State tomography for two qubits using reduced densities
  The optimal state determination (or tomography) is studied for a composite system of two qubits when measurements can be performed on one of the qubits and interactions of the two qubits can be implemented. The goal is to minimize the number of interactions to be used. The algebraic method applied in the paper leads to an extension of the concept of mutually unbiased measurements. 
D. Petz, K.M. Hangos, A. Szanto and F. Szollosi