Spontaneous parametric down conversion (SPDC) in a nonlinear crystal generates photon pairs, entangled in various parameters. Their polarization entanglement used for various quantum information experiments, like quantum teleportation, quantum key distribution (QKD), etc. Their wave vector entanglement has been used in quantum imaging, quantum lithography and so on.
In this talk, we are going to report three topics about the application of the entanglement of the SPDC photon pairs. First topic is about the application of their frequency entanglement for spectroscopy. Frequency correlation in each photon pair enabled us to perform absorption spectroscopy of a sample without frequency resolving the transmission spectrum. In the second topic, we analyze two-photon quantum interference of collinear SPDC photon pairs in Michelson interferometer. Third topic is the quantum key distribution with a heralded single photon source (HSPS) with a photon number resolving detection system. It enables us to operate QKD in a longer distance than the case when we use weak coherent pulselight (WCP).