Impact of the local valley splitting on the coherence of conveyor-belt spin shuttling in ²⁸Si/SiGe

Silicon quantum chips offer a promising path toward scalable, fault-tolerant quantum computing, with the potential to host millions of qubits. However, scaling up dense quantum-dot arrays and enabling qubit interconnections through shuttling are hindered by uncontrolled lateral variations of the valley splitting energy E_VS. We map E_VS across a 40 nm x 400 nm region of a ²⁸Si/Si_0.7Ge_0.3 shuttle device and analyze the spin coherence of a single electron spin transported by conveyor-belt shuttling. We observe that the E_VS varies over a wide range from 1.5 μeV to 200 μeV and is dominated by SiGe alloy disorder. In regions of low E_VS and at spin-valley resonances, spin coherence is reduced and its dependence on shuttle velocity matches predictions. Rapid and frequent traversal of low-E_VS regions induces a regime of enhanced spin coherence explained by motional narrowing. By selecting shuttle trajectories that avoid problematic areas on the E_VS map, we achieve transport over tens of microns with coherence limited only by the coupling to a static electron spin entangled with the mobile qubit. Our results provide experimental confirmation of the theory of spin-decoherence of mobile electron spin-qubits and present practical strategies to integrate conveyor-mode qubit shuttling into silicon quantum chips.