| Summary: | Many observers can simultaneously measure different parts of an environment of a quantum system in order to find out its state. To study this problem we generalize the formalism of conditional master equations to the multiple observer case. To settle some issues of principle which arise in this context (as the state of the system and of the environment are ultimately correlated), we consider an example of a system qubit interacting through controlled nots (CNOTs) with environmental qubits. The state of the system is the easiest to find out for observers who measure in a basis of the environment which is most correlated with the pointer basis of the system. In this case the observers agree the most. Furthermore, the more predictable the pointers are, the easier it is to find the state of the system, and the better is the agreement between different observers.
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