Quantitative characterization of highly efficient correlated photon-pair   source using biexciton resonance
  A high efficiency method for the generation of correlated photon pairs accompanied by a reliable means to characterize the efficiency of that process is needed in the study of correlated photon pairs and entangled states involving more than three photons, which have important potential applications in quantum information and quantum communication. In this study, we report the first characterization of the efficiency of generation of correlated photon pairs emitted from a CuCl single crystal using the biexciton-resonance hyper-parametric scattering (RHPS) method which is the highly efficient method of generation of correlated photon pairs. In order to characterize the generation efficiency and signal-to-noise ratio of correlated photon pairs using this method, we investigated the excitation density dependence on the photon counting rate and coincidence counting rate under resonant excitation. The excitation density dependence shows that the power characteristics of the photon counting rates changes from linear to quadratic dependence of the excitation density. This behavior represents a superposition of contributions from correlated photon pairs and non-correlated photons. Photon counting signals in this study were recorded as time-tag data, which provide the photon counting rate and coincidence counting rate simultaneously, and improve the coincidence counting rate by the factor of 200 compared to that obtained in previous work [K. Edamatsu $\it{et\ al}$. Nature $\bf{431}$, 167-170 (2004)]. The analysis of the excitation density dependence shows that one photon-pair is produced by a pump pulse with $2 \times 10^6$ photons. Moreover, the generation efficiency of this method is improved by a factor of $10^7$ compared to that of several methods based on the $\chi^{(3)}$ parametric process. 
Yasuo Yamamoto, Goro Oohata, and Kohji Mizoguchi
quant-ph