  Research on Si quantum dot spin qubits is motivated by the long spin
coherence times measured in Si, yet the orbital spectrum of Si dots is
increased as a result of the valley degree of freedom. The valley degeneracy
may be lifted by the interface potential, which gives rise to a valley-orbit
coupling, but the latter depends on the detailed structure of the interface and
is generally unknown a priori. These facts motivate us to provide an extensive
study of spin qubits in Si double quantum dots, accounting fully for the valley
degree of freedom and assuming no prior knowledge of the valley-orbit coupling.
For single-spin qubits we analyze the spectrum of a multivalley double quantum
dot, discuss the initialization of one qubit, identify the conditions for the
lowest energy two-electron states to be a singlet and a triplet well separated
from other states, and determine analytical expressions for the exchange
splitting. For singlet-triplet qubits we analyze the single-dot spectrum and
initialization process, the double-dot spectrum, singlet-triplet mixing in an
inhomogeneous magnetic field and the peculiarities of spin blockade in
multivalley qubits. We review briefly the hyperfine interaction in Si and
discuss its role in spin blockade in natural Si, including intravalley and
intervalley effects. We study the evolution of the double-dot spectrum as a
function of magnetic field. We address briefly the situation in which the
valley-orbit coupling is different in each dot due to interface roughness. We
propose a new experiment for measuring the valley splitting in a single quantum
dot. We discuss the possibility of devising other types of qubits in Si QDs,
and the size of the intervaley coupling due to the Coulomb interaction.
