  Two level quantum mechanical systems like spin 1/2 particles lend themselves
as a natural qubit implementation. However, encoding a single qubit in several
spins reduces the resources necessary for qubit control and can protect from
decoherence channels. While several varieties of such encoded spin qubits have
been implemented, accurate control remains challenging, and leakage out of the
subspace of valid qubit states is a potential issue. Here, we realize
high-fidelity single qubit operations for a qubit encoded in two electron spins
in GaAs quantum dots by iterative tuning of the all-electrical control pulses.
Using randomized benchmarking, we find an average gate fidelity of $\mathcal{F}
= (98.5 \pm 0.1)\,\%$ and determine the leakage rate between the computational
subspace and other states to $\mathcal{L} = (0.4\pm0.1)\,\%$. These results
also demonstrate that high fidelity gates can be realized even in the presence
of nuclear spins as in III-V semiconductors.
