Light with zero second-order autocorrelation function, g (2) (0), emitted by single-photon source (SPS), for example, two-level system (TLS), is usually considered as being in a Fock state with one photon in a certain electromagnetic mode of free space. However, real SPSs have finite linewidths and excite all modes lying within the linewidth. It is shown that the zero value of g (2) (0) of light emitted by an incoherently pumped TLS is a consequence of the quantum interference of electromagnetic modes lying within the linewidth. Applying the quantum regression theorem for out-of-time-ordered correlation functions, an analytical expression for g (2) (0) is obtained for the light emitted by a TLS and passed through a spectral filter. It is shown that narrowing the spectral width of the band-pass filter inevitably leads to an increase in g (2) (0). g (2) (𝝉) is found and it is demonstrated that certain spectral filters lead to its nonmonotonic time dependence. These results open up the possibility of controlling the statistics of the emitted light using a spectral filter, which can find applications in quantum communication and cryptography.

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Light with high mutual correlations at different frequencies can be used to create heralded single-photon sources, which may serve as the basic elements of existing quantum cryptography and quantum teleportation schemes. One of the important examples in natural systems of light with high mutual correlations is the light produced by spontaneous Raman scattering on an ensemble of molecules. In this paper, we investigate the possibility of using Raman light to create a heralded single-photon source. We show that when using Stokes scattered light for postselection of anti-Stokes scattered light, the latter may possess single-photon properties. We analyze the influence of various negative factors on the characteristics of such a heralded single-photon source, which include a time delay between Stokes and anti-Stokes photons, the finiteness of the correlation radius of an external source, and background radiation. We show that the low value of the second-order autocorrelation function of the single-photon source is preserved even when the flow of uncorrelated photons exceeds the flow of correlated photons in scattered Raman light by an order of magnitude.

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