(3 EPP based on spatial entanglement Mirror al bl PBS PBS a2 a2 b2 b2 Pump HPDC=y(aibi +aibi+red(a2hbrH+a2vb2v)]+Hc 单对光子:(aPb1B+a1biv+a2b2+a2yb2)0) 两对光子:(ab+n+aib+a2b21+a2b2y2 C. Simon, Jian-Wei Pan, Phys. Rev. Lett. 89, 257901(2002)(3) EPP based on spatial entanglement 单对光子： 两对光子： C. Simon, Jian-Wei Pan, Phys. Rev. Lett. 89, 257901 (2002)