Bad Qubits — Learn Quantum by Doing

Multiple choice

In the fault-tolerance problem, the comforting story of quantum error correction (encode, measure syndrome, recover) relies on one fatal implicit assumption. Which assumption is it?

That measurement outcomes are always discarded immediately after use
That qubits never decohere while idling between operations
That the encoding circuit, syndrome measurement, and recovery are themselves performed perfectly (noiselessly)
That the code distance d is always an even number