sentences:  As the electromagnetic field is coupled to the state of the atom, and the atom can only absorb a single photon before having to decay, the most basic case then is if the field only contains a single photon.Thus spontaneous decay occurs when the excited state of the atom emits a photon back into the vacuum Fock state of the field formula_11.During this process the decay of the expectation values of the above operators follow the following relations
  
  formula_12,
  
  formula_13.