  Electron spins in silicon quantum dots are excellent qubits because they have
long coherence times, high gate fidelities, and are compatible with advanced
semiconductor manufacturing techniques. The valley degree of freedom, which
results from the specific character of the Si band structure, is a unique
feature of electrons in Si spin qubits. However, the small difference in energy
between different valley levels often poses a challenge for quantum computing
in Si. Here, we show that the spin-valley coupling in Si, which enables
transitions between states with different spin and valley quantum numbers,
enables coherent control of electron spins in Si. We demonstrate coherent
manipulation of effective single- and two-electron spin states in a Si/SiGe
double quantum dot without ac magnetic or electric fields. Our results
illustrate that the valley degree of freedom, which is often regarded as an
inconvenience, can itself enable quantum manipulation of electron spin states.
