Although silicon is a promising material for quantum computation, the degeneracy of theconduction band minima (valleys) must be lifted with a splitting sufficient to ensure theformation of well-defined and long-lived spin qubits. Here we demonstrate that valleyseparation 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 varieslinearly with applied electric field, with a ratio in agreement with atomistic tight-bindingpredictions. We demonstrate single-shot spin read-out and measure the spin relaxation fordifferent valley configurations and dot occupancies, finding one-electron lifetimes exceeding2 s. Spin relaxation occurs via phonon emission due to spin–orbit coupling between the valleystates, a process not previously anticipated for silicon quantum dots. An analytical theorydescribes the magnetic field dependence of the relaxation rate, including the presence of adramatic rate enhancement (or hot-spot) when Zeeman and valley splittings coincide.
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