  The assemblies of either quantum dots or magnetic nanoclusters are studied.
It is shown that such assemblies can produce coherent radiation. A method is
developed for solving the systems of nonlinear equations describing the
dynamics of such assemblies. The method is shown to be general and applicable
to systems of different physical nature. Despite mathematical similarities of
dynamical equations, the physics of the processes for quantum dots and magnetic
nanoclusters is rather different. In a quantum dot assembly, coherence develops
due to the Dicke effect of dot interactions through the common radiation field.
For a system of magnetic clusters, coherence in the spin motion appears due to
the Purcell effect caused by the feedback action of a resonator. Self-organized
coherent spin radiation cannot arise without a resonator. This principal
difference is connected with the different physical nature of dipole forces
between the objects. Effective dipole interactions between the radiating
quantum dots, appearing due to photon exchange, collectivize the dot radiation.
While the dipolar spin interactions exist from the beginning, yet before
radiation, and on the contrary, they dephase spin motion, thus destroying the
coherence of moving spins. In addition, quantum dot radiation exhibits
turbulent photon filamentation that is absent for radiating spins.
