Two single-photon sources in an HOM interferometer. S_A and S_B are two independent single-photon sources. BS is a 50:50 non-polarizing beam splitter. D₁ and D₂ are two single photon detectors. CC is a two-photon coincidence count system. l_A and l_B are the transverse sizes of S_A and S_B, respectively. z A 1 is the lenth between S_A and D₁ via BS. z_A2, z_B1, and z_B2 are defined similarly.