Novel permanent magnet array geometries for scalable trapped-ion quantum computing in a laser-free entanglement architecture

Abstract

A novel design is presented for a permanent magnet array to address specific challenges with scalable trapped-ion quantum computing systems. Design and optimization of this magnet geometry is motivated by concepts for large-scale Quantum Charge-Coupled Device (QCCD) architectures. This proposal is relevant to magnetic field gradient schemes for laser-free entanglement using long-wavelength radiation, and individual addressing based on spatially dependent, magnetic field sensitive qubits. This configuration generates a localized, asymmetric magnetic field, yielding a region for ion transport into and out of a strong magnetic field gradient, while minimizing the absolute field experienced by the ion. This is a distinct improvement for scalability over dipolar magnet geometries where a strong magnetic field surrounds a magnetic field nil in three dimensions, which is problematic for ion transport applications. The design also relaxes the alignment constraints for experimental setup by allowing greater tolerance to misalignment in two dimensions. Additionally, the potential to scale a permanent magnet scheme in QCCD systems circumvents engineering challenges associated with using large electrical currents to generate the field gradient. Finally, a conceptual discussion is given for incorporating the design into a scalable QCCD type architecture.

Figures (13)

• Figure 1: Diagram demonstrating the geometry of the dual-layer magnet configuration. The lower layer is comprised of the Halbach array, and the upper layer is a geometrically identical magnet array aligned in the negative y-axis (North pole pointing vertically towards the Halbach array). The red arrows indicate the direction of the magnetic flux domains, while the red crosses and dots indicate arrows aligned into and out of the page respectively.
• Figure 2: Illustration of a linear, surface ion-trap orientation to magnet arrays showing principal axes for simulation and analysis. (a) Perspective view. (b) Side view.
• Figure 3: Contour plot showing the magnetic flux density, using the complete design, at a surface $0.5$ mm above the Halbach magnet array, representing a typical ion height plus the physical thickness of the surface trap. (a) Magnetic flux density in the axial direction (b) absolute magnitude of the magnetic field. The color maps are given in units of Gauss.
• Figure 4: (a) Magnetic flux density contribution in the three axes: axial (z), vertical (y) and transverse (x), marked in blue, green and red respectively with magnetic field gradient in the axial direction (z) overlaid (magenta) showing the low field edge on the right side. The intersection of the plane of the magnet edge is at $16$ mm in the simulation space. (b) Zoomed region of interest where at $\approx 17.6$ mm the magnetic field has an effective null in three axes with a strong axial gradient, $1.6$ mm from the aligned edge of the magnet arrays.
• Figure 5: 3D model of the magnet array configuration as presented in the COMSOL Multiphysics simulation package. This adapted geometry features identical Rhombic prism for the central magnet in the two arrays with cross-section dimensions of $1$ mm x $0.5$ mm, and height $1$ mm. The other magnets in the array maintain cuboid $0.5$ mm x $1$ mm x $1$ mm dimensions.