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Physical Review Letters

Miniaturized Wire Trap for Neutral Atoms

József Fortágh, Alexander Grossmann, Claus Zimmermann, Theodor W. Hänsch

Published Dec 14, 1998• 120 citations
Publisher

Abstract

We demonstrate the loading of a miniaturized magnetic guide for neutral atoms made of small-size current conductors. Cold rubidium atoms are initially stored in a shallow spherical quadrupole field and subsequently transferred into the magnetic guide by gradually transforming the trapping potential. Density, temperature, and atom number have been monitored in situ by absorption imaging with a charge-coupled device camera. A fraction of $14%$ of the initially trapped atoms has been transferred into the linear microtrap at a temperature of $39\ensuremath{\mu}\mathrm{K}$.

DOI
10.1103/physrevlett.81.5310

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•May 6, 2014•0 citations
We demonstrate the loading of a miniaturized magnetic guide for neutral atoms made of small-size current conductors. Cold rubidium atoms are initially stored in a shallow spherical quadrupole field and subsequently transferred into the magnetic guide by gradually transforming the trapping potential. Density, temperature, and atom number have been monitored in situ by absorption imaging with a charge-coupled device camera. A fraction of 1414%14 of the initially trapped atoms has been transferred into the linear microtrap at a temperature of 39\ensuremathμK39\ensuremath{\mu}\mathrm{K}39\ensuremathμK.
We demonstrate efficient loading of neutral atoms into a magnetic waveguide produced by the magnetic fields of microfabricated current-carrying conductors. The lithographically patterned conductors on this ``atom chip'' can be used to make a variety of guiding and trapping structures for manipulating cold atoms and Bose-Einstein condensates. A three-chamber vacuum apparatus collects atoms in a magneto-optical trap, precools them via evaporative cooling, and delivers them to the final chamber containing the atom chip. We describe in detail how the precooled atomic cloud is transferred from a macroscopic magnetic Ioffe-Pritchard trap to the microscopic magnetic waveguide on the atom chip 21cm21\phantom{\rule{0.3em}{0ex}}\mathrm{cm}21cm away. Permanent magnets provide a confining two-dimensional quadrupole field to guide the atoms between the two chambers while longitudinally the cloud is allowed to freely expand during the transfer. Strategically placed coils are used to control the longitudinal size and speed of the atomic cloud as it is loaded on the atom chip.
We demonstrate the direct loading of cold atoms into a microchip 2-mm Z-trap, where the evaporative cooling can be performed efficiently, from a macroscopic quadrupole magnetic trap with a high loading efficiency. The macroscopic quadrupole magnetic trap potential is designed to be moveable by controlling the currents of the two pairs of anti-Helmholtz coils. The cold atoms are initially prepared in a standard six-beam magneto-optical trap and loaded into the macroscopic quadrupole magnetic trap, and then transported to the atom chip surface by moving the macroscopic trap potential. By means of a three-dimensional absorption imaging system, we are able to optimize the position alignment of the atom cloud in the macroscopic trap and the microchip Z-shaped wire. Consequently, with a proper magnetic transfer scheme, we load the cold atoms into the microchip Z-trap directly and efficiently. The loading efficiency is measured to be about 50%. This approach can be used to generate appropriate ultracold atoms sources, for example, for a magnetically guided atom interferometer based on atom chip.
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.
Abstract We propose two novel schemes for guiding cold, neutral atoms using static magnetic fields generated by several cylindrical arrays of current‐carrying wires or a few different‐shaped (V‐, or U‐shaped, etc.) current‐carrying conductors, and calculate the corresponding magnetic‐field contours and their trapping potentials. Our study shows that some guiding schemes can be used to form an atomic funnel, or an atomic beam‐splitter, or even construct an atomic interferometer, and some guiding schemes can be used to realize a single‐mode wave guide of ultracold atoms (or coherent matter waves, BEC) under certain conditions. We also propose a few new schemes to microscopically trap neutral atoms by using various planar arrays of current‐carrying wires and calculate the spatial distribution of the magnetic fields. Our research shows that these microtrap arrays can be used to form 1D, 2D or 3D atomic magnetic lattices, and then to prepare 1D, 2D or 3D photonic crystals, even to realize a 1D, 2D or 3D array of magnetically‐trapped BECs or Bose Clusters.
A microtrap consisting of two concentric circular wire loops having radii of 300 and 660 μm, respectively, is demonstrated. The three-dimensional trap has a maximum depth of more than 1 mK, and the trap center position as measured below the atom chip surface can be adjusted by applying a small-bias magnetic field. More than 105 Rb87 atoms were transferred into the microtrap from a magneto-optical trap and remained trapped for several hundred milliseconds, which is limited by the background pressure. The loading of a linear array of three microtraps is also demonstrated. The trap dimensions are readily scaled to micrometer size, which is of interest for creating a one- and two-dimensional array of neutral atom traps on a single atom chip.
A novel kind of magnetic microtrap is demonstrated for ultracold neutral atoms. It consists of two concentric loops of radii r1r_1r1​ and r2r_2r2​ having oppositely oriented currents. A magnetic field minimum is generated in three dimensions that can be used to trap the atoms with a trap depth of about 1 mK using a current of 2.6 A. The condition r2/r1r_2/r_1r2​/r1​ = 2.2 maximizes the restoring force on the atoms toward the trap center. Unlike conventional magnetic microtraps, an external bias field is not required. Moreover, a one dimensional array of double-loop microtraps can be created by daisy chaining single microtrap circuits. \nA linear array of three microtraps having r1r_1r1​ = 300 mu\\mumum was fabricated as part of an atom chip. The following three techniques were developed to load the microtrap array: 1) atoms initially contained in a magneto-optical trap (MOT) were transported to the atom chip by a conventional magnetic trap, 2) atoms were first loaded into a mirror MOT and 3) atoms were initially loaded in a far off resonance optical dipole trap (FORT). Each technique loaded greater than 10510^5105 87^{87}87Rb atoms into the microtrap array. The lowest temperature of 30 mu\\mumuK for the microtrapped atom cloud was achieved using FORT loading. The strength and the position of the microtrap could be precisely adjusted over a range from 300 to 50 mu\\mumum above the atom chip surface by applying an external bias magnetic field. The lifetime of the atoms trapped in the microtrap array was measured to be 350 ms which was limited by the collisions with residual background gas.