ABSTRACT
In systems theory, the study of the observability of a system describes to what extent it is possible to determine the initial state by a combined action of dynamics and output measurement. This chapter briefly summarizes the main ideas of quantum state tomography which is the technique used to determine a quantum state via a series of measurements. It discusses a system theoretic treatment of the problem of state determination namely to the study of the observability of quantum systems. The chapter partitions the space of density matrices into equivalence classes of states that cannot be distinguished by appropriate control and measurement.
