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We systematically studied the temporal profile of biphotons, i.e., pairs oftime-correlated single photons, generated from a hot atomic vapor via thespontaneous four-wave mixing process. The measured temporal width of biphotonwave packet or two-photon correlation function against the coupling power wasvaried from about 70 to 580 ns. We derived an analytical expression of thebiphoton's spectral profile in the Doppler-broadened medium. The analyticalexpression reveals that the spectral profile is mainly determined by the effectof electromagnetically induced transparency (EIT), and behaves like aLorentzian function with a linewidth approximately equal to the EIT linewidth.Consequently, the biphoton's temporal profile influenced by the Dopplerbroadening is an exponential-decay function, which was consistent with theexperimental data. Employing a weak input probe field of classical light, wefurther measured the EIT spectra under the same experimental conditions asthose in the biphoton measurements. The theoretical predictions of the biphotonwave packets calculated with the parameters determined by the classical-lightEIT spectra are consistent with the experimental data. The consistencydemonstrates that in the Doppler-broadened medium, the classical-light EITspectrum is a good indicator for the biphoton's temporal profile. Besides, themeasured biphoton's temporal widths well approximated to the predictions of theanalytical formula based on the biphoton's EIT effect. This study provides ananalytical way to quantitatively understand the biphoton's spectral andtemporal profiles in the Doppler-broadened medium.

Temporal profile of biphotons generated from a hot atomic vapor and spectrum of electromagnetically induced transparency