Fig. 1. (a) Sketch of the experimental setup and (b) the energy diagram of SFWM. (PBS, polarizing beam splitter; BS, beam splitter; SMF, single mode fiber; Fs and Fas, filters in the Stokes path and anti-Stokes path; SPCM, single-photon counting module; AL, auxiliary laser.)

Fig. 2. Spectral scan of the fluorescence in the Stokes path. (a) Scan the first etalon with the second etalon removed. The strong peak near the null point is the on-resonance fluorescence noise, and the weak peak near the −2.7  GHz detuning is the Stokes signal. (b) Scan the second etalon. Clearly, the fluorescence noise is significantly filtered. (The solid lines in all of the figures are guides for the eyes.)

Fig. 3. Fluorescence intensity and cross correlation function versus the optical pumping beam power.

Fig. 4. (a) Photon pairs coincidence counts with 90 s collection time. (b) The conditional autocorrelation function gc(2) of heralded single anti-Stokes photons with 35 min acquisition time.

Fig. 5. Cross correlation function as a function of (a) pump power and (b) coupling power.

Fig. 6. Cross correlation function and coincidence counts versus the vapor temperature. The power of pump, coupling, and optical pumping beams are 20, 29, and 170 mW, respectively.