Compact source of narrow-band counterpropagating polarization-entangled   photon pairs using a single dual-periodically poled crystal
  We propose a scheme for the generation of counterpropagating polarization-entangled photon pairs from a dual-periodically poled crystal. Compared with the usual forward-wave type source, this source, in the backward-wave way, has a much narrower bandwidth. With a 2-cm-long bulk crystal, the bandwidths of the example sources are estimated to be 3.6 GHz, and the spectral brightnesses are more than 100 pairs/(s GHz mW). Two concurrent quasi-phase-matched spontaneous parametric down-conversion processes in a single crystal enable our source to be compact and stable. This scheme does not rely on any state projection and applies to both degenerate and non-degenerate cases, facilitating applications of the entangled photons. 
Yan-Xiao Gong, Zhen-Da Xie, Ping Xu, Xiao-Qiang Yu, Peng Xue, and   Shi-Ning Zhu
quant-ph