Quantum correlations of two qubits interacting with a macroscopic medium
  We consider two particles of spin-1/2 interacting with a one-dimensional N-spin array, which is an exactly solvable model. The dynamics of entanglement and quantum discord (QD) of the spins of the two particles is investigated by regarding the 1D N-spin array as the environment. It is found that although the entanglement may suffers a sudden death and a sudden birth in the evolution, it can neither be generated nor become larger than its initial value. Different from the entanglement dynamics, QD can be amplified, and even be generated by the interaction between particles and the common environment. We also observe that QD decays asymptotically to zero and later experiences a rival when the average number of excitation in the 1D N-spin array becomes larger in the case of nonzero inter-distance between two particles. 
Yan Liu, Jing Lu, Lan Zhou
quant-ph