Superconducting Magnetic field Generator

Abstract

Color centers in diamond offer exciting opportunities for implementing an effective solid-state spinphoton interface. Access to and control of spin-photon interaction can help realize a scalable, modular quantum computer using the color center spins. A large magnetic field is needed for splitting the color center spin states, enabling definition of the qubits. In addition to the large, global magnetic field (∼1T), applying a small and tunable magnetic field (order of mT) in the vicinity of individual color centers is advantageous as it allows for external field inhomogeneity compensation and multiplexing of the qubit driving frequency. Local tuning of the magnetic field magnitude might be achieved by driving DC currents through microscopic coils located in close proximity of the respective color centers. Since the systems must be located at cryogenic temperatures, cooling power is limited. Superconducting coils are therefore considered for local tuning instead of conventional metal. Design requirements for the local tuning coils depend strongly on design choices of the external magnet, externally applied field magnitude, number of FDM channels and coil distance from the color center. In order to find a design process that easily accounts for changes in these choices, the utilization of FEM physics simulations is studied. Two simulation methods are considered and tested in COMSOL Multiphysics. The first method is called 𝐻-𝜙-formulation, which is considered for optimizing the coil geometry. This method is based on a classical description of superconductivity and is shown here to have potential for simulating large coil structures. The second method allows for 3D simulation of superconductors based on the time-dependent Ginzburg-Landau (TDGL) equations. This method is considered with the goal of predicting changes in critical current density due to differences in cross-section dimensions. It is shown that this method can qualitatively reproduce transport characteristics of microscopic wires when no magnetic field is applied. When a magnetic field is applied, the observed vortex dynamics and resistivity correspond to that of material with no impurities or defects. Adding impurities in the form of geometrical deformations is shown here to be possible. However, further development on the method is required in order to overcome computational limitations and to determine unknown factors such as the impurity density and pinning forces. Until such time, the design procedure recommended in this thesis is based on 𝐻-𝜙-formulation simulations in combination with measurements of straight transport lines for critical current density characterization. Fabrication of NbTiN films is successfully tested by patterning coil-shaped structures, followed by DC magnetron sputtering and liftoff. The coil structures will be used to measure deviations from the expected critical current density due to geometry and to validate the recommended design procedure. Straight lines NbTiN geometries are fabricated with the purpose of finding the critical current density as a function of cross-section dimensions and magnetic field. These samples will be used to select the cross-section dimensions that deliver optimal critical current density, which will consequently be used in simulation to optimize the geometry.