  We demonstrate accurate single-qubit control in an ensemble of atomic qubits
trapped in an optical lattice. The qubits are driven with microwave radiation,
and their dynamics tracked by optical probe polarimetry. Real-time diagnostics
is crucial to minimize systematic errors and optimize the performance of
single-qubit gates, leading to fidelities of 0.99 for single-qubit pi
rotations. We show that increased robustness to large, deliberately introduced
errors can be achieved through the use of composite rotations. However, during
normal operation the combination of very small intrinsic errors and additional
decoherence during the longer pulse sequences precludes any significant
performance gain in our current experiment.
