Fig. 1. Experimental configuration. (a) HBT experiment. (b) SOI with two independent unbalanced Mach–Zehnder interferometers.

Fig. 2. Superradiant photons from Doppler-broadened cascade-type Rb87 atoms. (a) Cascaded three-level atomic system of 5S1/2−5P3/2−5D5/2 transition of Rb87 atoms. (b) Superradiant photon generation via SFWM process in the Rb87 atomic vapor cell with counterpropagating pump and coupling lasers. (c) Temporal statistical spectrum of signal photons obtained via HBT setup for accumulation time of 180 s.

Fig. 3. Experimental setup for second-order interference with thermal light. SOI obtained with the use of unbalanced Michelson interferometers with large path difference: M, mirror; POL, polarizer; Q, quarter-wave plate; H, half-wave plate; PBS, polarizing beam splitter; SPD, single-photon detector.

Fig. 4. Temporal waveform of real thermal light in Franson-type interferometer from Doppler-broadened cascade-type Rb87 atoms.

Fig. 5. Second-order interference with thermal light in two unbalanced Michelson interferometers. (a) SOI fringe of thermal light as a function of Δx1 with fixed Δx2 (coincidence detection of both SPDs). (b) Absence of first-order interference fringes in both SPD1 (blue circles) and SPD2 (red circles) as a function of Δx1 or Δx2. (c) SOI fringe as a function of Δx1 when Δx1 and Δx2 are varied equally in opposite directions (Δx2=−Δx1). (d) Absence of SOI fringe when Δx1 and Δx2 are varied equally along the same direction (Δx1=Δx2).