(b) The charge-stability diagram measured by performing charge readout as a function of plunger-gate voltages after being prepared in a random (3,1) state. The DQD and the sensor location are labeled as yellow and orange dots, respectively. (a) A SEM image of an overlap-style device. This could potentially simplify the scaling up of quantum information processing in the Si x Ge 1 − x platform by removing the requirement for magnetic field gradients, which are difficult to engineer. We measure several interdot transitions and find that this valley-introduced Δ g is universal and electrically tunable. This Δ 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 find a finite probability of valley states to flip during interdot transitions, which in turn provides a g-factor difference Δ g between two dots. We measure the valley spectrum in each dot using magnetic field spectroscopy of Zeeman split triplet states. In this work, we demonstrate that valley states can serve as an asset that enables full two-axis control of a singlet-triplet qubit formed in a double quantum dot. Spins in Si x Ge 1 − x quantum dots are promising candidates for quantum bits but are also challenging due to the valley degeneracy and weak spin-orbital coupling.

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