Orthogonality relations in Quantum Tomography
  Quantum estimation of the operators of a system is investigated by analyzing its Liouville space of operators. In this way it is possible to easily derive some general characterization for the sets of observables (i.e. the possible quorums) that are measured for the quantum estimation. In particular we analyze the reconstruction of operators of spin systems. 
G. M. D'Ariano, L. Maccone and M.G.A. Paris