Quantum Measurement

Quantum measurement is the process by which a quantum system's superposition of possible states is collapsed to a definite classical outcome. It is the most thermodynamically and conceptually contentious step in quantum computation: unlike unitary evolution — which is reversible — measurement is irreversible. The information in the unmeasured superposition is destroyed, and by Landauer's Principle, this destruction has a thermodynamic cost.

The measurement problem — why and how superposition collapses — remains foundationally unresolved. The major interpretations (Copenhagen Interpretation, Many-Worlds Interpretation, Decoherence) agree on what measurement produces but disagree on what it is. A theory of quantum computation that ignores the thermodynamics of measurement is not a complete theory — it describes the output while hiding the physics of the process that produces it.