  Selected problems of fundamental importance for spintronics and
spin-polarized transport are reviewed, some of them with a special emphasis on
their applications in quantum computing and coherent control of quantum
dynamics. The role of the solid-state environment in the decoherence of
electron spins is discussed. In particular, the limiting effect of the
spin-orbit interaction on spin relaxation of conduction electrons is carefully
examined in the light of recent theoretical and experimental progress. Most of
the proposed spintronic devices involve spin-polarized transport across
interfaces in various hybrid structures. The specific example discussed here,
of a magnetic semiconductor/superconductor interface, displays many intricacies
which a complex spin-dependent interface introduces in the spin-polarized
transport. It is proposed that pairs of entangled electrons in a superconductor
(Cooper pairs) can be transfered to a non-superconducting region, and
consequently separated for a transport study of the spin entanglement. Several
important theoretical proposals for quantum computing are based on electronic
and nuclear spin entanglement in a solid. Physical requirements for these
proposals to be useful are discussed and some alternative views are presented.
Finally, a recent discovery of optical control of nuclear spins in
semiconductors is reviewed and placed in the context of a long-standing search
for electronic control of nuclear dynamics.
