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Vortex based atom traps and atomic electrometry on a superconducting atom chip

K. S. Chan

Published Jan 1, 2014• 1 citation
Publisher

Abstract

This thesis focuses on the realization of the magnetic micro-trap using the vortex current in a superconducting structure and the measurement of the electric fi eld from the adsorbates. Persistent currents are induced in the type-II superconductor - yttrium barium copper oxide (YBCO) to generate a magnetic potential for the ultracold atoms. Various programmable magnetic potentials are accomplished through the manipulation of the multiple magnetic eld pulses sequence used in imprinting the vortices. A special example is the achievement of self-sustaining trap which its trapping potential is formed without any requirement for external magnetic field. A demonstration of a single trap to a four traps is also shown, indicating a possible interferometry type of experiment using\nsuperconductors. Afterwards, a focus is shifted from the magnetic fi eld towards the electric fi eld of the superconducting atom chip. By using the Rydberg state of the rubidium\natom, we perform Rydberg spectroscopy to measure the electric field above the atom chip at room temperature and cryogenic temperature. In addition, to study the temperature\ndependent adsorption, temperature is varied from room to cryogenic temperature. Model based on Langmuir equation is simulated to explain the experimental result. This thesis\nconcludes with a summary and the future outlook.

DOI
10.32657/10356/61882

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This thesis focuses on the realization of the magnetic micro-trap using the vortex current in a superconducting structure and the measurement of the electric fi eld from the adsorbates. Persistent currents are induced in the type-II superconductor - yttrium barium copper oxide (YBCO) to generate a magnetic potential for the ultracold atoms. Various programmable magnetic potentials are accomplished through the manipulation of the multiple magnetic eld pulses sequence used in imprinting the vortices. A special example is the achievement of self-sustaining trap which its trapping potential is formed without any requirement for external magnetic field. A demonstration of a single trap to a four traps is also shown, indicating a possible interferometry type of experiment using\nsuperconductors. Afterwards, a focus is shifted from the magnetic fi eld towards the electric fi eld of the superconducting atom chip. By using the Rydberg state of the rubidium\natom, we perform Rydberg spectroscopy to measure the electric field above the atom chip at room temperature and cryogenic temperature. In addition, to study the temperature\ndependent adsorption, temperature is varied from room to cryogenic temperature. Model based on Langmuir equation is simulated to explain the experimental result. This thesis\nconcludes with a summary and the future outlook.
We investigate superconducting chip structures for trapping and manipulating atoms. These structures are based on the average magnetic field of vortices induced in a type-II superconducting thin film. This magnetic field is the critical ingredient of the demonstrated vortex-based atom trap, which operate without transport current. We employ the hysteretic behavior of a superconducting thin film in the remanent state to generate different traps and flexible magnetic potentials for ultra-cold atoms. The experimental realization can be described by the Bean’s critical-state method to model the vortex field through mesoscopic super currents induced in the thin strip. Various vortex patterns can be obtained by programming different loading-field and transport current sequences. Furthermore we will discuss the expected enhanced lifetime of atoms trapped close to a superconducting surface in comparison with a metallic surface.
Atom chips are micro-fabricated devices which create versatile magnetic potentials for trapping, cooling and controlling ensembles of ultra-cold atoms. Advancing from the typical atom chips, currents from superconducting vortices are used to trap and manipulate the ultra-cold atoms in our experiments. The superconducting vortex traps can be configured by controlling strength and pulse time of external magnetic fields together with bias fields. In order to fully utilize reconfigurability, superconducting vortices are optically manipulated with laser light to generate desired vortex patterns with this tailored trapping potentials for ultra-cold atoms are created. We have carried out heat diffusion simulations of laser patterns and characterized laser power and pulse time to effectively configure the desired vortex potentials. We have also successfully manipulated vortex matter optically to create arbitrary vortex patterns and ring potentials for cold atoms to utilize in quantum simulations and on-chip matter wave interferometry applications. Moreover, cold atoms could be used as a sensitive probe to examine magnetic field distribution of the superconducting vortices and we have used this technique to experimentally verify the resulting magnetic potentials of superconducting vortices after optical manipulation. v First of all, I would like to express my heartfelt gratitude and appreciation to my teacher and mentor, Prof.
We investigate superconducting chip structures for trapping and manipulating atoms. These structures are based on the average magnetic field of vortices induced in a type-II superconducting thin film. This magnetic field is the critical ingredient of the demonstrated vortex-based atom trap, which operate without transport current. We employ the hysteretic behavior of a superconducting thin film in the remanent state to generate different traps and flexible magnetic potentials for ultra-cold atoms. The experimental realization can be described by the Bean's critical-state method to model the vortex field through mesoscopic super currents induced in the thin strip. Various vortex patterns can be obtained by programming different loading-field and transport current sequences. Furthermore we will discuss the expected enhanced lifetime of atoms trapped close to a superconducting surface in comparison with a metallic surface.
We investigate the behavior of electric fields originating from adsorbates deposited on a cryogenic atom chip as it is cooled from room temperature to cryogenic temperature. Using Rydberg electromagnetically induced transparency, we measure the field strength versus distance from a 1 mm square of yttrium barium copper oxide (YBCO) patterned onto a yttria stabilized zirconia chip substrate. We find a localized and stable dipole field at room temperature and attribute it to a saturated layer of chemically adsorbed rubidium atoms on the YBCO. As the chip is cooled towards 83 K we observe a change in sign of the electric field as well as a transition from a localized to a delocalized dipole density. We relate these changes to the onset of physisorption on the chip surface when the van der Waals attraction overcomes the thermal desorption mechanisms. Our findings suggest that through careful selection of substrate materials, it may be possible to reduce the electric fields caused by atomic adsorption on chips, opening up experiments to controlled Rydberg-surface coupling schemes.
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.
We store and control ultra-cold atoms in a new type of trap using the magnetic fields of vortices in a high-temperature superconducting micro-structure. We generate the attractive trapping potential for the atoms by combining the magnetic field of a superconductor in the remanent state with external homogeneous magnetic fields. We show the control of crucial atom trap characteristics such as an efficient intrinsic loading mechanism, spatial positioning of the trapped atoms and the vortex density in the superconductor. The measured trap characteristics are in good agreement with our numerical simulations.