Entanglement

Two qubits are entangled when their joint state cannot be written as one state for each. Measure one and the other's outcome is fixed, in every basis, in a way no local story explains.

Two-qubit gates create entanglement, and two-qubit gates are what a machine gets wrong most. That is why ROX OS counts them: the load of a circuit is, to first order, its two-qubit gates. The 12-spin ring on the record carries 432 of them at rung three.