Fig. 1. Experimental setup for observation of Hong–Ou–Mandel (HOM) interference with two completely independent continuous-wave coherent photons (CWCPs). (a) Schematic of the experimental setup for HOM interference between two independent CWCPs (P, polarizer; H, half-wave plate; Q, quarter-wave plate; BS, beam splitter; SPDs, single-photon detectors; TCSPC, time-correlated single-photon counter; Rb, atomic vapor cell of Rb85; SAS, saturated absorption spectroscopy; PS, polarization spectroscopy). (b) SAS spectrum (blue curve) of ECDL1 and (c) PS spectrum (red curve) of ECDL2 for the 5S1/2(F=3)−5P1/2 (F′=2,3) transition of Rb85.

Fig. 2. Spectral properties of two independent continuous-wave coherent photons. Spectral density spectrum of (a) frequency-stabilized ECDL1 upon error-signal locking of the SAS with frequency modulation and (b) frequency-stabilized ECDL2 obtained by offset locking of the PS.

Fig. 3. Hong–Ou–Mandel interference fringe with two independent continuous-wave coherent photons. Experimental result and theoretical curve fitting under the conditions of a bandwidth of 3.3 MHz and Δω=0.

Fig. 4. Two-photon beat fringes for different offset frequencies. (a) PS spectra (blue: Δω/2π=+3.5  MHz; black: Δω=0; and red: Δω/2π=−3.5  MHz) of ECDL2 frequency-stabilized to the 5S1/2(F=3)−5P1/2(F′=3) transition of Rb85; the horizontal dashed line (gray) indicates the DC-offset reference. (b)–(d) The HOM-type two-photon beating fringes are measured for different offset frequencies.

Fig. 5. Hong–Ou–Mandel interference fringes with arbitrary time delay between two independent CWCP sources. Three single-mode fiber (SMF) spools are employed as relative optical delay lines (ODLs). (a) SMF of 0  m (black curve), (b) SMF of 200  m (red curve), (c) SMF of 400  m (green curve), and (d) SMF of 600  m (blue curve).