Error correction

Quantum error correction spreads one logical qubit, the protected choice you meant to compute with, across many physical qubits. The extra qubits let a scrambled phase or a flipped bit be noticed and repaired without measuring the logical state itself, which would destroy its superposition.

The pile of physical qubits per protected one is the cost printed on the diagram. Keith finds the pile gloomy, and Malcolm finds it the most useful sketch in the hardware papers, because the price of keeping a sign is drawn to scale.