Integrated optical addressing of an ion qubit

The long coherence times and strong Coulomb interactions afforded by trapped ion qubits have enabled realizations of the necessary primitives for quantum information processing (QIP), and indeed the highest-fidelity quantum operations in any qubit to date. But while light delivery to each individual ion in a system is essential for general quantum manipulations and readout, experiments so far have employed optical systems cumbersome to scale to even a few tens of qubits. Here we demonstrate lithographically defined nanophotonic waveguide devices for light routing and ion addressing fully integrated within a surface-electrode ion trap chip. Ion qubits are addressed at multiple locations via focusing grating couplers emitting through openings in the trap electrodes to ions trapped 50 $\mu$m above the chip; using this light we perform quantum coherent operations on the optical qubit transition in individual $^{88}$Sr$^+$ ions. The grating focuses the beam to a diffraction-limited spot near the ion position with a 2 $\mu$m 1/$e^2$-radius along the trap axis, and we measure crosstalk errors between $10^{-2}$ and $4\times10^{-4}$ at distances 7.5-15 $\mu$m from the beam center. Owing to the scalability of the planar fabrication employed, together with the tight focusing and stable alignment afforded by optics integration within the trap chip, this approach presents a path to creating the optical systems required for large-scale trapped-ion QIP.