Generation of narrow-band photon pairs for quantum memory

We demonstrate a narrow-band photon pair source on a rubidium (Rb) D2 line (780 nm) using a periodical poled KTiOPO4 crystal via a degenerate optical parametric oscillator far below threshold. The single longitudinal mode is selected by a spectral filtering cavity. The measured cross correlation between two photons shows that the generated photon has a linewidth of 21 MHz, which is comparable to the typical linewidth of an atomic-based quantum memory. The detected photon pair rate is about 2.7/s/MHz/mW. This system could be utilized for many applications in quantum information field, such as the storage and retrieval of a photon in Rb atomic ensemble and so on.