Quantum measurement theory describes a mechanism where unitary interactions between a system and an apparatus generate entangled states in Schmidt form, linking orthogonal projectors on the system to basis shifts on the apparatus. This framework realizes orthogonal measurements acting on closed quantum systems by leveraging ancillary degrees of freedom without assuming direct access for prior knowledge, governed strictly by axioms derived from inner product preservation via unitary operators $U$ where $UU^\dagger = I$. The domain belongs to foundational Quantum Mechanics and operational physics, distinguishing between measurement in the apparatus eigenbasis versus arbitrary orthogonal bases which induce specific post-measurement state projections dependent on the choice of Schmidt basis.
Quantum measurement theory describes a mechanism where unitary interactions between a system and an apparatus generate entangled states in Schmidt form, linking orthogonal projectors on the system to…