  A fundamental goal in the manipulation of quantum systems is the achievement
of many coherent oscillations within the characteristic dephasing time T2*[1].
Most manipulations of electron spins in quantum dots have focused on the
construction and control of two-state quantum systems, or qubits, in which each
quantum dot is occupied by a single electron[2-7]. Here we perform quantum
manipulations on a system with more electrons per quantum dot, in a double dot
with three electrons. We demonstrate that tailored pulse sequences can be used
to induce coherent rotations between 3-electron quantum states. Certain pulse
sequences yield coherent oscillations with a very high figure of merit (the
ratio of coherence time to rotation time) of >100. The presence of the third
electron enables very fast rotations to all possible states, in contrast to the
case when only two electrons are used, in which some rotations are slow. The
minimum oscillation frequency we observe is >5 GHz.
