Photon-count fluctuations of spontaneous parametric down conversion exhibit long-range correlations

We investigate the fluctuation dynamics of photons generated by spontaneous parametric down conversion (SPDC) using detrended fluctuation analysis (DFA). SPDC photon pairs were generated via a periodically poled potassium titanyl phosphate waveguide, pumped by a 405 nm continuous-wave laser. The scaling exponents obtained from DFA reveal that SPDC photons exhibit long-range correlations at high pump power regimes. The average scaling exponent of DFA increases from 0.67 to 0.97 as the laser diode drive current increases, indicating the existence of long-range correlations. Our analysis concluded that the scaling exponent of aggregated time series is correlated to the number of SPDC photon pairs.