  We propose an efficient mechanism for the operation of writing spin in a
quantum dot, which is an ideal candidate for qubit. The idea is based on the
Andreev reflection induced spin polarization (ARISP) in a ferromagnetic /
quantum-dot / superconductor system. We find that on the resonance of Andreev
reflection, the spin polarization of quantum dot strongly denpends on the
magnetization of ferromagnetic electrode, and the sign of the spin polarization
is controllable by bias voltage. In the presence of intradot Coulomb
interaction, we show that ARISP effect can still survive as long as the
charging energy is comparable to the superconducting gap. Detailed conditions
and properties of ARISP are also discussed.
