  Rapid coherent control of electron spin states is required for implementation
of a spin-based quantum processor. We demonstrate coherent control of
electronic spin states in a double quantum dot by sweeping an initially
prepared spin singlet state through a singlet-triplet anti-crossing in the
energy level spectrum. The anti-crossing serves as a beam splitter for the
incoming spin singlet state. Consecutive crossings through the beam splitter,
when performed within the spin dephasing time, result in coherent quantum
oscillations between the singlet state and a triplet state. The all-electrical
method for quantum control relies on electron-nuclear spin coupling and drives
single electron spin rotations on nanosecond timescales.
