Can we build large-scale fault-tolerant quantum computers?

Quantum error correction in principle allows arbitrarily long computations from imperfect qubits. Demonstrations (Google, IBM, Quantinuum) have shown break-even and below-threshold codes on small numbers of logical qubits.

Scaling to thousands of logical qubits — required for industrially relevant algorithms — needs ~million physical qubits with current codes. Engineering challenges and possible new code families are very active research areas.