Coherent transfer of the transverse momentum of an optical vortex beam to the motion of a single trapped ion

We demonstrate the excitation, using a structured light beam carrying orbital angular momentum, of the center of mass motion of a single atom in the transverse direction to the beam’s propagation. This interaction is achieved with a vortex beam carrying one unit of orbital angular momentum and one unit of spin/polarization angular momentum. Using a singly charged ${}^{40}$Ca${}^{+}$ ion, cooled near the ground state of motion in the 3D harmonic potential of a Paul trap, we probe the narrow S${}_{1\mathrm{/}2}$ to D${}_{5\mathrm{/}2}$ transition near $729\mathrm{n}\mathrm{m}$ on its motional sidebands to quantify the momentum transfer. Exchange of quanta in the perpendicular direction to the beam’s wave vector $\mathbf{k}$ is observed in case of the vortex shaped beam, in strong contrast to the absence of this spin-motion coupling for the case of a Gaussian beam. We characterize the coherent interaction by an effective transverse Lamb-Dicke factor ${\eta }_{\perp }^{\mathrm{e}\mathrm{x}\mathrm{p}}=0.0062(5)$ which is in agreement with our theoretical prediction ${\eta }_{\perp }^{\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{o}}=0.0057(1)$