  In a large scale trapped atomic ion quantum computer, high-fidelity two-qubit
gates need to be extended over all qubits with individual control. We realize
and characterize high-fidelity two-qubit gates in a system with up to 4 ions
using radial modes. The ions are individually addressed by two tightly focused
beams steered using micro-electromechanical system (MEMS) mirrors. We deduce a
gate fidelity of 99.49(7)% in a two-ion chain and 99.30(6)% in a four-ion chain
by applying a sequence of up to 21 two-qubit gates and measuring the final
state fidelity. We characterize the residual errors and discuss methods to
further improve the gate fidelity towards values that are compatible with
fault-tolerant quantum computation.
