Entanglement in Weisskopf-Wigner theory of atomic decay in free space
  In this paper, we use the Weisskopf-Wigner theory to study the entanglement in the state of the free-space radiation field produced from vacuum due to atomic decay. We show how bipartite entanglement is shared between different partitions of the radiation modes. We investigate the role played by the size of the partitions and their detuning with the decaying atom. The dynamics of the atom-field entanglement during the atomic decay is also briefly discussed.   From this dynamics, we assert that such entanglement is the physical quantity that fix the statistical atomic decay time. 
J. F. Leandro and F. L. Semi\~ao