  Junctions are fundamental elements that support qubit locomotion in
two-dimensional ion trap arrays and enhance connectivity in emerging
trapped-ion quantum computers. In surface ion traps they have typically been
implemented by shaping radio frequency (RF) electrodes in a single plane to
minimize the disturbance to the pseudopotential. However, this method
introduces issues related to RF lead routing that can increase power
dissipation and the likelihood of voltage breakdown. Here, we propose and
simulate a novel two-layer junction design incorporating two perpendicularly
rotoreflected (rotated, then reflected) linear ion traps. The traps are
vertically separated, and create a trapping potential between their respective
planes. The orthogonal orientation of the RF electrodes of each trap relative
to the other provides perpendicular axes of confinement that can be used to
realize transport in two dimensions. While this design introduces manufacturing
and operating challenges, as now two separate structures have to be precisely
positioned relative to each other in the vertical direction and optical access
from the top is obscured, it obviates the need to route RF leads below the top
surface of the trap and eliminates the pseudopotential bumps that occur in
typical junctions. In this paper the stability of idealized ion transfer in the
new configuration is demonstrated, both by solving the Mathieu equation
analytically to identify the stable regions and by numerically modeling ion
dynamics. Our novel junction layout has the potential to enhance the
flexibility of microfabricated ion trap control to enable large-scale
trapped-ion quantum computing.
