A measurement concept defining how outcomes are modeled and how state descriptions are updated after an outcome is recorded. It governs outcome probabilities, information extraction, and the disturbance introduced by the measurement interaction. It does not yield reliable inference without adequate calibration, sufficient data, and appropriate estimation procedures. It supports reconstruction and validation of state and process descriptions from experimental statistics. The concept is generally stable, though practical implementations and estimation methods evolve over time.

Principle

Readout fidelity organizes the reliability of the measurement mapping from quantum state space to recorded classical outcomes; it quantifies stochastic assignment errors and systematic bias in the final digitized result.

Demonstration

Single-qubit dispersive readout: prepare |0⟩ and |1⟩ many times, collect analog signals, apply the assignment rule (threshold or classifier) and compute fraction of correct assignments. If 98% of preparations of |0⟩ are recorded as 0 and 97% of |1⟩ as 1, readout fidelity is commonly reported as the average 97.5%.

Misapplication

Treating readout fidelity as a measure of gate performance or assuming a reported readout fidelity holds under different bias conditions or measurement bases without recharacterization.

Consequence

High readout fidelity reduces bias in state tomography, improves syndrome extraction for error correction, and tightens confidence intervals in benchmarking; it sets a lower bound on observable process infidelity when measurement is the final step.

Reversal

Readout infidelity inverts the mapping: systematic misassignment or randomization of measurement outcomes makes recorded data diverge from actual state populations and can mask coherent gate errors.

Boundary

Readout fidelity refers specifically to the measurement-and-assignment stage (analog signal → classical outcome) and excludes upstream contributions from imperfect state preparation and gate errors unless explicitly combined; it is basis- and operating-point-dependent.

Semantic Tension

The term competes with related notions such as measurement efficiency, assignment error, and QND fidelity; average assignment fidelity can hide asymmetric error rates (false-positive vs false-negative) that matter in many protocols.

Synthesis

Readout fidelity concisely measures how reliably a detector converts a quantum state into the correct classical label under a defined preparation and assignment procedure; it is a pragmatic, basis-dependent metric that must be reported with operating conditions and error asymmetries.