Quantum properties and dynamics of X states
  X states are a broad class of two-qubit density matrices that generalize many states of interest in the literature. In this work, we give a comprehensive account of various quantum properties of these states, such as entanglement, negativity, quantum discord and other related quantities. Moreover, we discuss the transformations that preserve their structure both in terms of continuous time evolution and discrete quantum processes. 
Nicol\'as Quesada, Asma Al-Qasimi and Daniel F. V. James
quant-ph