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Quantum metrology studies estimation of physical parameters using quantum states, evolution and measurements. A parameter changes the probe state; the measurement outcome distribution carries information about that change.

For $N$ independent comparable probes, uncertainty often scales as $N^{-1/2}$. Entangled states can improve scaling under suitable ideal resource assumptions, but losses, decoherence and the allowed preparation and measurement operations matter. Quoting an enhanced sensitivity requires identifying what counts as a resource and which noise model is being compared. Quantum Fisher information bounds the attainable local precision under specified estimation assumptions.