Confinement Drives Valley Splitting above 4 K in Buried Silicon Quantum Wells

Controlling the energy scales of a quantum system is essential for defining robust qubits. In silicon, the nearly degenerate conduction-band valleys create a leakage channel from the single-spin computational basis. Here, we perform spectroscopic measurements of single- and two-electron states in buried silicon quantum wells. Across a linear array of four quantum dots with an average orbital energy of 2.4(2) meV, we report an average single-electron valley splitting of 0.40(6) meV and an average two-electron singlet-triplet splitting of 0.24(7) meV. In three dots, we observe a strong correlation between valley splitting and orbital energy, with an average linear coefficient of ≈0.22 (meV/meV). In contrast, the remaining dot exhibits the highest valley splitting of 0.76(2) meV and low correlation. Our findings show that strong confinement can effectively enhance valley splitting in buried quantum wells, thereby establishing a viable path toward shuttling and sparse-occupation-based architectures in low-disorder heterostructures.