Topological quantum error correction — X and Z chains, syndrome detection
The surface code arranges qubits on a 2D lattice with stabilizer measurements (X-type plaquettes and Z-type vertices). Physical errors create chains of flips; only chains crossing the boundary flip the logical qubit. Syndrome qubits detect the error endpoints without measuring the logical information. The threshold theorem guarantees that below ~1% error rate, the logical error rate decreases exponentially with code distance d.