  Semiconductor architectures hold promise for quantum information processing
(QIP) applications due to their large industrial base and perceived scalability
potential. Electron spins in silicon in particular may be an excellent
architecture for QIP and also for spin electronics (spintronics) applications.
While the charge of an electron is easily manipulated by charged gates, the
spin degree of freedom is well isolated from charge fluctuations. Inherently
small spin-orbit coupling and the existence of a spin-zero Si isotope
facilitate long single spin qubit coherence times. Here we consider the
relaxation properties of localized electronic states in silicon due to donors,
quantum wells, and quantum dots, including effects due to phonons and Rashba
spin-orbit coupling. Our analysis is impeded by the complicated, many-valley
band structure of silicon and previously unaddressed physics in silicon quantum
wells. We find that electron spins in silicon and especially strained silicon
have excellent decoherence properties. Where possible we compare with
experiment to test our theories. We go beyond issues of coherence in a quantum
computer to problems of control and measurement. Precisely what makes spin
relaxation so long in semiconductor architectures makes spin measurement so
difficult. To address this, we propose a new scheme for spin readout which has
the added benefit of automatic spin initialization, a vital component of
quantum computing and quantum error correction. Our results represent important
practical milestones on the way to the design and construction of a
silicon-based quantum computer.
