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conventional quantum cryptographic technologies.e common element required to realize the optimized quantum receiver and quantum repeater is the technology for controlling quantum states of photons at will in a highly accurate and precise fashion, which in turn requires novel quantum mechanical tools that are not yet in our inventory. Toward realization of such photon control technologies, we are developing a quantum entanglement source, a basis technology, as well as conducting principle verication experiments of quantum-domain specic com-munication protocols.2Development of a high-speed, high-purity quantum entanglement sourceQuantum entanglement represents a correlation among particles that appears only in the quantum mechanical domain and is totally inexplicable in terms only of conven-tional mechanics and electromagnetism (i.e. classical phys-ics in contradistinction to quantum mechanics). For example, we generate photon pairs whose polarization is quantum mechanically entangled. To explain the basic nature of quantum entanglement, let us rst consider clas-sical correlation without resort to quantum mechanical tools. We assume the existence of a light source that, ac-cording to a random selection of longitudinal/horizontal polarization, continues to generate two photons at a time with the selected polarization. e two photons thereaer maintain the same polarization, i.e. correlation exists be-tween them. is correlation can be detected, for example, when measurement is made of each photon by passing it through a lter capable of distinguishing longitudinal and horizontal polarization. However, measurement of dierent polarization base, for example right-hand/le-hand circu-larly polarized light, cannot give denite correlation be-cause each individual photon rotates in a random manner. is represents an example of classical correlation. In contrast, if photons are entangled, the correlation always appears, which is irrespective of the choice of the measure-ment, longitudinal/horizontal polarization measurement or circular polarization measurement. e results of measure-ment are not aected even if the choice of measurement method is made aer the entangled state is formed. Preservation of correlation, independent of the choice of measurement technique, is the most quintessential feature of quantum entanglement. Quantum entanglement is ac-knowledged as being a basic resource in many areas of quantum information technologies.Up to the present, the research and development on a quantum entanglement source has been largely limited to the near infrared region (around 800 nm). In view of future applications for quantum information communication, we have worked on the development of a quantum entangle-ment source in telecom wavelengths (around 1.5 µm), and successfully developed a high-purity single photon source and quantum entanglement source[1]–[3].For practical application of a quantum entanglement source in quantum information processing protocols, cre-ation of high-purity quantum entanglement states, as well as its high-generation and detection rate, is a high hurdle to surmount. Research eorts pursuing higher generation rates are actively underway worldwide. Up to now, these attempts have focused mainly on enhancing the pulse in-tensity of driver lasers to attain a higher rate in the gen-eration of quantum entangled photon pairs. Higher pulse intensity, however, entails increased noise, giving rise to deterioration of correlation between the entangled pair. Alternative approaches include the method of enhancing FiF1　High-speed, high-purity quantum entanglement source Left: Experimental setup Right: Results from measurementsMirrorFilterLensPhoton detectorSize=30*2*1 mmPPKTP crystal（Quantum entangled photon-pair generationdevice)Frequency-comb laser sourceOperation rate: 2.5GHzWavelength: 1553nmPulse width: ~2.5psIntensity of the driving laser (mW)Interference visibilityThis workPrevious work4 Quantum Node Technology50　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)