Fig. 2: Measurements of the first degree of coherence \({g}_{{{{\rm{S}}}}}^{(1)}(\tau )\) of Stokes light.

The coherence of heralded QNG light \({g}_{{{{\rm{h,S}}}}}^{(1)}(\tau )\) and of unconditioned Stokes field is shown as blue and gray plots, respectively. The solid curves correspond to the sum of a two Gaussian functions used for an effective quantitative evaluation of the temporal widths of the measured of g⁽¹⁾(τ). The insets show the heralded intensity correlation function \({g}_{{{{\rm{h,S}}}}}^{(2)}(\tau )\) evaluated from its direct measurement as black data points and a scheme of Michelson interferometer with a tunable relative path difference Δs covering expectable heralded Stokes temporal wavepackets on the order of ten ns. The measured \({g}_{{{{\rm{h,S}}}}}^{(2)}(\tau )\) can be plausibly reproduced by simulation employing the measured \({g}_{{{{\rm{S,AS}}}}}^{(2)}(\tau )\) and Stokes intensity profile I_S(τ) estimated from the \({g}_{{{{\rm{S}}}}}^{(2)}(\tau )\) measurement in an unheralded regime, what suggests a close to unity overlap between the unconditioned and heralded temporal intensity functions of Stokes field. Error bars correspond to a single standard deviation and, where not illustrated, their size is smaller than the size of the corresponding plot symbols.