= Quantum nondemolition measurement
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= Non-demolition measurement
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An ideal measurement preserving the measured eigenspaces. In the projective setting, the selective instrument $\rho\mapsto P_j\rho P_j$ preserves every vector within outcome $j$, including coherence inside a degenerate eigenspace. Repeated measurements agree in the absence of intervening dynamics changing the measured quantity. Measuring a finer observable can destroy degeneracy coherence and need not realize the same <Lüders rule> instrument.
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