Evaluation of two-photon polarization density matrix of polarization-entangled photon-pairs generated through biexciton resonant hyper-parametric scattering

We have investigated the excitation-power dependence of polarization-entangled photon-pairs generated from a CuCl single crystal using biexciton resonant hyper-parametric scattering. The measured two-photon polarization density matrix which corresponds to the two-photon polarization state changes with increasing the excitation power. The evaluation of the tangle and the linear entropy obtained from the measured density matrix indicates that the two-photon polarization density matrix can be expressed as the mixed state of an ideal state and a totally mixed state, and the mixture ratio of the totally mixed state increases as the excitation density increases. The variations of the density matrix and the mixture ratio with the excitation power originate from the generation of the multiple photon pairs.