Generation of a macroscopic entangled coherent state using quantum   memories in circuit QED
  $W$-type entangled states can be used as quantum channels for, e.g., quantum teleportation, quantum dense coding, and quantum key distribution. In this work, we propose a way to generate a macroscopic $W$-type entangled coherent state using quantum memories in circuit QED. The memories considered here are nitrogen-vacancy center ensembles (NVEs), each located in a different cavity. This proposal does not require initially preparing each NVE in a coherent state instead of a ground state, which should significantly reduce its experimental difficulty. For most of the operation time, each cavity remains in a vacuum state, thus decoherence caused by the cavity decay and the unwanted inter-cavity crosstalk are greatly suppressed. Moreover, only one external-cavity coupler qubit is needed. This method is quite general and can be applied to generate the proposed $W$ state with atomic ensembles or other spin ensembles distributed in different cavities. 
Tong Liu, Qi-Ping Su, Shao-Jie Xiong, Jin-Ming Liu, Chui-Ping Yang,   Franco Nori
quant-ph