We have identified a class of many body problems with analytic solution
beyond the mean-field approximation. This is the case where each body can be
considered as an element of an assembly of interacting particles that are
translationally frozen multi-level quantum systems and that do not change
significantly their initial quantum states during the evolution. In contrast,
the entangled collective state of the assembly experiences an appreciable
change. We apply this approach to interacting three-level systems.Comment: 5 pages, 3 figures. Minor correction