Środowiskowe Seminarium z Informacji i Technologii Kwantowych

Second-order autocorrelation function of the optical field, g(2)(τ), where τ is a time delay, is a powerful tool for determining the physical properties of light beams and their sources. In particular, the value of this function at τ = 0 reveals the statistics of the photons: g(2)(0) = 2 for a Gaussian statistics, g(2)(0) = 1 for a Poissonian one, and the value g(2)(0) < 1 indicates the phenomenon of photon antibunching, which is a signature of field nonclassicality. In my report, I discuss a possibility of measuring the time-resolved second-order autocorrelation function of one of two beams generated in type-II parametric downconversion by means of temporal magnification of this beam, bringing its correlation time from the picosecond to the nanosecond scale, resolvable by modern photodetectors. I show that such a measurement enables one to infer directly the degree of global coherence of that beam, which is linked by a simple relation to the number of Schmidt modes characterizing the entanglement between the two generated beams. I illustrate the proposed method by the photon pairs generated in a periodically poled KTP crystal with a symmetric group velocity matching for various durations of the pump pulse, resulting in different numbers of modes.