultimately lead to fragmentation of ultracold atom clouds. Measurements are also provided for the magnetic field produced by a thin current-carrying wire with small geometric modulations along the edge. Comparisons of our measurements with a full numeric calculation of the current flow in the wire and the subsequent magnetic field show excellent agreement. Our results highlight the use of scanning MR microscopy as a convenient and powerful technique for precisely characterizing the magnetic fields produced near the surface of atom chips.
Potential roughness has been reported to severely impair experiments in magnetic microtraps. We show that these obstacles can be overcome as we measure disorder potentials that are reduced by two orders of magnitude near lithographically patterned high-quality gold layers on semiconductor atom chip substrates. The spectrum of the remaining field variations exhibits a favorable scaling. A detailed analysis of the magnetic field roughness of a $100\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-wide wire shows that these potentials stem from minute variations of the current flow caused by local properties of the wire rather than merely from rough edges. A technique for further reduction of potential roughness by several orders of magnitude based on time-orbiting magnetic fields is outlined.