Quantum errors can be corrected without looking at the data. Spread one qubit over nine, tied together so that checking how they relate to each other shows whether one of them went wrong, and how to undo it, without revealing what the qubit holds.

Quantum errors can be corrected faster than they arise, as long as each step errs less often than a certain threshold. Nest codes inside codes, and a quantum computation can run as long as needed, as accurately as wanted.