Frontier Communication Laboratory

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Recent advances in computing power are increasing the risk that current cryptography methods will be broken. In particular, since classical cryptography methods can be compromised by quantum computers now under development, the introduction of post-quantum cryptography (PQC) is anticipated, but over the long term, there is also the risk that PQC itself will be compromised. To mitigate this long-term risk, we will establish secure encrypted communications technology that can be applied to even high-speed communications exceeding 100 Gbps as in IOWN APN.

• Elastic Key Control: Technology that enables flexible switching of cryptography methods such as PQC and combining of multiple cryptography methods. It can swiftly switch from a compromised cryptography method to a secure method, and by combining multiple cryptography methods, it can maintain security even if a single method becomes compromised.
• Disaggregation: A configuration that separates key exchange, key management, and data encryption functions and that can provide the same level of security on various platforms (optical transponder, SmartNIC, etc.).

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The optical network is expanding across wide areas and becoming increasingly complex as the societal demand for communications increases owing to the spread of smartphones and emerging AI-related services. Automating the design and maintenance operations of the optical network will make data communications more affordable for everyday use. In addition, highly reliable optical communication services can be achieved by proactively addressing the risk through the prediction of failures in communication facilities. In areas with a particularly high occurrence of natural disasters such as Japan, it is important to accurately estimate the potential damage to communication facilities in the case of a disaster. To achieve highly precise and fully automated design and maintenance operations, our group is researching an "optical-network digital twin" scheme that reproduces the state of the optical network in virtual space and reflects the simulation results in the actual facilities. To achieve large-capacity, low-latency connections between data centers, we have automated design and configuration operations that typically require 2-3 hours by experienced experts. The automation technology reduces the time to complete such tasks to just a few minutes. We have also established a model that can analyze the risk of failure in the communication network in the event of a disaster. Through these research activities, we are actively collaborating with research institutions in Japan and abroad, and conducting experiments on overseas testbeds.

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New applications involving AI and machine learning have been spreading widely in recent years, and to train them efficiently with massive amounts of data, it is common to connect multiple servers over the network and perform data communications among those servers. The network has so far been using packet switches to interconnect such servers. However, these switches require electrical processing to process and exchange packets, which means high power consumption and large network delays. In response to these issues, we are researching and developing an optical switch network for AI clusters that implements optical switches to directly exchange data using light without the need for electrical processing. With these switches, we can configure a low-power and low-latency network.

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The recent evolution of quantum communication technology holds the possibility of fundamentally changing the future of information communications. Our research group is working on advanced photonic communication technology with the aim of achieving a high-speed and secure quantum network. We present our recent efforts in this field focusing on key elemental technologies.

The first is research on Time-Correlated Single Photon Counting (TCSPC). Detecting a single photon is essential for accurately reading a quantum state, and TCSPC equipment is expected to be applied to quantum computing, quantum cryptographic communication, and quantum sensing. At the Frontier Communication Laboratory, we are also researching TCSPC as a key technology for application to quantum teleportation and developing associated equipment (Fig. 1).

The second is research of high-speed photon distribution technology. As one effort of this research, we proposed a method using two Mach-Zehnder modulators to generate short optical pulses. We applied the proposed technology to quantum key distribution (QKD) and achieved a secure key rate of 668 kbps over a 100 km optical fiber under the assumption of a general individual attack bring key distillation. This achievement is a step toward the commercialization of long-distance quantum communication, and the proposed technology is expected to be a fundamental technology toward the construction of a future quantum network (Fig. 2).

Through this research, we aim to make quantum communication technology practical and expand the scope of its application.

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Multi-party quantum entanglement distribution is a technology that enables the sharing of quantum entanglement among multiple users. It will enable quantum computers at different geographical locations to cooperate with each other to solve difficult problems, making it a promising technology for efficiently extending computing power. We present our research and development initiatives toward a multi-party quantum entanglement communication system.

The first initiative is research of a system configuration for a multi-party quantum entanglement protocol. In this research, we are proposing a practical framework for achieving multi-party quantum entanglement under controlled conditions using entangled photon pairs in time bins. The proposed system includes components and methodology that consider large-capacity photon transmission, stable real-time operation, and scalability to accommodate future extensions and increases in throughput (Fig. 1).

The second initiative is research of quantum key distribution (QKD) security. In this research, we are focusing on BB84 protocol using time bins that are not easily affected by polarization in optical fiber while also using passive optical devices to avoid side-channel attacks. Specifically, for time-bin BB84 protocol using a practical delay interferometer and threshold detector on the receive side, we identified a potential eavesdropping method when using "satellite time bins" generated via the delay interferometer as a key, and by proposing an additional operation for preventing that from happening, proved that the protocol was secure (Fig. 2).

Through this research, we aim to make the concept of quantum communication a viable technology that will enrich our lives in the future.

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