  Prospect of building spintronic devices in which electron spins store and
transport information has attracted strong attention in recent years. Here we
present some of our representative theoretical results on three fundamental
aspects of spintronics: spin coherence, spin entanglement, and spin transport.
In particular, we discuss our detailed quantitative theory for spin relaxation
and coherence in electronic materials, resolving in the process a long-standing
puzzle of why spin relaxation is extremely fast in Al (compared with other
simple metals). In the study of spin entanglement, we consider two electrons in
a coupled GaAs double-quantum-dot structure and explore the Hilbert space of
the double dot. The specific goal is to critically assess the quantitative
aspects of the proposed spin-based quantum dot quantum computer architecture.
Furthermore, we discuss our theory of spin-polarized transport across a
semiconductor-metal interface. In particular, we study Andreev reflection,
which enables us to quantify the degree of carrier spin polarization and the
strength of interfacial scattering.
