History, progress and prospects of quantum cryptography

Quantum technologies are in a phase of rapid advancement and exploration, they hold the promise of stirring revolution in computing, cryptography and communications, among other fields. Although still in its nascent stages, progress in quantum computing has been remarkable, with companies and research labs moving toward building more powerful and stable quantum computers. In parallel, quantum cryptography is emerging as a robust solution to emerging security challenges, especially with regard to privacy and data protection. Quantum communication, through teleportation and quantum entanglement, has also shown tremendous potential to enable ultra-secure and efficient communication channels. Despite the enthusiasm and advances, quantum technologies still face significant challenges in terms of scalability, bugs and technical control, requiring a sustained effort in research and development to achieve their full transformative potential.

The Bankinter Innovation Foundation held its latest think tank Future Trends Forum under the slogan Quantum Computing and Artificial Intelligence: The Silent Revolution. It brought together more than forty experts: scientists, researchers, entrepreneurs and executives to analyze the state of the art of these technologies and future opportunities.

Among the experts gathered, Artur Ekert is Professor of Quantum Physics and Cryptography at the University of Oxford, Founding Director of CQT Singapore and co-inventor of quantum cryptography. He received the 1995 Maxwell Medal and Prize from the British Institute of Physics, the 2007 Hughes Medal from the English Royal Society and the Chinese Micius Quantum Prize 2019 for his discovery of quantum cryptography. He is also a co-recipient of the 2004 European Union Descartes Prize. In 2016 he was elected a fellow of the Royal Society.

In this webinar, Prof. Ekert gives us a masterclass on quantum cryptography: after a historical review of cryptography and scientific research, he explains the importance of Bell’s inequalities since they were proposed in 1964 until their recognition with the Nobel Prize in 2022. He explores their importance in quantum physics and cryptography, as well as the pioneering experiments that revealed their importance. Ekert delves into how these inequalities went from being a mere philosophical curiosity to fundamental to the security of quantum communication.

From curiosity to cryptography: a historical journey to secure communications

At the beginning of his lecture, Artur Ekert explains the interconnection between two seemingly different areas: cryptography, i.e. the art of secure communication, and curiosity-driven discoveries in physics, specifically in the quest to understand nature at a fundamental level. Through a historical journey, Ekert takes us from the earliest attempts by humans to protect their communications, to contemporary advances that seek to merge cryptography with quantum physics.

He begins his account with a flashback to around 400 B.C., when the Greeks invented the scytale to swap characters and encrypt messages. He moves forward through the centuries to the Renaissance, where he highlights Alberti’s cipher wheel as a significant effort to improve secret communication. Ekert also mentions the Enigma machine in the 20^th century, which although considered impenetrable, was deciphered thanks to the efforts of Polish mathematicians and later, finally deciphered by Alan Turing.

He introduces the concept of perfect encryption through Claude Shannon’s ‘One Time Pad’s system that guarantees impenetrable security as long as the keys used are truly random, secret and never reused. However, Ekert points out that the crucial challenge lies in the effective generation and distribution of these keys between the communicating parties, represented by the usual characters in cryptography: Alice and Bob.

He then goes into the evolution of public-key cryptographic systems, which offer a partial solution to the problem of key distribution by allowing anyone to encrypt messages, but only the correct recipient to decrypt them. Despite the beauty of these systems, Ekert emphasizes their vulnerability, since their security depends on the difficulty of certain mathematical problems that could be solved efficiently with the advent of quantum computers. He mentions how this has led the cryptographic community to search for systems resistant to quantum attacks.

Ekert highlights the importance of curiosity-driven research in the advancement of cryptography, and how the intersection of these disciplines has generated a promising new entity in the quest for secure communication: quantum cryptography could be the answer to the quest for positively secure communications.

From Antiquity to Quantum: exploring randomness and its encounter with cryptography.

In the second part of the webinar, Artur Ekert continues to explore the relationship between cryptography and physics, focusing now on curiosity-driven research into the nature of randomness. He begins with a historical debate between ancient philosophers, Epicurus and Democritus, on whether randomness is objective or subjective. It touches on the theological aspect of randomness and how it impacts the notion of free will, with implications for moral responsibility.

Ekert then exposes Albert Einstein’s dissatisfaction with quantum theory, particularly with its probabilistic nature that defies deterministic predictability. He exemplifies the randomness inherent to quantum physics with an experiment involving a beam splitter and individual photons, highlighting how the behavior of photons defies classical expectations.

