 In Ozawa’s approach, we assume that a system \(\cal S\) of interest in state \(\ket{\psi}\) is coupled to a measurement device \(\cal M\) in state \(\ket{\chi}\), and their interaction is governed by a unitary operator \(U\). On the Hilbert space of the joint system the observable \(\bQ\) of the system \(\cal S\) we are interested in is represented by