  We present a unified description of cooling and manipulation of a mesoscopic
bath of nuclear spins via coupling to a single quantum system of electronic
spin (quantum bit). We show that a bath cooled by the quantum bit rapidly
saturates. Although the resulting saturated states of the spin bath (``dark
states'') generally have low degrees of polarization and purity, their symmetry
properties make them a valuable resource for the coherent manipulation of
quantum bits. Specifically, we demonstrate that the dark states of nuclear
ensembles can be used to coherently control the system-bath interaction and to
provide a robust, long-lived quantum memory for qubit states.
