The nines: why gate fidelity matters
99.9% vs 99.99% looks like a rounding error but means 10× fewer mistakes. Error correction only pays off above a threshold near “three nines”; crossing “four nines” buys real headroom.
Gate fidelity is how often a quantum operation does what it should. The numbers look almost identical — 99.9% versus 99.99% — but the gap is enormous: the second means roughly ten times fewer errors, and quantum computers run so many operations that those tenfold differences compound dramatically.
There is a reason people count the nines. Error correction only pays off once gates are good enough — above a threshold near “three nines” (99.9%). Cross into “four nines” (99.99%), as IonQ reported for a two-qubit gate in 2025, and you buy real headroom: fewer physical qubits needed per logical qubit, and more room for everything else to go slightly wrong.
Fidelity is quieter than qubit counts in the headlines, but it is just as decisive for whether a machine can ever be useful. A processor with many qubits and mediocre fidelity may be less capable than a smaller one with excellent gates — which is why we track fidelity as its own metric rather than folding it into a single score.