Overview

New research and development in the third mid-term plan, the Quantum Phase III (2011-2015), has successfully completed on March 2016. The goals of the Quantum Phase III are (1) to make practical use cases of quantum cryptographic network, (2) to develop quantum node technologies for quantum communications network, and (3) to apply techniques and devices developed in those researches to new sensing technologies and metrology. The research and development were carried out by the NICT Research Laboratories and the NICT Commissioned Research teams in collaboration with other relevant projects and institutes in Japan and foreign countries. (see figure below).

Part of the R&D activities are evolved to the "Quantum Secure Network" project in “Advanced Information Society Infrastructure Linking Quantum Artificial Brains in Quantum Network” under the ImPACT program conducted by Cabinet Office, Government of Japan.

Quantum cryptography

These days, security threats against state-secret networks have become reality. The worst scenario of code breaking will also be found Medical service over the network (virtual hospitals) in future. Eavesdropping of electric medical chart, including genetic information, would damage not only the clients but also their offspring too, such as abuse in job employment, and discrimination of a family for a long time.

After the Great East Japan Earthquake, the importance of business continuity planning and constructing data backup networks for it has been widely recognized. Data backup communications should be protected tightly especially when transmitting data concerning individuals' lives and property.

The ultimately secure communications such as one-time pad with keys shared by quantum key distribution (QKD) are really needed to protect communications used in the scenes mentioned above, even at high costs. The aim of Phase III is to construct a prototype of QKD network for these high-end applications and start test services in NICT.

We update the Tokyo QKD Network by installing novel QKD systems and node technologies, and by tightly integrating quantum, optical-transport, and IP layers. QKD secure key can be used not only for one-time-pad encryption but also for node authentication, to prevent falsification and spoofing attacks. We take a track record of long-term operation for quality guarantee. In the next phase, this kind of network test-bed will evolve to "secure photonic network", where various security technologies including modern and quantum cryptography will be combined in a photonic network, and multi-purpose security services will be provided depending on needs and costs, as depicted in Fig. 1.

Fig. 1Secure photonic network

In Phase III, we develop viable QKD systems for mission critical applications. Stabilization techniques for reliable operation and countermeasures against side-channels should be implemented into a compact QKD system without degrading the QKD performance currently achieved. After testing the QKD systems in the Tokyo QKD Network for the long-term reliability, they will be used in the NICT intra-network first, and will then be applied to dedicated secure communication lines. Next generation QKD systems for core and access networks will also be developed. The above tasks are mainly carried out by the Commissioned Research as summarized below. The NICT Research Laboratories conduct R&D on network-layer integration design hybridizing quantum and modern cryptography. The NICT Research Laboratories and the Commissioned Research teams collaborate to widen applications of QKD to the upper layers of the network, and to pursue the standardization and commercialization of them.

Commissioned research : Secure photonic network technology

Final reports of the projects

Project	Research team (Leader)	Team ID
Subject A : QKD network control technology
Application-enhanced QKD systems	Mitsubishi Electric Corporation (Mitsuru Matsui)	157A-T01
Interim report page Original page
Next generation QKD core- and access-networks	Toshiba Corporation (Andrew Shields)	157A-T02
Interim report page Original page
QKD technology for mission critical applications	NEC Corporation (Akio Tajima)	157A-T03
Interim report page Original page
Subject B : Quantum cryptographic theory
Security certification and efficient key distillation	Nippon Telegraph and Telephone Corporation (Kiyoshi Tamaki) Mitsubishi Electric Corporation Hokkaido University Nagoya University Tokyo Institute of Technology	157B-T01
Interim report page Original page
Subject C : Continuous variable QKD and its application
QKD and optical secure communications with quandrature amplitude modulation technology	Gakushuin University (Takuya Hirano) Tohoku University	157C-T01
Interim report page Original page
Subject D : Secure photonic network architecture
Architecture to install QKD systems into existing mission-critical-secure networks	NEC Corporation (Shione Asami) Hokkaido University	157D-T01
Interim report page Original page

Quantum node technologies

Quantum node technologies include two subjects. One is quantum decoder to extract the maximum information from optical signals, and to realize the maximum-capacity communications with given transmission power. The other is quantum repeater to extend QKD distances and to make entanglement networks for new quantum protocols.

Quantum decoder is mainly studied by the NICT Research Laboratories. It consists of quantum information processing (QIP) and the final measurement. We work on QIP for the optimal signal carriers, that is, the coherent states, and novel photon detectors including superconducting nano-wire single photon detector and photon-number-resolving detector using transition edge sensor.

