  To date, individual addressing of ion qubits has relied primarily on local
Rabi or transition frequency differences between ions created via
electromagnetic field spatial gradients or via ion transport operations.
Alternatively, it is possible to synthesize arbitrary local one-qubit gates by
leveraging local phase differences in a global driving field. Here we report
individual addressing of $^{40}$Ca$^+$ ions in a two-ion crystal using axial
potential modulation in a global gate laser field. We characterize the
resulting gate performance via one-qubit randomized benchmarking, applying
different random sequences to each co-trapped ion. We identify the primary
error sources and compare the results with single-ion experiments to better
understand our experimental limitations. These experiments form a foundation
for the universal control of two ions, confined in the same potential well,
with a single gate laser beam.
