Observation of frequency-uncorrelated photon pairs generated by counter-propagating spontaneous parametric down-conversion. - Liu, Yi-Chen; Guo, Dong-Jie; Ren, Kun-Qian; Yang, Ran; Shang, Minghao; Zhou, Wei; Li, Xinhui; Sun, Chang-Wei; Xu, Ping; Xie, Zhenda; Gong, Yan-Xiao; Zhu, Shi-Ning (Scientific Reports, 2021)