Quantum error correction assumes that errors on different qubits are approximately independent. If the noise is correlated — if a fluctuation that flips one qubit also flips its neighbor — the error correction codes designed for independent errors can fail. In dense silicon spin qubit arrays, where qubits are separated by tens of nanometers, noise correlations are expected. The question is whether they are fatal.

Two distinct noise sources dominate, and they have opposite correlation structures.

Global magnetic field drifts produce perfectly correlated fluctuations across the entire array. Every qubit sees the same frequency shift simultaneously. This is the worst case for error correction: a code designed to correct one error per syndrome cycle is overwhelmed when all qubits drift together. But magnetic drifts are slow and predictable — they can be tracked and compensated by active feedback, converting a correlated error into a tracked systematic shift.

Charge noise from two-level fluctuators — defects in the oxide or at interfaces that randomly switch between two configurations — produces short-range correlations that decay within neighboring qubits. This is the noise that scales with density: pack more qubits closer together, and each fluctuator affects more qubits simultaneously. The measurement shows these correlations are moderate, electrically tunable, and compatible with fault-tolerant operation with minimal overhead. The charge noise is not benign, but it is not a scaling barrier.

The correlated noise that matters most (magnetic) is controllable. The noise that scales with density (charge) is tolerable. The combination means silicon spin qubits can potentially scale to fault-tolerant operation — not because the noise is small, but because each type of noise admits a different mitigation strategy. The solution is two fixes, not one.