The Science Newswise — Qubits are the quantum counterpart to the bits used in conventional computers. Bits have a “0” and “1” state that is defined by electric charge. In a type of qubit called a silicon spin qubit, the “0” and “1” states are defined by electron spin. This spin can point either up or down in a magnetic field, analogous to a tiny compass needle. Scientists build silicon spin qubits by trapping a single electron inside a thin layer of silicon. The thin layer of silicon (called a quantum well) is sandwiched between another semiconductor material. In addition to spin, electrons in silicon also have a quantum property called a valley state. The energy difference between these valley states is called valley splitting. Valley splitting competes with the spin states used for computation. If the valley splitting is too small, the electron can leak into unwanted valley states. This leakage causes errors and loss of fidelity. In this study, researchers examined how the quantum well affected valley splitting. The Impact Because silicon spin qubits build on the same technology that underpins today’s semiconductor industry, they are one of the most promising platforms for scalable quantum computing. For years, researchers knew that defects and inconsistencies in the materials used in silicon quantum devices reduce valley splitting and cause failure. This valley splitting has long been known to vary from device to device, but its origin remained unclear. This study revealed that disorder on the atomic scale in the quantum well is the main source of variability of valley splitting. By identifying this root cause, the work turns a long-standing challenge into a tractable materials problem. It gives industry and National Laboratories a clear path towards building more reliable, higher-fidelity silicon qubits. Summary This study was enabled by a unique