Quantum error correction
Encoding quantum information redundantly so errors can be detected and corrected faster than they accumulate.
Quantum error correction spreads one unit of quantum information across many physical qubits and measures the system in a way that reveals what went wrong without revealing - and therefore destroying - the state itself. Surface codes are the most studied family; the code distance determines how many simultaneous errors can be tolerated, and increasing it costs quadratically more physical qubits.
Its central result is a threshold: below a certain physical error rate, adding more physical qubits makes the encoded logical qubit more reliable rather than less. Above it, redundancy makes things worse. Crossing that threshold and then demonstrating that reliability improves as the code grows is the milestone that actually distinguishes progress from a larger chip.
This is why a roadmap should be read against demonstrated code distances and logical error rates rather than against device announcements, and why the timeline for a cryptographically relevant machine is an error-correction question rather than a qubit-count one.
Evidence that keeps pace with the decision.
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