Gate fidelity

The probability that one quantum operation does exactly what was intended, usually quoted as a percentage such as 99.9%.

• 99.9% means about one error per thousand operations, and errors compound: 1,000 operations at 99.9% all succeed only about 37% of the time.
• Two-qubit gates, which create entanglement, are harder and usually the limiting figure; single-qubit gates often have about 10× fewer errors.
• Error-correction codes need gates past a threshold of roughly 99%, and only become practical well above it, at 99.9% or better.
• Check how it was measured: the best pair of qubits or the chip average, and whether readout and idle errors are counted.