  We show that the mixing between spin and valley degrees of freedom in a
silicon quantum bit (qubit) can be controlled by a static electric field acting
on the valley splitting $\Delta$. Thanks to spin-orbit coupling, the qubit can
be continuously switched between a spin mode (where the quantum information is
encoded into the spin) and a valley mode (where the the quantum information is
encoded into the valley). In the spin mode, the qubit is more robust with
respect to inelastic relaxation and decoherence, but is hardly addressable
electrically. It can however be brought into the valley mode then back to the
spin mode for electrical manipulation. This opens new perspectives for the
development of robust and scalable, electrically addressable spin qubits on
silicon. We illustrate this with tight-binding simulations on a so-called
"corner dot" in a silicon-on-insulator device where the confinement and valley
splitting can be independently tailored by a front and a back gate.

Mesoscale and Nanoscale Physics