Silicon quantum computers are plagued by a hidden disorder that affects their reliability and fidelity, specifically in the valley splitting of spin-qubits. Researchers have pinpointed atomic-scale disorder in silicon quantum wells as the primary cause of this variability. Using electrical spectroscopy, a team mapped valley splitting across a 12-qubit-class silicon quantum dot processor fabricated by Intel at the Argonne National Laboratory's Chicago Quantum Computing Testbed. This discovery reframes valley-splitting variability as a materials-engineering issue, rather than a fundamental limitation of quantum mechanics1. The findings could inform manufacturing improvements to produce more consistent and reliable silicon quantum computers. As companies like Intel continue to advance quantum computing capabilities, the need for cryptographic migration to post-quantum cryptography (PQC) becomes increasingly urgent, underscoring the importance of addressing these materials-engineering challenges to ensure the security and fidelity of quantum computing systems.