Experimental setup for the second-order interference of two tunable single-mode lasers. L₁ and L₂ are two identical grating stabilized tunable single-mode diode lasers (DL100, Toptica Photonics). The central wavelength and frequency bandwidth of the laser are 780 nm and 100 kHz, respectively. P₁ and P₂ are two linearly polarizers to ensure that the polarizations of these two light beams are identical. D_F is a fast amplified silicon detector (ET-2030A, Electro-Optics Technology, Inc.). S is a spectrum analyzer (Agilent E441B) to monitor the frequency difference between these two lasers. D₁ and D₂ are two single-photon detectors (SPCM-AQRH-14-FC, Excelitas Technologies) and CC is two-photon coincidence counting system (SPC630, Becker & Hickl GmbH). BS is a 1:1 non-polarizing beam splitter. A is an optical attenuator to decrease the intensity of light so that the single-photon counting rates of both detectors are around 50000 c/s. FBS is a 1:1 non-polarizing fiber beam splitter, which is employed to ensure that the positions of these two detectors are identical. The optical distance between the laser and D_F is equal to the one between the laser and the collector of FBS, which is 525 mm. The length of FBS is 2 m.