  Although silicon is a promising material for quantum computation, the
degeneracy of the conduction band minima (valleys) must be lifted with a
splitting sufficient to ensure formation of well-defined and long-lived spin
qubits. Here we demonstrate that valley separation can be accurately tuned via
electrostatic gate control in a metal-oxide-semiconductor quantum dot,
providing splittings spanning 0.3 - 0.8 meV. The splitting varies linearly with
applied electric field, with a ratio in agreement with atomistic tight-binding
predictions. We demonstrate single-shot spin readout and measure the spin
relaxation for different valley configurations and dot occupancies, finding
one-electron lifetimes exceeding 2 seconds. Spin relaxation occurs via phonon
emission due to spin-orbit coupling between the valley states, a process not
previously anticipated for silicon quantum dots. An analytical theory describes
the magnetic field dependence of the relaxation rate, including the presence of
a dramatic rate enhancement (or hot-spot) when Zeeman and valley splittings
coincide.
