  The gate fidelity and the coherence time of a qubit are important benchmarks
for quantum computation. We construct a qubit using a single electron spin in a
Si/SiGe quantum dot and control it electrically via an artificial spin-orbit
field from a micromagnet. We measure an average single-qubit gate fidelity of
$\approx$ 99$\%$ using randomized benchmarking, which is consistent with
dephasing from the slowly evolving nuclear spins in substrate. The coherence
time measured using dynamical decoupling extends up to $\approx$ 400 $\mu$s for
128 decoupling pulses, with no sign of saturation. We find evidence that the
coherence time is limited by noise in the 10 kHz $-$ 1 MHz range, possibly
because charge noise affecting the spin via the micromagnet gradient. This work
shows that an electron spin in a Si/SiGe quantum dot is a good candidate for
quantum information processing as well as for a quantum memory, even without
isotopic purification.
