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Abstract

The nonclassical nature of a multi-photon wave packet is a key resource for overcoming the limitations of the present optical technologies governed by classical wave optics. Specifically, quantum entanglement of photons, which is the most striking nonclcassical feature of the multi-photon wave packet, plays a pivotal role in quantum optical technologies, including quantum information and communication, quantum imaging and quantum metrology. Thus far, there are many reports on the entanglement generation in polarization degree of freedoms because it provides a superior environment in which to demonstrate quantum information and communication protocols in quantum mechanical 2-level states (qubits). On the other hand, entanglement in frequency degree of freedom also attracts much attention in recent quantum optical experiments. Extended phase matching in spontaneous parametric down-conversion process allows us to control the frequency correlation of the photons in two-photon wave packets, and the control of frequency correlations strongly affect the temporal correlations as a result of Fourier duality. Such new technique are useful not only in the entanglement generation but also in the generation of spectrally pure single photons heralded by their sister photons. Control of frequency correlations between photons is one of the key issues to achieve efficient generation of photon pairs that satisfy the required conditions. We discuss our recent results on the generation of photon pairs having controlled frequency correlation.