  Experiments with trapped ions and neutral atoms typically employ optical
modulators in order to control the phase, frequency, and amplitude of light
directed to individual atoms. These elements are expensive, bulky, consume
substantial power, and often rely on free-space I/O channels, all of which pose
scaling challenges. To support many-ion systems like trapped-ion quantum
computers or miniaturized deployable devices like clocks and sensors, these
elements must ultimately be microfabricated, ideally monolithically with the
trap to avoid losses associated with optical coupling between physically
separate components. In this work we design, fabricate, and test an optical
modulator capable of monolithic integration with a surface-electrode ion trap.
These devices consist of piezo-optomechanical photonic integrated circuits
configured as multi-stage Mach-Zehnder modulators that are used to control the
intensity of light delivered to a single trapped ion on a separate chip. We use
quantum tomography employing hundreds of multi-gate sequences to enhance the
sensitivity of the fidelity to the types and magnitudes of gate errors relevant
to quantum computing and better characterize the performance of the modulators,
ultimately measuring single qubit gate fidelities that exceed 99.7%.
