  Laser-controlled entanglement between atomic qubits (`spins') and collective
motion in trapped ion Coulomb crystals requires conditional momentum transfer
from the laser. Since the spin-dependent force is derived from a spatial
gradient in the spin-light interaction, this force is typically longitudinal --
parallel and proportional to the average laser $k$-vector (or two beams'
$k$-vector difference), which constrains both the direction and relative
magnitude of the accessible spin-motion coupling. Here, we show how momentum
can also be transferred perpendicular to a single laser beam due to the
gradient in its transverse profile. By controlling the transverse gradient at
the position of the ion through beam shaping, the relative strength of the
sidebands and carrier can be tuned to optimize the desired interaction and
suppress undesired, off-resonant effects that can degrade gate fidelity. We
also discuss how this effect may already be playing an unappreciated role in
recent experiments.
