Abstract The decay of quantum system translated into the excited state depends on the manner of excitation and the system parametres as well [1]. In this case when the system was let to itself after the excitation its next development may be two-fold under the influence of spontaneous transitions of particles from the excited state to the ground state. The first one is the uncoherent spontaneous radiation. In this case all particles of the system are independently radiating. The total power of this radiation is the sum of spontaneous radiation powers of isolated particles. The second case is the coherent radiation, i.e. the particles are radiating into the coherence volume νωh = (c·T′2)3 with a single phase (C is the field velocity in the medium and T′2 is the phase relaxation time of the system). In the latter case the amplitude of the radiated field is propotional to the particle number N and the power is propotional to the square of particle number. However the second case is realized under the specifi...