  Spins in SiGe quantum dots are promising candidates for quantum bits but are
also challenging due to the valley degeneracy which could potentially cause
spin decoherence and weak spin-orbital coupling. In this work we demonstrate
that valley states can serve as an asset that enables two-axis control of a
singlet-triplet qubit formed in a double quantum dot without the application of
a magnetic field gradient. We measure the valley spectrum in each dot using
magnetic field spectroscopy of Zeeman split triplet states. The interdot
transition between ground states requires an electron to flip between valleys,
which in turn provides a g-factor difference $\Delta g$ between two dots. This
$\Delta g$ serves as an effective magnetic field gradient and allows for qubit
rotations with a rate that increases linearly with an external magnetic field.
We measured several interdot transitions and found that this valley introduced
$\Delta g$ is universal and electrically tunable. This could potentially
simplify scaling up quantum information processing in the SiGe platform by
removing the requirement for magnetic field gradients which are difficult to
engineer.