Subsequently, Ekert introduces Bell’s inequalities (basically, a theorem evidencing the impossibility of explaining quantum phenomena with classical theories), formulated by John Bell, which provided a testable proposition for the debate on randomness. Describing experiments that seek to verify these inequalities, Ekert mentions the pioneering work of John Clauser and the more conclusive experiment of Alain Aspect, which evidenced the violation of Bell’s inequalities, suggesting an inherent randomness in nature.

Ekert then brings together the narratives of cryptography and quantum physics. He highlights how the search for a perfect cipher in cryptography requires true randomness at two different locations, and how findings in quantum physics provide that randomness. He mentions the merging of these fields by Gilles Brassard y Charles Bennett although he points out that the true convergence of cryptography and the foundations of physics occurred later. The merging of these fields not only advanced the fundamental understanding of nature, but also inspired practical applications in cryptography. The experiments that were conducted to validate Bell’s inequalities were essential for designing secure cipher systems. This confluence of cryptography and quantum physics, Ekert points out, has raised the bar, expanding research from a purely theoretical approach to exploring practical applications in secure communications.

Quantum cryptography: bridging quantum randomness and cyber security

In the third part of the webinar, Artur Ekert highlights the development and actual practice of quantum cryptography. Ekert highlights how the violation of Bell’s inequalities proved that photons do not carry predetermined polarization values prior to measurement, which translates into immunity against eavesdropping. The relevance of this milestone lies not only in its theoretical significance, but in that it enables device-independent cryptography, an almost utopian scenario for cryptographers, as it minimizes assumptions in secure communication and allows the integrity of cryptographic devices to be verified, even if they are provided by an untrusted source.

Ekert cites the first experiments he conducted in 1991 at the Australian Defence Science and Technology Group and how it was a challenge to persuade the authorities to explore semi-philosophical issues for practical applications. He acknowledges the work of Rotem Arnon-Friedman, Renato Renner and Thomas Vidick who provided solid proof of the security of this form of cryptography, which in turn has been corroborated by recent experiments demonstrating the viability of device-independent quantum cryptography.

A notable milestone in experimentation was made by Chinese scientists in 2019, who used a dedicated satellite to distribute keys using entangled photons over more than 1,000 kilometers. Ekert also mentions the aforementioned Alain Aspect and John Clauser Nobel Prize awardalong with Anton Zeilinger, which marks a recognition of the confluence between curiosity-driven research and practical cryptographic applications.

Question and Answer (Q&A) section:

On breaking cryptographic keys with a quantum computer: Ekert assures that quantum encryption techniques are immune to quantum attacks, so having or not a quantum computer would not affect the security of quantum key distribution.

Promising applications of quantum cryptography: Ekert mentions that quantum cryptography is especially useful for point-to-point communication. Although he acknowledges that there are still areas where classical cryptography may be more applicable, he sees a promising future for quantum cryptography in the secure synchronization of databases between different locations.

Most commercial Quantum Key Distribution (QKD) systems are based on methods where particles are prepared and then measured for information exchange. However, he wonders if there is another system based on entanglement (a special connection between particles) that could be more useful: Ekert mentions that, for a long time, creating high-quality entanglement between particles has been very difficult due to technological limitations. But he highlights an important difference with a new approach: device-independent cryptography. This approach makes it possible to verify that devices used for secure communication are working properly, even if you don’t know where they came from or don’t trust who gave them to you. He explains that someone could give you devices for secure communication, and even if you don’t trust that person, you could use those devices as long as they pass certain statistical tests, which are linked to Bell inequalities.

The crucial point is that this type of verification and security can only be achieved with entanglement-based cryptography, and not with the traditional method of preparing and measuring particles. Therefore, if perfect security is sought, where even the devices used are not trusted, entanglement-based cryptography is the way to go.

Generation of random encryption keys using computational devices: Ekert points out that, although pseudo-random number generators exist, quantum cryptography allows the generation of random keys in different locations that are identical and secure, a need not satisfied by classical methods. He also mentions that there have been recent mathematical developments in the expansion and amplification of randomness that are leveraged in quantum cryptography.

If you would like to learn more about quantum technologies and what they may represent in the short, medium and long term, we invite you to read the Future Trends Forum report Quantum and Artificial Intelligence: The Silent Revolution.