Frequency-up conversion and quantum swap gate in an optical cavity with   atomic cloud
  A scheme is presented for realizing frequency-up conversion and a two-qubit quantum swap gate for intracavity fields. In the scheme, a V-type atomic ensemble prepared in their ground states collectively mediates the interaction between the two cavity modes. Under certain conditions, the cavity-field degree of freedom is decoupled from the atomic degrees of freedom, and the effective coupling strength between the two cavity modes scales up with $\sqrt{N}$ ($N$ is the number of atoms). The numerical simulation shows that the quantum swap gate still has a high fidelity under the influence of the atomic spontaneous emission and the decay of the cavity modes. 
Gong-Wei Lin, Xu-Bo Zou, Ming-Yong Ye, Xiu-Min Lin, Guang-Can Guo