Catches classical-style errors: a qubit that should be 0 accidentally becomes 1.
These are the row checks — the same ones we just saw in the Sudoku grid.
Catches purely quantum errors: a qubit's phase flips — invisible in the classical world, but fatal for quantum communication.
These are the column checks — a second identical grid, rotated 90°.
Alice locks the quantum message with her operator
CSS activeBob adds his own lock and sends it back
CSS activeBoth locks removed — message delivered
CSS active
At every stage, CSS error correction runs automatically —
detecting and fixing both bit-flips and phase-flips
without ever reading the message.