A Constructive Unitary Model of Quantum Measurement: Born-Consistent Outcomes and Stable Records

Quantum measurement combines several logically distinct tasks: a microscopic source sector must become correlated with a physical output, the record-conditioned sector weights must become effectively extremal, the apparatus must register the information in one mutually exclusive macroscopic basin, and the record must remain stable. Classical actuality is a separate network-level record-ontology claim rather than an additional term in the microscopic dynamics. We construct a finite, calculable realization of the dynamical chain using a quantum-nondemolition qubit coupled to a two-photon-driven Kerr parametric oscillator (KPO) with continuously monitored homodyne output. A unitary qubit–pointer–field dilation makes the origin of decoherence, output noise, and conditioning explicit; the stochastic master equation is a filter relative to the specified field record, not an objective-collapse law. Exact QND commutation makes the conditioned source-sector weight a bounded martingale. Under asymptotic output identifiability, the induced source-sector measure becomes extremal almost surely, with terminal hitting probabilities equal to the initial Born sector weights. This establishes Born consistency of the instrument, not a derivation of the Hilbert-space probability rule from dynamics alone. The nonlinear KPO supplies biased symmetry-breaking-like amplification and metastable single-basin registration. Its pointer separation scales as √N while the effective semiclassical noise scale is N^(−1/2). Full quantum trajectories show synchronization between conditional extremalization and pointer locking. Over the simulated finite-size range, wrong registration and later record corruption decrease approximately exponentially with distinct rates; the corruption exponent is independently supported by reduced first-passage, Liouvillian-metastability, and Freidlin–Wentzell minimum-action calculations to within a few percent. The model therefore realizes conditional extremalization, single-basin registration, and metastable storage within standard unitary-background quantum mechanics, without deleting unconditioned global components or using a fundamental stochastic selector. Within the companion Framework, this stored KPO record is a local building block: if it is later embedded with other persistent co-recordable records in one commutative record algebra, its probabilities become compatible restrictions of the single classical record measure induced by the universal quantum state, while coherent single-valued actuality remains the separate ontological step.