Application of Quantum Cryptography to Free Space Optical Communications

1. Introduction

In communication services, encryption technology for securing transmitted content is crucial, and this technology has continued to evolve.
Current encryption technology is said to become insecure around 2030 (known as cryptographic degradation), and with general-purpose quantum computers expected to emerge in the 2030s, research and standardization of encryption techniques that cannot be decrypted even by quantum computing are underway.

Apr 12, 2024

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SoftBank's PQC: Preparing for the Arrival of Quantum Computers

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As mentioned in the article above, although there is time until 2030, there is a risk of 'harvesting attacks' (also called 'Store Now Decrypt Later'), where valuable information even after 10 years is intercepted and stored to be decrypted and misused when technology matures in the future. As countermeasures to such attacks, ‘Post Quantum Cryptography (PQC)’ and ‘Quantum Key Distribution (QKD)’ are drawing attention.

2. What is QKD?

Quantum cryptography is an encryption technology that utilizes a shared encryption key (hereafter ‘secret key’), applying the principles of quantum mechanics. As it is theoretically impossible to intercept and decode the encryption keys, and their safety is proven (information-theoretical security), even if the performance of quantum computers improves in the future, data will not be decoded by third parties. This quantum cryptography consists of ‘One-Time Pad (OTP)’ and ‘Quantum Key Distribution (QKD).’ OTP is a method that uses one-time disposable keys for encryption/decryption, and QKD is a technology that applies the principles of quantum mechanics to share cryptographic keys. By using the secret keys shared in advance by QKD, the sender and receiver of data can perform secure communication by encrypting the data through OTP.

There are several different methods of QKD, broadly categorized into three types: Continuous Variable QKD (CV-QKD), Discrete Variable QKD (DV-QKD), and those using ‘quantum entanglement’. Among these, this article will focus on the BB84 protocol, the most practically used DV-QKD method.

In the BB84 protocol, the sender first generates a random number that serves as a seed for a secret key and transmits it through an optical fiber, each bit carried by a single photon (Figure 1). If an eavesdropper on the communication channel tries to incept it by ‘man-in-the-middle attacks’, anomalously large errors occur when the receiver generates a secret key, making it possible to detect the interception. Moreover, even if the eavesdropper intercepts the information and sends it to the receiver, it is impossible to perfectly reproduce the state of the photon due to the ‘no-cloning theorem’ of quantum dynamics. Therefore, frequent key generation errors occur at the receiver in such cases, and we can find the interception.

Figure 1: Schematic diagram of QKD (BB84 protocol)

3. SoftBank’s activities towards the implementation of QKD

SoftBank has been conducting experiments toward the practical application of QKD technology. To introduce QKD, not only is it necessary to install a dedicated device to carry random numbers on photons, but it is also crucial to directly connect this device with an optical fiber. Generally, the contract for an optical line comes with a device for sending and receiving electrical signals, such as optical terminal devices, and it is almost impossible to use the optical fiber alone. Moreover, even with specialized knowledge and the ability to contract for only optical fiber, it would be difficult for anyone except a telecommunications operator to contract and manage all optical fibers to the data center or to connect distant offices. Therefore, it is considered that there are significant challenges in introducing and utilizing QKD in the current network.

At SoftBank, we have been conducting various experiments with the future practical application of services in mind, believing that if we make the 'inner part of the network,' which is not visible to our customers, compatible with QKD and operate it, our customers should be able to perform secure communication by installing QKD devices in their offices.

In 2023, in collaboration with Toshiba Digital Solutions Corporation (hereafter 'Toshiba Digital Solutions'), we demonstrated inter-base virtual private network (VPN) communication using QKD and successfully achieved stable communication on an actual network.

Sep 20, 2023

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SoftBank Corp. and Toshiba Digital Solutions Successfully Complete Field Experiment of IPsec QKD-VPN

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4. Application of QKD to Free Space Optical Communications

So far, we have explained QKD technology using optical fibers. However, as the existing optical fiber network ages in the future, it is expected that a mesh network based on Free Space Optical (FSO) communications will be established as a replacement for optical fiber. FSO is a technology that directly radiates laser light into the air and exchanges optical signals between opposing devices. The beam width of the transmitted light is extremely narrow, so it is possible to minimize interference with other signals and make interception difficult.

To improve the security of such a future FSO network, SoftBank and Toshiba Digital Solutions introduced a QKD system designed for optical fibers to a test environment combining optical fiber and FSO and conducted a demonstration experiment.

Mar 19, 2024

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SoftBank Corp. and Toshiba Digital Solutions Successfully Demonstrate QKD Operation with Optical Wireless Communications

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In the experiment, we confirmed that the encryption keys can be stably shared by QKD, even when there was an FSO interval in the middle of the optical fiber transmission path. To evaluate only the operational characteristics of QKD and prevent the effects of external influences such as sunlight, the evaluation was conducted inside a laboratory shielded from light (Figure 2).

Figure 2: QKD system with the FSO interval

FSO communication has a characteristic that the polarization plane of light is less likely to change compared to optical fiber. In this experiment, we verified how this characteristic of FSO communication affects the performance of QKD.

In the experiment, we compared the case where QKD was applied to FSO communication (wireless QKD) with the case where transmission was done only through optical fiber (wired QKD). We evaluated their characteristics by measuring the key generation speed against the amount of light attenuation for each. In the wireless QKD experiment, we adjusted the light attenuation by changing the distance of the wireless communication section.

The experimental system for this study is shown in Figure 3.

Figure 3: Experimental system of the wireless QKD

The results obtained from this experiment are as follows (Figure 4).

Figure 4: Key generation rates of the wired and wireless QKD systems

From this experiment, we obtained similar attenuation characteristics for the key generation speed in both wireless QKD and wired QKD. As a result, we found that in designing a QKD network system incorporating optical wireless communication, it is feasible to use the same design approach as QKD using optical fiber in terms of light attenuation and key generation speed.

5. Concluding Remarks

SoftBank will continue research and development to realize a safe and secure society, enhancing the scalability of the QKD secure network, such as the extension of areas by long-distance transmission through optical fibers and optical wireless towards the practical application of QKD.