New aspects of the purity and information of an entangled qubit pair
  In this article, we investigate the purity dynamics of entangled 2 two-level atoms interacting with a single quantized electromagnetic field. We show that the purity of the qubit pairs depends on the initial state of the atomic system. It is found that the superposition case is the best choice to generate entangled states with high purity and hence high entanglement. It is clear that the purity of one qubit can be purified at the expense of the other pair through the phenomena of purity swapping. The mean photon number plays an important role in increasing the purity. The robustness of the quantum channel is investigated in the presence of individual attacks, where we study the separability of these channels and evaluate its fidelity. Finally, we use the partial entangled states as quantum channels to perform the original coding protocol. We find that Bob can obtain the coded information with reasonable percentage. The inequality of security is tested, where we determine the interval of times in which Alice and Bob can communicatesecurely. These intervals depend on the type of error and the structure of the initial atomic system 
Nasser Metwally