Technical Definition
Consider a full Von Neumann measurement by giving a quantum description to both the system and the measuring apparatus . Suppose the dynamics can be described by a Hamiltonian of the form
where Hself is the self-interaction of the system and Hint is the coupling between the system and the measuring apparatus. Generally, the Hint term is modeled as "turning on" for only a brief time and yielding the ideal (weak) Von Neumann measurement:
This measurement is a QND measurement if
Read more about this topic: Quantum Nondemolition Measurement
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