On the meaning of entanglement in quantum measurement

Abstract

Measurement interaction between a measured object and a measuring instrument, if both are initially in a pure state, produces a (final) bipartite entangled state vector. The quasi-classical part of the correlations in it is connected with transmission of information in the measurement. But, prior to "reading" the instrument, there is also purely quantum entanglement in the final state vector. It is shown that in repeatable measurement quantitatively the entanglement equals the amount of incompatibility between the measured observable and the final state. It also equals the amount of incompatibility of the observable and the initial state of the object.