Sudden death and sudden birth of entanglement

We compare the evolution of entanglement for a system of two two-level atoms interacting with (i) a common reservoir being in the vacuum state and (ii) a common thermal reservoir at non-zero temperature. The Markovian master equation is used to describe the evolution of entanglement which is quantified by concurrence. Phenomena of sudden death, revival, and sudden birth of entanglement are discussed. It is shown that when the atoms behave collectively and the reservoir is the vacuum, the entanglement evolution exhibits quite a rich structure. For thermal reservoir with non-zero mean number of photons this structure is gradually degraded as the temperature of the reservoir increases. For thermal reservoir the phenomenon of sudden death of entanglement becomes a standard behavior, and no asymptotic evolution can be observed. As the temperature of the reservoir increases the sudden birth of entanglement, which is a signature of collective behavior of the atoms in the vacuum, gradually disappears. The results are illustrated graphically for a number of initial states of the two-atom system.