Cooperative Two-Quantum Interaction of Excited System with Bath
  The problem of the two-photon coherent generation of entanglement photon pairs in Quantum Optics has been intensively studied for the last years. It is important to note that the two-quantum cooperative effects play a main role in other fields of physics as well. The collective processes in Condensed Matter have many analogical proprieties with cooperative radiation effects in Quantum Optics. Recently the two-quantum generation of entanglement photons and its applications in information technology and communication has been intensively studied. The new cooperative emission phenomenon for dipole-forbidden transitions of inverted system of radiators can be observed in the processes of the two-photon spontaneous emission. 
Nicolae A. Enaki and Vitalie Eremeev