  We experimentally demonstrate an ultrafast method for preparing spin states
of donor-bound electrons in GaAs with single laser pulses. Each polarization
state of a preparation pulse has a direct mapping onto a spin state, with
bijective correspondence between the Poincar\'{e}-sphere (for photon
polarization) and Bloch-sphere (for spin) state representations. The
preparation is governed by a stimulated Raman process and occurs orders of
magnitude faster than the spontaneous emission and spin dephasing. Similar
dynamics governs our ultrafast optical Kerr detection of the spin coherence,
thus getting access to spin state tomography. Experiments with double
preparation pulses show an additive character for the preparation method.
Utilization of these phenomena is of value for quantum information schemes.
