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Answer

quantum error correction

A physical qubit is a fragile thing; heat, vibration and stray electromagnetic noise all destroy its state. Error correction spreads one logical value across many physical qubits so that errors can be detected and reversed faster than they accumulate. Crossing that threshold is what separates a physics experiment from a computer, and several groups have now crossed it repeatedly rather than once.

• Quantum chemistry: catalyst and battery electrolyte simulation where classical approximations get expensive.
• Materials science: modelling superconductors and magnetic materials from first principles.
• Optimisation: narrow logistics and portfolio problems, though classical solvers remain stubbornly competitive.
• Sensing: an underrated cousin, already commercial in navigation and medical imaging.