We report the temporal intensity correlation (TIC) of scattered photons (SPs) generated via a two-photon transition in a Doppler-broadened warm atomic vapor of the 5S1/2 − 5P3/2 − 5D5/2 transition of 87Rb atoms. Through the investigation of the TICs of the SPs obtained via both one- and two-photon transitions, the second-order correlation values g(2)(0) (i.e., at zero time delay) of both SPs were measured as approximately 1.75, respectively. The widths of the g(2)(τ) spectra were measured as 26 ns (corresponding to the natural lifetime of the 5P3/2 state) for the one-photon transition and 1.8 ns (corresponding to the Doppler width of the warm atomic vapor) for the two-photon transition. We confirmed that the coherence time of the SPs can vary in accordance with the photons emitted from the one- or two-photon transitions in the ladder-type atomic system. The correlated SPs obtained via the two-photon transition contributed to almost all the velocity classes of the atoms in the Doppler-broadened atomic ensemble.
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