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Laser Spectroscopy

BOSE-EINSTEIN CONDENSATES ON MAGNETIC FILM MICROSTRUCTURES

Mandip Singh, S. Whitlock, Robert W. Anderson, Saeed Ghanbari, B. V. Hall, Michael Volk, A. M. Akulshin, R. Mclean, A. SIDOROV, Peter Hannaford

Published Apr 1, 2008• 1 citation
Publisher

Abstract

We report on recent experiments with BECs and ultracold atoms in magnetic microtraps created by magnetic film microstructures on an atom chip. We describe the use of RF spectroscopy plus absorption imaging of ultracold atoms as a sensitive high resolution technique to map the magnetic field topology of the magnetic film. We find that the distribution of condensate atoms between the wells of an asymmetric double-well potential on the magnetic film can provide a highly sensitive technique for determining potential gradients, such as gravitational gradients, using an atom chip. Finally, we report on the use of periodic magnetic film microstructures to generate a magnetic lattice for manipulating BECs and ultracold atoms.

DOI
10.1142/9789812813206_0020

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We report on recent experiments with BECs and ultracold atoms in magnetic microtraps created by magnetic film microstructures on an atom chip. We describe the use of RF spectroscopy plus absorption imaging of ultracold atoms as a sensitive high resolution technique to map the magnetic field topology of the magnetic film. We find that the distribution of condensate atoms between the wells of an asymmetric double-well potential on the magnetic film can provide a highly sensitive technique for determining potential gradients, such as gravitational gradients, using an atom chip. Finally, we report on the use of periodic magnetic film microstructures to generate a magnetic lattice for manipulating BECs and ultracold atoms.
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 present a permanent magnetic film atom chip based on perpendicularly magnetized TbGdFeCo films. This chip routinely produces a Bose-Einstein condensate (BEC) of 10^5 87Rb atoms using the magnetic film potential. Fragmentation observed near the film surface provides unique opportunities to study BEC in a disordered potential. We show this potential can be used to simultaneously produce multiple spatially separated condensates. We exploit part of this potential to realize a time-dependent double well system for splitting a condensate.
We present a hybrid atom chip which combines a permanent magnetic film with a micromachined current-carrying structure used to realize a Bose–Einstein condensate (BEC). A novel TbGdFeCo material with large perpendicular magnetization has been tailored to allow small scale, stable magnetic potentials for ultracold atoms. We are able to produce 87 Rb BEC in a magnetic trap based on either the permanent magnetic film or the current-carrying structure. Using the condensate as a magnetic field probe we perform cold atom magnetometry to profile both the field magnitude and gradient as a function of distance from the magnetic film surface. Finally, we discuss future directions for our permanent magnetic film atom chip.
This thesis reports the realization of Bose-Einstein condensation (BEC) of 87Rb F=1 atoms in multiple sites of a one-dimensional 10 μm-periods magnetic lattice. The magnetic microtraps are created by a perpendicularly magnetized, multi-layered structure of TbGdFeCo film integrated on to an atom chip, together with uniform bias magnetic fields. Clear signatures for the onset of quantum degeneracy in multiple sites of the the magnetic lattice are provided by in-situ site-resolved radio-frequency spectroscopy
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.
We describe the fabrication and construction of a setup for creating lattices of magnetic microtraps for ultracold atoms on an atom chip. The lattice is defined by lithographic patterning of a permanent magnetic film. Patterned magnetic-film atom chips enable a large variety of trapping geometries over a wide range of length scales. We demonstrate an atom chip with a lattice constant of 10 μm, suitable for experiments in quantum information science employing the interaction between atoms in highly excited Rydberg energy levels. The active trapping region contains lattice regions with square and hexagonal symmetry, with the two regions joined at an interface. A structure of macroscopic wires, cutout of a silver foil, was mounted under the atom chip in order to load ultracold (87)Rb atoms into the microtraps. We demonstrate loading of atoms into the square and hexagonal lattice sections simultaneously and show resolved imaging of individual lattice sites. Magnetic-film lattices on atom chips provide a versatile platform for experiments with ultracold atoms, in particular for quantum information science and quantum simulation.