摘要
Instead of an infinite number of atoms in the thermodynamic limit, we study the ground states of a finite number of three-level atoms in electromagnetically induced transparency media. With the help of a classical control field, critical coupling strengths are derived analytically for the existence of quantum phase transitions in the ground states of this extended Dicke model. Compared to the classical limit, evolution of a finite-sized excitation during the storage and retrieval process is also illustrated, as well as atom–field entanglement. The results derived in this work provide the connection not only to the Dicke model, but also to the Lipkin–Meshkov–Glick model.