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cryptography. ese include communication with drones and the Internet of ings (IoT), where one or more of the element technologies for constructing a quantum cryptog-raphy system — very week photonic communication, key distillation, physical random number generation, and oth-ers — have eect. We call these technologies collectively “quantum photonic network technology” (Fig.1).3Quantum node technologyWe are now enjoying a seemingly sucient amount of information transfer provided by optical communication. However, as mentioned in the introduction paper, concerns are rising over the possibility that the rapidly growing communication volume may hit the theoretical ceiling sooner or later. Our current technology is faced with an-other challenge: in ultralong distance communication paths in outer space, where optical ampliers are not available, technology to extract the maximum amount of information from ultraweak signals buried in quantum noise is not yet within reach. Turning our eye to security issues, quantum cryptography described in the previous section can be the agent to realize ultimate security. However, because it in-volves transmission and reception of photonic level signals, the QKD protocols now under development toward practi-cal implementation impose heavy restrictions in terms of communication distance and key generation rate.A drastic solution to address these problems can be derived from a technology that measures, controls and preserves the quantum attributes of the photonic signal at each relay point (node) of the network. In reality, extreme fragility of quantum mechanical attributes means the technology is still in need of several innovations. We call these technologies in need collectively “quantum node technology,” and we are now engaged in basic research with a view toward the long-term perspective (Fig.2). Specically, NICT is conducting research focusing on three major themes:Quantum optical control technology: full control of the quantum state of light and establishment of a quantum-based information-communication protocol, which lies beyond the scope of conventional electromagnetic theory based optics (called classical optics in contrast to quantum optics).Quantum metrology: control of atoms and ions on a particle-by-particle basis in view of applying them in quantum communication and the next generation fre-quency standards.Superconducting quantum circuit technology: precise control of photon-matter interaction on a photon-by-photon basis on the superconducting circuit, which can be considered itself as a macroscopic embodiment of an arti-cial atom, in view of shedding light on hitherto unknown quantum physical phenomena. All these themes represent future technologies that directly involve cutting-edge de-velopments in quantum physics, leading to the way to real-ize technologies still out of reach of the human race.Quantum entanglement is one of the key concepts in the development of quantum technologies. It represents a correlation between particles that appears only in the quantum mechanical domain and is totally inexplicable in terms of classical physics (i.e. conventional mechanics and electromagnetism). For example, if two photons are pre-pared to have the same longitudinal polarization, correla-tion (in the classical meaning) is formed between the photonic polarization. is correlation can be detected, for example, when measurement is made for each photon by passing it through a lter capable of distinguishing longi-tudinal and horizontal polarization. However, measurement of dierent polarization bases, for example right-hand/le-hand circularly polarized light, cannot give denite correlation because each individual photon rotates in a random manner. In contrast, if photons are quantum me-chanically entangled, the correlation always appears, which is irrespective of the choice of the measurement, longitu-dinal/horizontal polarization measurement or circular polarization measurement. A noteworthy feature of this measurement is the fact that we get the same result even if we choose the measurement method aer the quantum state was prepared.is retention of correlation under operation of any measurement method is the most peculiar feature of quan-tum entanglement. By monitoring the entangled photon pair using more than one measurement method, any eavesdropping attempt done to the pair by a third person can be detected 100% even aer it has been delivered to two, mutually far apart receivers. A drawback of quantum entanglement is its fragility, which makes it unt for direct long-distance transmission. is can be xed, however, by implementing a regenerative function at the relay points in the network: partially corrupted quantum entanglement can be restored at the relay points for long-distance trans-mission. Realization of such quantum repeater technology will enable ultralong distance delivery of entangled photons with the same assurance of security using the eld-proven QKD methods. Quantum entanglement is also known to 72 Quantum Info-Communication Technology -Overview-