A cat-and-mouse game around cryptography

Professor Yamamoto says: “From the optical fiber networks currently used, third parties can eavesdrop by extracting light along the way. You may think that the information is flowing in an encrypted form through these networks, posing no problem. However, the encryption system currently used is very likely to be broken easily if a quantum computer with a markedly higher capability of calculation is developed. If so, safe communication will not be possible.”

Cryptography has a long history. It was used in the ancient Roman Empire governed by General Caesar, occasionally playing a role determining the fate of a country. Due to such nature, cryptography became more sophisticated and complicated over time. During World War II, each country gathered able mathematicians in the active attempt of decoding the enemy country’s cryptography. It is said that many of the research outcomes which later led to the development of computers were derived from cryptography-developing efforts. However, as the ciphers became sophisticated, the technology aimed at breaking them also became sophisticated. Such a cat-and-mouse game has been repeated by humans.

In the Internet-prevailing modern age, cryptography serves as an indispensable component of social infrastructure. A huge volume of personal information, confidential business information and financial information is flowing and exchanged via the Internet. The virtual currency, which has the potential to significantly change the financial world, is also based on the confidence that unauthorized falsification by third parties is impossible.

In modern society where the Internet has permeated economic activities, a type of cryptography that can be used by anyone but unable to be broken is required. On the other hand, there is a dilemma that complicated ciphers are more difficult to use. For these reasons, the ciphers currently used for communication in 2020 are quite simple. They are designed on the premise that decoding takes tens of years even with a supercomputer and is thus unfeasible. The strength (complexity) of the cipher is now set through a trade-off between ease of use and safety.

However, if a computer with a markedly improved calculation capability becomes available, this premise will lose its validity immediately. “Even if a universal quantum computer does not become available, a new algorithm (calculation method) may break the cipher. There is no guarantee in principle with the current cryptography technology that it cannot be solved,” adds Professor Yamamoto.

Ultimate safety from quantum wonders

Is there a type of cryptography valid against universal quantum computers with a markedly high calculation capability that may appear in the future? Professor Yamamoto says: “A cipher principally unbreakable can be produced if the properties of quantum mechanics dealing with the very small world (elementary particles, atoms, etc.) are utilized.”

“If a message to be sent is converted into digital data composed of 0 and 1 and a certain random number is applied as a key to the digital data, a random numeric string will arise. This string is transmitted to the receiver, who then restores the original message with the use of a secret key provided in advance. Even when a third party successfully eavesdrops the random numeric string, the message content cannot be read without the secret key. In this setting, a problem is how the secret key can be safely shared by the sender and the receiver.”

A quantum system is noteworthy as a possible carrier of the secret key.

Electrons, atoms, photons (light), etc. are particles that obey quantum mechanics. Quantum mechanics is strange in that it possesses the nature of waves in addition to that of particles. Quantum mechanics which arose during the first half of the 20th century enables precise calculation of the quantum system’s behavior, which cannot be explained by existing Newtonian mechanics. Quantum mechanics revealed that the state possessed by quantum systems (photon polarization, electron spin, etc.) changes once it is observed.

Therefore, if a secret key is transmitted with the use of quantum systems serving as a carrier, eavesdropping (observation) midway causes a change in the state, thus leaving a sign of eavesdropping. Thus, checking for the presence of such a sign will enable confirmation of the security of the secret key.

Safety can be secured by the principle of quantum mechanics (rather than by the calculation capability compared with existing computers) as long as the quantum mechanisms are not principally incorrect. This will stop the conventional cat-and-mouse game and enable ultimately encrypted communication.

Active competition over development across the world

Quantum mechanics involves strange phenomena termed “quantum entanglement” and “quantum teleportation.”

Using the example of photons, it is possible to create a pair of firmly bound Quantum systems always assuming a state of horizontal vibration (polarization) on the other side if vibration in a vertical direction prevails on one side. This is termed “quantum entanglement.”

For such a pair of entangled quantum systems, the state of the other quantum systems can be determined without further observation once the stage of one quantum system is observed. This means that the state (=information) of one quantum system constituting this part looks like it is being teleported to the other quantum system located far away.
Professor Yamamoto explains: “This phenomenon, suggesting a link between two quantum systems through telepathy or the like, is difficult to understand. However, if a repeater apparatus making use of quantum teleportation (quantum repeater) is connected to the network, a safe communication network having the potential of markedly advancing the current Internet may be created. That is the Quantum Internet.”

As such, next-generation quantum information technology directly affects not only industry, but also state security. There is now active competition in terms of developmental efforts in this field among countries. Among others, China is one step ahead of other countries through the launch of its first satellite “Micius” for quantum-encoded communication in 2016, and establishment of an approximately 2,000-km-long optical fiber communication path between Beijing and Shanghai. The USA, feeling a sense of crisis in maintaining its worldwide supremacy, is also accelerating research in this field with a huge budget. Japan has also positioned quantum technology in its “national strategy” and plans to launch a satellite for quantum-encoded communications in the latter half of the 2020s.

↑An experimental apparatus to confirm the principle of quantum relay

A time when quantum internet is “norm”

What world will we face if a quantum network permeates the world? Professor Yamamoto provides the following answer:

“If the Quantum Internet with ultimate safety against eavesdropping is realized, it can be used for summit meetings, referendums, financial transactions, and exchange of genetic and other biological information. With the current Internet network, the capability of calculation is unlikely to be markedly improved even with the connection of multiple quantum computers. However, the Quantum Internet is expected to be able to process a huge amount of data for calculation in a dispersive manner with the use of connected quantum computers. High-level calculation will be possible while preserving confidentiality, thus promoting product development by enterprises. If an atomic clock is precisely synchronized with this network, it will be applicable to establishing a navigation system with high precision. There is also an idea of combining telescopes with such an Internet system to advance astronomy. However, there is also a risk of it being used for the wrong purposes by criminal syndicates. Extensive social discussions are required as to how the Quantum Internet should be used. The current Internet system was born in 1969. In those days, there was a voice ‘Will this be of any use?” Now, the Internet is indispensable infrastructure. If the Quantum Internet is introduced, it may be utilized in a way not imagined in advance by people.”

●Professor Takashi Yamamoto

Osaka University School/Graduate School of Engineering Science/ Institute for Open and Transdisciplinary Research Initiatives/ Center for Quantum Information and Quantum Biology

• 2003: Completion of Ph.D. course at the Graduate University of Advanced Studies, School of Advanced Science, Department of Optical Science. Ph.D. (Science). Researcher at the same university.
• 2004: Specially Appointed Assistant Professor at Osaka University Graduate School of Engineering Science.
• 2007: Assistant Professor at the same school.
• 2011: Associate Professor at the same school.
• Since October 2018: Current position.

Majors: quantum optics, quantum information processing

(Interviewed in March 2020)