Entanglement Changes under Various Spin-Environment Models

We have studied the effects of various environments on the degree of entanglement of a two-qubit system with no direct interaction. The interaction between the system and the environment is considered as a two-spin interaction. We have focused on the case in which each system qubit interacts with different environment qubits so that indirect interactions are not involved. The results show that the two-qubit system with no initial entanglement that has no direct interaction can be entangled only under interactions with entangled elements in a common bath. This implies that an entanglement of the environment can be transferred to an entanglement of a two-qubit system that has no direct and indirect interactions.