Fig. 1. (a) SFWM experimental setup. The Rb85 atomic ensemble is prepared in a two-dimensional MOT. Energy level for photon pairs in Rb85 atomic ensemble. |1〉=|5S1/2, F=2〉, |2〉=|5S1/2, F=3〉, |3〉=|5P1/2,F=3〉, |4〉=|5P3/2, F=3〉. HOM interferometer: single photons from MOT1 and MOT2 mix at a 50:50 BS. We use four SPCMs to detect the photons. (b) Timing control for write and read lasers in MOT1 and MOT2. For each MOT, the write and read lasers are separated by an identical delay time and the total experiment time consists of N write-read cycles. In our experiment, we use the Δt between the starting edge of the pump pulses of MOT1 and MOT2 to separate the temporal waveforms of the independent single photons.

Fig. 2. (a) Normalized coincidence rates of MOT1 and MOT2, τ=tas−ts is the relative time delay of the anti-Stokes photons. (b) Fourfold coincidence as a function of δt, the time difference between the arrival of the Stokes photons. The experimental data show two cases: Δt=0  ns (blue triangles) and Δt=−150  ns (dark squares). A time step of δt axis is 30 ns.

Fig. 3. Normalized coincidence probability as a function of Δt. The accidental coincidence is also shown in the figure. The whole figure expresses the “HOM DIP.” The solid line is the Gaussian fit for normalized coincidence probability. From the fitted line, we get the visibility in the experiment=0.91.

Fig. 4. Visibility versus time bin. The visibility is consistent, with error bars shown. The HOM dip can be measured by any single-photon counting module.