  An electron spin qubit in silicon quantum dots holds promise for quantum
information processing due to the scalability and long coherence. An essential
ingredient to recent progress is the employment of micromagnets. They generate
a synthetic spin-orbit coupling (SOC), which allows high-fidelity spin
manipulation and strong interaction between an electron spin and cavity
photons. To scaled-up quantum computing, multiple technical challenges remain
to be overcome, including controlling the valley degree of freedom, which is
usually considered detrimental to a spin qubit. Here, we show that it is
possible to significantly enhance the electrical manipulation of a spin qubit
through the effect of constructive interference and the large spin-valley
mixing. To characterize the quality of spin control, we also studied spin
dephasing due to charge noise through spin-valley mixing. The competition
between the increased control strength and spin dephasing produces two
sweet-spots, where the quality factor of the spin qubit can be high. Finally,
we reveal that the synthetic SOC leads to distinctive spin relaxation in
silicon, which explains recent experiments.
