Parametric down-conversion in a nonlinear crystal-generated photon pair in which photons have orthogonal polarization. The pair was emitted in a cone shape for each polarization and collimated by a lens to be doughnut shaped. The doughnutlike photon pair runs collinear to the pump laser. The group delay and walk-off between the orthogonal polarization photons in the pair were compensated passing through a half-wave plate and a nonlinear crystal. Then, the photon doughnut pair was coupled into a 1 x 2 fiber together with a pump laser, i.e., the intense pump laser was used as a guide for alignment. After the alignment was finished, a cut filter was inserted in the optical path to cut the pump laser transmitting the photon doughnut pair. The coincidence measurement of the output of the 1 x 2 fiber showed that the photon pairs are entangled in polarization.
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机译：非线性晶体生成的光子对中的参数下转换，其中光子具有正交极化。该对以每种偏振的圆锥形发射，并通过透镜准直成甜甜圈形。环形的光子对与泵浦激光器共线。通过半波片和非线性晶体补偿了成对的正交偏振光子之间的群延迟和走离。然后，将光子圆环对与泵浦激光器一起耦合到1×2光纤中，即，将强泵浦激光器用作对准的引导。对准完成后，将切割滤光片插入光路中，以切割传输光子圆环对的泵浦激光器。 1 x 2光纤输出的重合测量表明，光子对纠缠在偏振中。
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