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Physical Review A

Microscopic magnetic traps for neutral atoms

Jonathan D. Weinstein, Kenneth G. Libbrecht

Published Nov 1, 1995• 164 citations
Publisher

Abstract

We describe planar and mostly planar current geometries for constructing microscopic magnetic traps for neutral atoms. These geometries are well-suited for fabrication from superconductors using standard microfabrication techniques. Magnetic-field gradients greater than 5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ G/cm, and field curvatures greater than ${10}^{8}$ G/${\mathrm{cm}}^{2}$ can be produced in microscopic traps. Trap loading could be accomplished by constructing a nested series of traps, and compressing atom clouds from larger traps into smaller ones. In the smallest magnetic microtraps the motional ground-state energy of atoms in the trap can exceed the recoil energy from resonant photons, which may allow direct laser cooling to the trap ground state. If a number of atoms can be simultaneously cooled to the ground state, the resulting ``Bose clusters'' should exhibit interesting optical behavior, since their spatial extent would be less than the resonant light wavelength.

DOI
10.1103/physreva.52.4004

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Journal of the Optical Society of America B•Dec 2, 2002•10 citations
We describe planar and mostly planar current geometries for constructing microscopic magnetic traps for neutral atoms. These geometries are well-suited for fabrication from superconductors using standard microfabrication techniques. Magnetic-field gradients greater than 5\ifmmode\times\else\texttimes\fi{}105{10}^{5}105 G/cm, and field curvatures greater than 108{10}^{8}108 G/cm2{\mathrm{cm}}^{2}cm2 can be produced in microscopic traps. Trap loading could be accomplished by constructing a nested series of traps, and compressing atom clouds from larger traps into smaller ones. In the smallest magnetic microtraps the motional ground-state energy of atoms in the trap can exceed the recoil energy from resonant photons, which may allow direct laser cooling to the trap ground state. If a number of atoms can be simultaneously cooled to the ground state, the resulting ``Bose clusters'' should exhibit interesting optical behavior, since their spatial extent would be less than the resonant light wavelength.
A single microscopic magnetic trap for neutral atoms using planar current-carrying wires was proposed and studied theoretically by Weinstein et al.In this paper,we propose three structures of composite current-carrying wires to provide 1D,2D and 3D arrays of microscopic magnetic traps for cold alkali atoms.The spatial distributions of magnetic fields generated by these structures are calculated and the field gradient and curvature in each single microtrap are analysed.Our study shows that arrays of microscopic magnetic traps can be used to provide 1D,2D or 3D atomic magnetic lattices,and even to realize 1D,2D and 3D arrays of magneto-optical traps,and so on.
A single microscopic magnetic trap for neutral atoms using planar current-carrying wires was proposed and studied theoretically by Weinstein et al . In this paper, we propose three structures of composite current-carrying wires to provide 1D, 2D and 3D arrays of microscopic magnetic traps for cold alkali atoms. The spatial distributions of magnetic fields generated by these structures are calculated and the field gradient and curvature in each single microtrap are analysed. Our study shows that arrays of microscopic magnetic traps can be used to provide 1D, 2D or 3D atomic magnetic lattices and even to realize 1D, 2D and 3D arrays of magneto-optical traps and so on.
A microscopic magnetic quadrupole trap for neutral atoms has been realized with a combination of permanent magnets, coils, and ferromagnetic pole pieces. The attainable magnetic field gradients of $3\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0ex}{0ex}}\mathrm{G}/\mathrm{cm}$ infer a spatial extension of the ground state much smaller than the wavelength of optical transitions. The field gradient can be varied over a wide range which allows for efficient loading of $4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ lithium atoms from a shallow potential by adiabatic transport and compression. During compression the observed 275-fold density increase implies a collision rate increase by a factor of 1700.
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.
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.
We propose two double-well magnetic traps for cold neutral atoms that use current-carrying wires. These traps can be used to trap two different atomic species. The spatial distributions of the magnetic fields, gradients, and curvatures from these two wire configurations were calculated and analyzed. Our study shows that the double-well traps proposed here can be continuously changed into single-well traps by reduction of the current in a wire (or a coil) and vice versa and that a maximum field gradient greater than 5×104 G/cm and a maximum field curvature (at each trap center) greater than 2.5×107 G/cm2 can be generated in our double-well traps and used to achieve two-species Bose–Einstein condensations and study the properties of double-well Bose–Einstein condensations or to achieve sympathetic cooling between two atomic samples or even to achieve two-species magneto-optical traps and study cold collisions between two atomic samples.