Entanglement generation in double-$\Lambda $ system
  In this paper, we study the generation of entanglement in a double-$\Lambda $ system. Employing standard method of laser theory, we deduce the dynamic evolution equation of the two-mode field. We analyze the available entanglement criterion for double-$\Lambda $ system and the condition of entanglement existence. Our results show that under proper parameters, the two-mode field can entangled and amlified. 
Ling Zhou, Yong Hong Ma, Xin Yu Zhao