Generation of Narrow-Band Polarization-Entangled Photon Pairs for Atomic   Quantum Memories
  We report an experimental realization of a narrow-band polarization-entangled photon source with a linewidth of 9.6 MHz through cavity-enhanced spontaneous parametric down-conversion. This linewidth is comparable to the typical linewidth of atomic ensemble based quantum memories. Single-mode output is realized by setting a reasonable cavity length difference between different polarizations, using of temperature controlled etalons and actively stabilizing the cavity. The entangled property is characterized with quantum state tomography, giving a fidelity of 94% between our state and a maximally entangled state. The coherence length is directly measured to be 32 m through two-photon interference. 
Xiao-Hui Bao, Yong Qian, Jian Yang, Han Zhang, Zeng-Bing Chen, Tao   Yang, Jian-Wei Pan