  Complete control of single-electron states in a two-dimensional semiconductor
quantum-ring model is established, opening a path into coherent laser-driven
single-gate qubits. The control scheme is developed in the framework of optimal
control theory for laser pulses of two-component polarization. In terms of
pulse lengths and target-state occupations, the scheme is shown to be superior
to conventional control methods that exploit Rabi oscillations generated by
uniform circularly polarized pulses. Current-carrying states in a quantum ring
can be used to manipulate a two-level subsystem at the ring center. Combining
our results, we propose a realistic approach to construct a laser-driven
single-gate qubit that has switching times in the terahertz regime.
