fault-tolerant

Describes a quantum computer that keeps computing correctly even though its parts keep making errors, because it detects and corrects them as it goes.

Fault tolerance is not the absence of errors. It is the ability to get the right answer anyway. A fault-tolerant quantum computer assumes its hardware will keep slipping, and is built so that those slips are caught and repaired faster than they accumulate, including slips in the correction machinery itself.

Whether that is possible turns on a threshold. As the Nature paper on fault-tolerant quantum memory puts it, error correction becomes practically realisable once the physical error rate is below a threshold value, and that value depends on the choice of quantum code, the syndrome measurement circuit and the decoding algorithm. Below the threshold, adding more hardware to each logical qubit makes the machine steadily more reliable. Above it, the correction introduces more errors than it removes, and adding hardware makes things worse.

This is why the phrase carries weight in research announcements. A demonstration described as fault-tolerant is claiming to be on the right side of that line, which is a different and much stronger claim than having built a machine that happened to run without visible errors.

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