Quantum premeasurement

The coherent interaction that correlates a system observable with detector records before selecting an outcome. An ideal coupling maps $|s_j⟩|d_0⟩$ to $|s_j⟩|d_j⟩$. Linearity then maps a superposition to ${\sum }_j{c}_j|s_j{d}_j⟩$. The full state can remain pure and entangled, whereas a nonselective projective measurement removes coherence between different record sectors. Conditioning on one observed record is a third, selective operation.