Physical Realizability of Multi-Level Quantum Systems
  This paper considers the physical realizability condition for multi-level quantum systems having polynomial Hamiltonian and multiplicative coupling with respect to several interacting boson fields. Specifically, it generalizes a recent result the authors developed for two-level quantum systems. For this purpose, the algebra of SU(n) was incorporated. As a consequence, the obtained condition is given in terms of the structure constants of SU(n). 
Luis A. Duffaut Espinosa, Zibo Miao, I. R. Petersen, V. Ugrinovskii   and M. R. James
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