Quantum measurement

A physical interaction that yields an outcome governed by quantum probabilities and updates the system’s state relative to that outcome.

Born rule: The rule assigning probabilities to measurement outcomes from a quantum state's amplitudes. It determines the probabilities of outcomes before measurement.
Many-worlds interpretation: An interpretation in which quantum evolution never collapses and all outcomes occur in branching components of the universal state. It rejects fundamental collapse while explaining why observers experience definite records.
Niels Bohr: A Danish physicist who helped establish quantum theory and emphasized the role of experimental arrangements in defining observations. His complementarity framework made measurement conditions central to quantum descriptions.
Quantum tomography: A collection of methods for reconstructing a quantum state from statistics of repeated measurements. It infers an unknown state by combining results from different measurement settings.
Quantum Zeno effect: The inhibition of a quantum system's evolution by sufficiently frequent measurements. It shows that measurement frequency can alter the evolution being measured.
Quantum state: A mathematical description encoding the possible outcomes and probabilities of a quantum system. Measurement outcomes are predicted from the state, which is then updated relative to the result.
Objective-collapse theory: A family of theories in which quantum state reduction is a real physical process with altered dynamics. It treats measurement-like collapse as an objective event rather than a state update alone.
Werner Heisenberg: A German physicist who formulated matrix mechanics and the uncertainty principle. His early account of measurement helped frame the relation between observation and quantum state.
Quantum nondemolition measurement: A measurement designed to determine an observable while preserving its value for later repeated measurements. It extracts information about selected quantities without destroying the measured value.
Leggett–Garg inequality: An inequality constraining correlations between measurements under assumptions of macroscopic realism and noninvasive measurability. Its experimental tests probe whether measurement disturbance is needed to explain quantum temporal correlations.

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