Model Identification for Spin Networks

We consider the problem of determining the unknown parameters of the Hamiltonian of a network of spin 1/2 particles. In particular, we study experiments in which the system is driven by an externally applied electro-magnetic field and the expectation value of the total magnetization is measured. Under appropriate system theoretic assumptions, we prove that, if it is possible to prepare the system in a known initial state, the above experiment allows to identify the parameters of the Hamiltonian. In the case where the initial state is itself an unknown parameter, we characterize all the pairs Hamiltonian-Initial State which give the same value of the magnetization, for every form of the driving electro-magnetic field. The analysis is motivated by recent results on the isospectrality of Hamiltonians describing magnetic molecules.