Probing the Variation of the Intervalley Tunnel Coupling in a Silicon Triple Quantum Dot*

The “valley splitting” of the two lowest-lying valleys in silicon quantum wells is known to be sensitive to atomic scale disorder. A large valley splitting is desirable to have a well-defined spin qubit. In addition, an understanding of the intervalley tunnel coupling that couples different valleys in adjacent quantum dots is extremely important, as the resulting gaps in the energy-level diagram may affect the fidelity of charge and spin-transfer protocols in silicon quantum-dot arrays. We use microwave spectroscopy to probe variations in the valley splitting, and the intra- and intervalley tunnel couplings (t_ij and t_ij’) that couple dots i and j in a triple quantum dot. We uncover large variations of t_ij’/t_ij and linear scaling of t_ij’ with t_ij, as expected from theory. The results indicate strong interactions between different valley states on neighboring dots, which we attribute to local inhomogeneities in the silicon quantum well.