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1Introductione concept that everyone can access information, so-called “Big Data,” via the internet to which PCs, home electric appliances, automobiles, robots smart meters etc. are connected, that is, IoT (Internet of ings), has been spreading very quickly these days. In the society where real space and cyberspace sophisticatedly merge through IoT, we have convenience, but at the same time, attackers can easily execute attacks in cyberspace using malicious so-ware (so-called “malware”). Actually, methods of high-level and systematic attack that are suspected to be from national entities emerged and became a cause of serious damage to life, economy and social activities. e threat to the security of our country has been increasing year by year. Public key encryption or common key encryption that we use for network services to guarantee security are based on dicult mathematical problems. e risk of safety of such mathematical cryptography increases as calculation technology progresses. Especially, once an encryption key is broken, the safety of all functions of encryption based on it collapses. For example, the safety of RSA cryptography that is the most typically used public key cryptography is based on the diculty of the prime decomposition prob-lem. Now, the specication of a 1,024-bit key length is at risk of being decrypted. erefore, the system is undergo-ing a shi to a 2,048-bit key length [1]. Here, we have to pay attention to the fact that updating a cryptography system forces increased hardware load. For example, com-paring 1,024 bits with 2,048 bits, the throughput of 2,048 bits uses 5-30 times more load than that of 1,024 bits. is may degrade performance for the user. Also, even if the upgrade to 2,048 bits is accomplished, the cryptography system would cease to be useful when new mathematical knowledge concerning decrypting of the cryptography al-gorithm is discovered. e possibility of using a cryptog-raphy system that has been broken is not zero as the worst case.Also, an eavesdropper (usually named Eve) may dupli-cate and save the data transmitted via a channel even though she cannot decrypt it now but may decrypt it in the future when she acquires the key used for the cryptog-raphy system by chance or acquires a new decryption technology that enables decryption. For example, Edward Snowden disclosed the fact in his so-called “Snowden Files” that the intelligence agencies in the United States record encrypted data on internet in order that they will be able to decrypt them in the future. Actually, it is well known that intelligence agencies in the United States and Europe have performed wiretapping on a large scale and for a long time via optical ber networks (e Guardian, Washington Post, 2013). e technology used for the purpose was a tapping device that is used for diagnosis of optical switches or optical ber. ese days, small commercial tapping devices are sold that can be used as optical wiretap-ping devices. It is needless to use special tapping devices for wiretapping because most advanced optical detectors can reveal signals inside optical bers [2]. is is the so-called “crosstalk” phenomenon between optical bers, in which optical signals leak easily between adjacent optical 3 Quantum Key Distribution Network3-1 Research and Development of Quantum Key Distribution Network in NICTMikio FUJIWARA and Masahide SASAKIWe report a quantum key distribution (QKD) system which enables us to communicate with information theoretical security. The security of the QKD is guaranteed by the low of physics. Basically, the QKD links provides point-to-point secure communication. However, we can extend application/service ranges by constructing a QKD network. NICT has developed the QKD network architecture which has a centralized control structure. In this paper, we introduce the QKD principle, QKD hardware, and the network architecture.113 Quantum Key Distribution Network