  The similarities between gated quantum dots and the transistors in modern
microelectronics - in fabrication methods, physical structure, and voltage
scales for manipulation - have led to great interest in the development of
quantum bits (qubits) in semiconductor quantum dots. While quantum dot spin
qubits have demonstrated long coherence times, their manipulation is often
slower than desired for important future applications, such as factoring.
Further, scalability and manufacturability are enhanced when qubits are as
simple as possible. Previous work has increased the speed of spin qubit
rotations by making use of integrated micromagnets, dynamic pumping of nuclear
spins, or the addition of a third quantum dot. Here we demonstrate a new qubit
that offers both simplicity - it requires no special preparation and lives in a
double quantum dot with no added complexity - and is very fast: we demonstrate
full control on the Bloch sphere with $\pi$-rotation times less than 100 ps in
two orthogonal directions. We report full process tomography, extracting high
fidelities equal to or greater than 85% for X-rotations and 94% for
Z-rotations. We discuss a path forward to fidelities better than the threshold
for quantum error correction.
