Surface Code
The leading quantum error-correction recipe: qubits on a flat grid whose neighbors check for errors. Practical, but it costs hundreds of physical qubits per logical one.
The surface code is the most studied recipe for quantum error correction. Qubits are arranged on a 2D grid, and neighboring 'measure' qubits repeatedly check parities among 'data' qubits, never reading the encoded information directly, only whether errors have occurred. Decode the pattern of alarms, fix what went wrong, repeat.
Its appeal is practical: it only requires nearest-neighbor interactions on a flat chip (convenient for superconducting hardware) and tolerates physical error rates near 1%, which real devices can actually hit. Its cost is overhead: error rates fall as the grid grows, so useful logical qubits demand hundreds of physical ones each.
Not every platform marries the surface code: trapped-ion and neutral-atom machines, with their flexible connectivity, can use leaner codes. Which code wins is one of the live questions the millikelvin watches.