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Bose-Einstein condensates on a magnetic film atom chip

S. Whitlock

Published Jan 1, 2007• 5 citations
Publisher

Abstract

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.

DOI
10.25916/sut.26276140

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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 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.
In this thesis I discuss work that has been carried out on the dynamics of a Bose Einstein condensate of Rb 87 produced near an atom chip. A Bose Einstein Condensate (BEC) is a quantum state of matter where a single quantum state becomes occupied by a macroscopic number of identical Bosons. In our case this is achieved by cooling a system of trapped identical rubidium 87 atoms to its ground state. To reach temperatures of condensation we initially laser cool atoms from room temperature, before loading them into a magnetic trap. The magnetic trap is produced through a combination of uniform magnetic fields from coils outside our vacuum chamber and currents running through wires on an atom chip. The atom chip is a microfabricated device, produced by a coating a silicon chip with a thin layer of gold and etching wires into it. Together, these fields create a magnetic field minimum 120μm from the surface of the chip which can be used to confine low field seeking hyperfine states of the atom in an elongated harmonic trap. Once the atoms are confined in the magnetic trap we used force evaporative cooling out to reach the phase space densities required for Bose Einstein condensation. The BEC is used to investigate the relative dynamics between the fraction of the atoms in the condensate to those not in the condensate. Our atom chip provided a suitable environment to investigate this due to fragmentation of the magnetic potential close to the chip. Small imperfections in the wires on our atom chip mean that the trapping potential isn’t smooth. Small regions of higher trapping frequency - or fragments - are formed. Due to the small size of these fragments it is possible to find a position where a condensate can form in the fragment, and see a potential of high frequency, whereas a non-condensed atom will see a lower frequency potential. We exploit this to set the condensed fraction moving relative to the non condensed part and investigate the subsequent damping of their motion relative to each other.
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 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.
Atom chips provide a convenient platform to study ultracold quantum gases and their interactions with electromagnetic fields, surfaces, micro-magnetic structures, superconducting circuits and nanomechanical systems. The research work presented in this thesis is classified in two major categories. The first part is focused on the development and construction of a permanent magnetic lattice on an atom chip and the study of trapped atoms in the permanent magnetic lattice potential. In the second part, a new experiment is proposed to create a macroscopic entanglement between the flux state of a superconducting loop and a Bose-Einstein condensate (BEC) on a superconducting atom chip. The permanent magnetic lattice potential is produced by applying a uniform bias magnetic field to a grooved silicon structure coated with a permanent magnetic film. The magnetic film consists of a multilayer structure of GdTbFeCo with perpendicular intrinsic magnetization. The permanent magnetic structure is mounted beneath a current carrying wire structure glued to a ceramic insulator. The current carrying wires assist in the preparation of the ultracold atoms and for loading into the magnetic lattice. In addition, the current carrying wires are used to produce a BEC in a Z- wire magnetic trap. The magnetic lattice potential is produced by a one-dimensional array of highly anisotropic and extremely tight magnetic microtraps of period 10 ¹m. A detailed study of the reflection dynamics of atoms in the magnetic lattice potential is presented along with numerical simulations. Finally, the trapping of atoms in the permanent magnetic lattice at a distance less than 5 ¹m from the surface of the atom chip is described. The properties of the trapped atoms including lifetime, temperature and trap frequency are discussed. A maximum radial trap frequency of about 90 kHz is measured in the magnetic lattice potential with no confinement applied along the axial direction. A new experiment which combines the quantum coherent dynamics in superconducting circuits and BEC manipulation on a superconducting atom chip is proposed and investigated. It is shown how a macroscopic entanglement can be produced between the flux state of a superconducting loop and the state of a BEC trapped in a magnetic trap. In addition, the detection of the macroscopic entanglement based on a quantum eraser and an entangled BEC interference is discussed. Thus, this proposal provides a platform to realise interferometry with an entangled BEC and to explore the quantum-classical interface.
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.