Generation of 1.5 {\mu}m discrete frequency entangled two-photon state   in polarization maintaining fibers
  In this Letter, the generation of 1.5 {\mu}m discrete frequency entangled two-photon state is realized based on a piece of commercial polarization maintaining fiber (PMF). It is connected with a polarization beam splitter to realize a modified Sagnac fiber loop (MSFL). Correlated two-photon states are generated through spontaneous four wave-mixing process along the two propagation directions of the MSFL, and output from the MSFL with orthogonal polarizations. The quantum interference of them is realized through a 45{\deg} polarization collimation between polarization axes of PMFs inside and outside the MSFL, while the phase difference of them is controlled by the polarization state of the pump light. The frequency entangled property of the two-photon state is demonstrated by a spatial quantum beating experiment with a fringe visibility of (98.2+/-1.3)%, without subtracting the accidental coincidence counts. The proposed scheme generates 1.5 {\mu}m discrete frequency entangled two-photon state in a polarization maintaining way, which is desired in practical quantum light sources. 
Qiang Zhou, Wei Zhang, Chenzhi Yuan, Yidong Huang, and Jiangde Peng
quant-ph