Possibility of Inhomogeneous Coupling Leading to Decoherence in an   Electromagnetically-Induced-Transparency Quantum-Memory Process
  The effect of inhomogeneous coupling between three-level atoms and external light fields is studied in the electromagnetically induced transparency (EIT) quantum memory techqnique. By introducing a subensemble-atomic system to deal with present inhomogeneous coupling case, we find there is a non-symmetric dark-state subspace (DSS) that allows the EIT quantum memory technique to function perfectly. This shows that such memory scheme can work ideally even if the atomic state is very far from being a symmetric one. 
Xiong-Jun Liu, Zheng-Xin Liu, Xin Liu and Mo-Lin Ge