Google's Willow chip demonstrated exponential suppression of errors below the surface-code threshold — the milestone that separates quantum processors that merely tolerate noise from ones that begin to correct it.
Why error correction matters
Qubits are fragile. Errors creep in from heat, interference, and the very act of measurement, and in a naive design they compound as you add qubits. Error correction inverts that logic: below a certain threshold, adding qubits actually reduces the error rate. Crossing that threshold is the difference between a physics experiment and a scalable machine.
What Willow showed
The result is the exponential suppression of errors as the code distance increases — the signature behaviour of a system operating below threshold. It is the strongest evidence to date that fault-tolerant quantum computing is not a matter of "if" but of engineering effort and scale.
The caveats
This is a milestone, not a product. A logical qubit built from many physical qubits is still far more expensive than the physical qubit it replaces, and scaling to the number of logical qubits needed for broadly useful computation remains a hard, unsolved engineering problem. The classical benchmarking debate is also worth watching: what "quantum advantage" means, and against what classical baseline, is still being defined.