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where long-term absolute security in condential com-munications is essential. Quantum cryptography is the branch of information and communication technology (ICT) that promises to deliver the hoped-for tools.e two major features of quantum cryptography in comparison with the existing methodologies can be de-scribed as follows: information-theoretic security that dees any eavesdropping attempts from any third party in pos-session of whatsoever huge computing power, and the detectability of any physical eavesdropping attempt (e.g. partial tapping of information from the optical ber). Quantum cryptography consists of two major elements: Quantum Key Distribution (QKD) that allows sharing of the secret key (sequence of random bits shared only among the senders and receivers), and the encrypted communica-tion using QKD. In the latter process, the information to be sent is encrypted using the QKD-shared secret key be-fore being transmitted through one of the standard paths (i.e. the internet).QKD requires special communication devices that be-come available only with the help of quantum characteris-tics. e sender transfers information in the form of specially randomized photons, i.e. particles of light. e receiver detects the state of the photons one by one, and rejects those with suspicion of eavesdropping with the help of an algorithm on the computer. is process (key distil-lation) enables the receiver to generate a secure secret key. Any eavesdropping attempt to a photon level signal infal-libly leaves a vestige of tapping (Heisenberg’s uncertainty principle), which can be used to detect the attack. In ad-dition, the use of physical random number generation (a stochastic physical phenomenon is used to generate random numbers) enables sharing of information-theoretically se-cure secret keys (i.e. security totally unaected by the level of computing capability on the side of the eavesdrop-per). is has been an overview of the principle that sup-ports quantum cryptography. In the era when quantum cryptography was invented (from the 1980s to 1990s), it was generally accepted that exactly one photon must be prepared to investigate the signal conveyed by the single-photon state. However, subsequent theoretical studies have shown that a very weak laser light (such that the laser pulse contains only one or less photons on average) and well-contrived transmitting/receiving devices can deliver a substitute for single-photon experiments. Entering this century, these ndings have signicantly accelerated re-search and development toward practical realization of quantum cryptography.NICT started basic research on quantum cryptography in 2001 in collaboration with industries and universities. Since 2006, NICT has been conducting application studies including demonstration of a quantum cryptography net-work based on a ground ber network, and toward practi-cal application thereof. In addition to the eort to realize quantum cryptography systems, the research in recent years has indicated the possibility of delivering new secu-rity technologies even to communication networks which defy, at least at present, implementation of quantum FiF1　Overview of quantum photonic network technology Quantum photonic network technologyQuantum photonic networkproviding critical security in government, defense, medical and financial infrastructuresThe quantum nature of photons (uncertainty principle) is exploited to construct a technology that enables secure distribution of cryptographic keys, which is secure against ANYcomputational attack by eavesdroppers＜Quantum cryptography＞QKD networkInformation-theoretic secure free-space optical communication (FSO) technology・Information-theoretic security・Security against physical eavesdroppingFSO testbed between NICT and the University of Electro-CommunicationsSecure control communicationDrone2 Quantum Info-Communication Technology -Overview-6　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)