Cooperative Spontaneous Emission as a Many Body Eigenvalue Problem
  We study emission of a single photon from a spherically symmetric cloud of N atoms (one atom is excited, N-1 are in ground state) and present an exact analytical expression for eigenvalues and eigenstates of this many body problem. We found that some states decay much faster then the single-atom decay rate, while other states are trapped and undergo very slow decay. When size of the atomic cloud is small compared with the radiation wave length we found that the radiation frequency undergoes a large shift. 
Anatoly Svidzinsky and Jun-Tao Chang