  We introduce a solid-state qubit in which exchange interactions among
confined electrons provide both the static longitudinal field and the
oscillatory transverse field, allowing rapid and full qubit control via rf
gate-voltage pulses. We demonstrate two-axis control at a detuning sweet-spot,
where leakage due to hyperfine coupling is suppressed by the large exchange
gap. A {\pi}/2-gate time of 2.5 ns and a coherence time of 19 {\mu}s, using
multi-pulse echo, are also demonstrated. Model calculations that include
effects of hyperfine noise are in excellent quantitative agreement with
experiment.
