Decoherence at constant excitation
  We present a simple exactly solvable extension of of the Jaynes-Cummings model by adding dissipation. This is done such that the total number of excitations is conserved. The Liouville operator in the resulting master equation can be reduced to blocks of $4\times 4$ matrices. 
Juan Mauricio Torres, Emerson Sadurni, Thomas H. Seligman
quant-ph