Readout fidelity
Readout fidelity is the probability that measuring a qubit reports the state it was actually in. Every shot pays this cost once per measured qubit, so it caps the accuracy of every result.
What is readout fidelity?
Also called measurement or assignment fidelity — often decomposed into P(read 0 | prepared 0) and P(read 1 | prepared 1), which typically differ. A 98% readout on 10 measured qubits means roughly one shot in five contains at least one misread qubit before any gate errors are counted.
How is it measured?
Usually measured by preparing computational basis states and tallying assignment errors; sometimes reported after software mitigation (readout-error correction), which is a different claim. Vendors do not always state whether a figure is raw or mitigated, per-qubit or device-median.
Protocols are covered in Krantz et al., 2019.When are two values NOT comparable?
Not comparable when one figure is raw and the other mitigated; when aggregations differ; or when measurement durations differ enormously (ion readout is slower but more accurate — another speed/quality trade). Where a vendor did not state the method, the QPU137 record says "unspecified by vendor" rather than guessing.
See it in the data: the sourced catalog · compare two processors · lesson: reading hardware specs