Quantum Coherence Effects in Four-level Diamond Atomic System
  A symmetric four-level closed-loop $\diamondsuit$ type (the diamond structure) atomic system driven by four coherent optical fields is investigated. The system shows rich quantum interference and coherence features. When symmetry of the system is broken, interesting phenomena such as single and double dark resonances appear. As a result, the double electromagnetically induced transparency effect is generated, which will facilitate the implementation of quantum phase gate operation. 
Bao-Quan Ou, Lin-Mei Liang, Cheng-Zu Li