Quantum decoder

The data traffic in optical fiber networks is doubling every year due to the rapid spread of broadband access. It is most likely that the throughput per fiber will reach an Peta (1015) bps level in a few decades. Dense wavelength division multiplexing (DWDM) technology and multi-level modulation technique have been sustaining to increase the transmission capacity. However, straightforward extension of those technologies are facing a serious bottleneck, namely the power limit of a fiber, or fiber fuse. When orders of several Watts of optical power is input into an optical fiber, the edge face of a fiber may be burn out. In order to circumvent this bottleneck, various approaches are taken, including new multi-core fibers and fiber-fuse protection devices. In a long-term perspective, however, it is strongly desired to investigate a totally new paradigm which is based on quantum information and communications technology to realize the ultimate resource-efficiency. It is nothing but to study a fundamental issue of communications, namely how to transmit the maximum information through an optical channel with a given finite amount of signal power and bandwidth.

Recent progress of quantum communication theory clarifies the ultimate limit of communication with electromagnetic fields through a linear-loss channel, and also a scheme to approach to the limit. In the optimal scheme, the encoding should be the coherent modulation, which is completely conventional technology, while the optimal decoding essentially requires quantum effect, namely controlling quantum interference among blocks of coherent state pulses and an appropriate final measurement. This is quantum decoder, a new technology to beat the limits and bottle necks of conventional communication technologies.

→Review of background and recent results

Quantum repeater is mainly studied by the Commissioned Research as follows.

Quantum repeater

Quantum repeaters are the core technology for any future quantum internet. The central purpose of such repeaters is the creation/establishment of entanglement over long distances. Entanglement is an essential resource for quantum communication and its applications as a maximally entangled Bell state can be used to teleport an arbitrary qubit state over a quantum network. Bell states are however fragile to photon loss and noise. The transmission of quantum signals through a fiber over long distances suffers from a huge amount of photon loss (similar to classical signal? degradation) and so we need mechanisms to circumvent this fundamental issue. In the case of classical signals we can use amplifiers, however this? is impossible in the quantum case. The solution is a series of quantum repeaters distributed over a network.

A quantum network consists of a series of nodes each containing a quantum repeater. Instead of trying to transmit one qubit from the Bell pair over a long distance, a quantum network uses the quantum repeaters to establish a Bell pairs between two adjacent network nodes and then uses entanglement swapping to create longer range Bell states. The quantum repeater is also used to maintain the quality (fidelity) of the Bell pairs being used in the system. Figure 2 depicts the concept of how quantum repeaters can be used to establish a high fidelity Bell pairs over long distances. As entanglement is the key resource for quantum communication and most of quantum information processing, quantum repeaters are the core technology for any future quantum internet.

To date a number of proposals have been made for how to realize a quantum repeater. Initially the main focus of this work was to demonstrate entanglement distribution between two remote qubits. A number of these entanglement distribution proposals have even been experimentally demonstrated. Dependent on the physical implementation, there are four entanglement distribution schemes which we show in Figure 3. The emission scheme is the simplest and was the first to be successfully implemented. The absorption, emission - absorption and scattering schemes appear more technologically demanding. However, it is not clear which is best as each scheme has its own advantage and disadvantages. Stepping further towards integrated quantum repeater systems, we need to consider the quantum state manipulation for entanglement swapping and purification. This requires integration of different quantum protocols making the quantum manipulation much harder. A demonstration of this integrated quantum manipulation is one of the most difficult challenges we currently face.

Many core elements of a quantum repeater have been individually demonstrated, however no integrated device has been demonstrated. This is one of the reasons why quantum repeater research is still at the fundamental research stage. The experimental challenge is to realise physical systems which perform all necessary functions for an operational quantum repeater node. To support the future of these experimental developments, a theory effort needs to be developed to guarantee that such physical system can be integrated into quantum devices capable of implementing scalable quantum networks. An explicit estimation of the accuracy for these quantum device operations needs to be provided to ensure their feasibility and scalability.

Commissioned research : Quantum repeater technology

Final reports of the projects

Project	Research team (Leader)	Team ID
Subject A : Quantum repeaters and their node technology
Quantum repeater network system architecture and node device requirements	National Institute of Informatics (Kae Nemoto)	158A-T01
Interim report page Original page
Subject B : Entanglement purification between distant nodes
Quantum repeater based on diamond NV centers	Yokohama National University (Hideo Kosaka) Osaka University Nippon Telegraph and Telephone Corporation National Institute of Informatics	158B-T01
Interim report page Original page
Quantum repeater based on optical-pulse-controlled quantum dot spins and single photons	Stanford University / National Information of Informatics (Yoshihisa Yamamoto / Shoko Utsunomiya)	158B-T02
Interim report page Original page
Subject C : Multi-bit entanglement control and optical interface with Superconducting circuits
Superconducting circuit-QED and quantum transducer	The University of Tokyo (Yasunobu Nakamura) Nippon Telegraph and Telephone Corporation Tokyo Medical and Dental University Tohoku University	158C-T01
Interim report page Original page

Quantum sensing and metrology

Quantum light sources and novel photon detectors developed in the above project will be used for realizing new sensing technologies. Prototype of small-scale quantum decoder will be applied to the receiver at the ground station for satellite-ground optical link. Various atomic optics are also studied for the mission of optical frequency standard technology.

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