Fig. 1: Experimental setup.
The probe beams E1 and E1′ counter- and co-propagates with the pump beams (E2 and E2′), respectively. The counter-propagating E1 and two pump beams can generate the pumping four-wave mixing (pumped FWM) signal (EF), which emits along the opposite direction of E2. The output probe and pumped FWM spectra are received by three APDs. PBS: polarization beam splitter; BS: 50/50 beam splitter; APD: avalanche photodiode detector; HW: half-wave plate; HR: high-reflectivity mirror. The inset marked by a dashed box shows the atomic configuration for the pumped FWM
