Toward the Realization of Quantum Communication in Outer Space -The “Key” is Optical Wireless Communication-： Latest Trends in Space Business: ISS 2024 Part Ⅱ

(This article can be read in 5 minutes.)

International Space Summit (ISS) 2024 was held in Seoul, South Korea from June 11–13, 2024. ISS is a conference organized by the South Korean space startup CONTEC, bringing together over 600 space business professionals. Warpspace CSO Hirokazu Mori participated in the event, delivering a keynote speech and participating in panel discussions on “optical communication” and “quantum communication.”

This article explains the relationship between optical communication which Warpspace is also engaged in and the currently prominent quantum communication, highlighting simply.

(For “Part 1: The Establishment of South Korea’s Space Agency, KASA,” click here)

Modern encryption, represented by the product of large prime numbers, can be broken by quantum computers!? In our conventional information society, most of the exchanged information’s security relies on keeping the encryption “key” secret.

For example, let’s consider the encryption process of a widely known Internet communication, HTTPS. When sending data via HTTPS, the data is encrypted according to a certain algorithm. This encryption method acts as the “lock,” and the method to decrypt it acts as the “key.” By sending the “locked” data separately from the “key,” unauthorized access to the data is prevented. This “lock” consists of a product of prime numbers with 617 digits, while the “key” is generated based on these prime numbers. Therefore, for a third party without the “key” to eavesdrop on the data, they would need to factorize the product of prime numbers in the “lock” to find the prime numbers in the “key.” However, current computers require extensive computation time to find these combinations, making decrypting the encryption extremely challenging.

Nevertheless, it’s known that the encryption systems utilizing these prime numbers can be unlocked rapidly by utilizing the quantum computers currently in development. With researchers worldwide advancing the development of quantum computers, it’s only a matter of time until modern encryption becomes vulnerable.

Quantum Communication — “Unbreakable, Next-Generation Encryption”

Amidst this, quantum communication has gained attention as the “unbreakable, next-generation encryption” technology. Quantum communication exchanges data through the following process:

1. The sender encrypts the data to be transmitted using a method called exclusive disjunction. Simultaneously, a “random number (key)” of the same bit number as the transmitted data is generated. (This “key” differs from the earlier prime numbers, being a random number with strong randomness and disposable nature. It’s mathematically proven that decryption without this “key” is impossible.)
2. The sender allocates information equal to 1 bit of the “random number of the same bit number as the transmitting data” to each photon and sends these photons to the receiver using optical fibers.
3. The receiver measures the state of the received photons and reads out the “key” information.

At this point, the photons transporting the “key” information abide by the laws of quantum mechanics. Quantum mechanics describe the physical phenomena in the very small world, such as electrons and photons. In quantum mechanics, actions we casually perceive like “observing” or “measuring” have significant meanings. For instance, when readers observe a plate of curry rice in front of them, the photons flying around the room bounce off the curry rice, reflect, and enter their eyes, allowing them to observe the presence of the curry rice. This interaction between photons and curry rice is what is being observed. If what is being observed is large such as curry rice, the quantum mechanics does not matter as the interaction is negligible compared with the whole object.

However, when the observed entity is not curry rice, but something as microscopic as a photon, a quantum mechanical substance, the photons flying around the room collide and reflect with the photons that should be observed, causing a change in the state of the photons that should be observed at the timing of collision. This is known as the observer effect.

The thought experiment known as “Schrödinger’s cat” is famous for interpreting the meaning of “observation” in quantum mechanics. When placing a cat inside a box with a device that releases poisonous gas at a random probability, Until the box is opened (observed), the cat’s chances of being dead or alive are superimposed. That is, the cat’s state of being dead or alive is not determined until it is observed, and it is the act of observation that finally determines the cat’s fate. This illustrates the strangeness of quantum mechanics and highlights the importance of the act of “observation” in quantum mechanics. (Image generation: Adobe Firefly)

Here, let’s return to the topic of quantum communication from curry rice and cats. If someone were to eavesdrop on the transport of the “key” converted into photon information, the quantum nature of the photons would cause a change from the “unobserved state” to the “observed state” due to the aforementioned observer effect, making it apparent to the receiver that it has been intercepted. Therefore, if there are signs of eavesdropping on the “key,” it should not be used, and another “key” should be prepared. This way, only secure “keys” can be shared. This is quantum communication.

Importance of In-Space Optical Wireless Communication for Quantum Communication

In quantum communication, a technology that ensures secure communication by using a “key” that changes if eavesdropped upon, there are still challenges to its practical implementation. Quantum communication primarily involves imparting information to infrared photons and transmitting these photons through optical fibers, but the transmission loss during this process must be reduced.

Currently, quantum communication technology is primarily assumed to be used on the ground, where communication via optical fibers is possible. However, in space communication networks where optical fibers cannot be connected, radio waves are mainly used for communication.

Compared to infrared light, radio waves have frequencies several orders of magnitude lower, making their beam diameter larger. As a result, as the distance between the transmitter and receiver increases, the energy density of the transmitted signal significantly decreases. In quantum communication, transmission loss when transmitting a single infrared photon through optical fibers is already a challenge, making it difficult to achieve quantum communication using radio waves, which have greater losses than infrared light.

Hence, technology gaining attention is optical wireless communication between satellites and space-to-ground, handled by Warpspace. This enables communication using infrared light in free space similar to optical fiber on the ground, opening up the possibility of utilizing “next-generation encryption technology that is indecipherable” in communication networks, including those in space.

At ISS 2024, discussions were held regarding the potential of optical wireless communication and quantum communication from this perspective. Quantum communication, which can protect information securely over the long term without the risk of being decrypted, and optical wireless communication, which serves as the “key” for it. It seems necessary to continue to pay attention to these innovative technologies that will shape the future of our information society.

Panel discussion “Session 10: FSO Communication/Quantum Key” at ISS 2024

The content of this article is designed to be extremely simple and qualitative so that individuals without a technical or specialized background can enjoy it. For those who wish to delve deeper, please refer to the following references.

(Writer: Junichiro Nakazawa)

References

[NEC] Quantum Cryptography — the Next Generation of Light-based Cryptographic Technology