  We report initialization, complete electrical control, and single-shot
readout of an exchange-only spin qubit. Full control via the exchange
interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns.
Measurement and state tomography are performed using a maximum-likelihood
estimator method, allowing decoherence, leakage out of the qubit state space,
and measurement fidelity to be quantified. The methods developed here are
generally applicable to systems with state leakage, noisy measurements, and
non-orthogonal control axes.
