What “below threshold” means (Google Willow)
For decades, adding qubits added errors faster than you could correct them. “Below threshold” is the turning point where making the code bigger makes it more reliable — the precondition for a useful machine.
For decades quantum error correction had a chicken-and-egg problem: adding more physical qubits to protect information also added more places for errors to creep in, often faster than correction could remove them. Below a certain quality “threshold,” bigger codes were worse, not better.
“Below threshold” is the moment that flips. When the underlying hardware is good enough, making the error-correcting code larger actually lowers the logical error rate — protection wins over added noise. Google's 2024 Willow result demonstrated exactly this: scaling the code from a smaller to a larger patch cut the logical error rate roughly in half.
It does not make a useful machine on its own. But it is the precondition for one, because it means scale finally works in your favor: adding qubits now buys reliability instead of eroding it. That is why the result drew attention out of proportion to its qubit count.