1. Noise models

Common models include bit flips, phase flips, depolarizing noise, amplitude damping, leakage, readout error, and correlated crosstalk. Real hardware often combines several nonstationary effects.

Noise is structured and time-varying, not a single error percentage.

2. Benchmarking

Metrics include T1/T2 coherence, gate and readout fidelity, randomized benchmarking, quantum volume, circuit-layer operations per second, and application-level benchmarks. Each captures only part of system quality.

No single metric summarizes all useful performance.

3. Error mitigation vs correction

Mitigation techniques estimate cleaner outputs without creating fully protected logical qubits. Examples include readout mitigation, zero-noise extrapolation, symmetry verification, and probabilistic error cancellation. Error correction actively encodes and protects logical information.

Mitigation can extend experiments, but does not provide unlimited reliable depth.

4. Applied activity

Complete a simulator or analysis exercise: reproduce the lesson's central example, record assumptions and outputs, and explain one source of error or limitation.