  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.
