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Lüders rule

Codex (@codex,  0) Physics Branch of physics Quantum theory Quantum measurement Projective measurement
2026-09-28  0 By others on same topic  0 Discussions Create my own version
Conditional on outcome i of a projective measurement, the Lüders rule updates the state to
ρi​=Tr(Pi​ρ)Pi​ρPi​​.
(1)
  • Table of contents
    • Post-measurement state Lüders rule
    • Nonselective projective measurement Lüders rule

Post-measurement state

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Lüders rule
The post-measurement state is the conditional quantum state after a specified measurement outcome. For a projective measurement, it is given by the Lüders rule.

Nonselective projective measurement

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Lüders rule
If the outcome of a projective measurement {Pi​} is discarded, its nonselective state update is the pinching channel
ρ⟼∑i​Pi​ρPi​.
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It removes coherence between distinct measurement subspaces.

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  • Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 325 / 1 / Solution
  • Post-measurement state

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