A method known as quantum key distribution has long held the promise of communication security unattainable in conventional cryptography. An international team of scientists has just experimentally demonstrated, for the first time, an approach to quantum key distribution based on high-quality quantum entanglement, offering much broader security guarantees than previous schemes.

The art of cryptography is to skillfully transform messages so that they become meaningless to everyone but the intended recipients. Modern cryptographic schemes, such as those underpinning digital commerce, prevent adversaries from illegitimately decrypting messages – for example, credit card information – by requiring them to perform mathematical operations that consume a prohibitive amount of power. Calculation. From the 1980s, however, ingenious theoretical concepts were introduced in which security does not depend on the spy’s finite number-processing capabilities. Instead, the fundamental laws of quantum physics limit how much information, if any, an adversary can ultimately intercept. In such a concept, security can be guaranteed with only a few general assumptions about the physical device used. Implementations of such “device-independent” schemes have long been sought after, but have remained out of reach. So far, that is. write in *Nature*an international team of researchers from the University of Oxford, EPFL, ETH Zurich, University of Geneva and CEA report the first demonstration of this type of protocol – a decisive step towards practical devices providing such exquisite security.

**The key is a secret**

Secure communication is about keeping information private. It may therefore come as a surprise that in real-world applications, a large proportion of transactions between legitimate users take place in public. The key is that the sender and receiver don’t have to keep their entire communication hidden. Basically, they just have to share a “secret”; in practice, this secret is a string of bits, known as a cryptographic key, which allows anyone in its possession to transform coded messages into meaningful information. Once the legitimate parties have ensured, for a given communication cycle, that they, and they alone, share such a key, virtually all other communication can take place in plain sight, so that everyone can see them. The question then is how to ensure that only legitimate parties share a secret key. The process to achieve this is known as “key distribution”.

In the underlying cryptographic algorithms, for example, RSA – one of the most widely used cryptosystems – key distribution is based on the (unproven) conjecture that some mathematical functions are easy to compute but hard to invert . More specifically, RSA relies on the fact that for today’s computers it is difficult to find the prime factors of a large number, while it is easy for them to multiply known prime factors to obtain this number. The secrecy is thus ensured by the mathematical difficulty. But what is incredibly difficult today could be easy tomorrow. Famously, quantum computers can find prime factors much more efficiently than classical computers. Once quantum computers with a large enough number of qubits become available, RSA coding is destined to become penetrable.

But quantum theory provides the basis not only for cracking the cryptosystems at the heart of digital commerce, but also for a potential solution to the problem: an entirely different way from RSA to distribute cryptographic keys — one that has nothing to do with hardness to perform mathematical operations, but with fundamental physical laws. Enter Quantum Key Distribution, or QKD for short.

**Quantum Certified Security**

In 1991, the Polish-British physicist Artur Ekert showed in a seminal article that the security of the key distribution process can be guaranteed by directly exploiting a property specific to quantum systems, without equivalent in classical physics: quantum entanglement. Quantum entanglement refers to certain types of correlations in the results of measurements made on distinct quantum systems. It is important to note that the quantum entanglement between two systems is exclusive, in the sense that nothing else can be correlated to these systems. In the context of cryptography, this means that the sender and the receiver can produce shared results between them via entangled quantum systems, without a third party being able to secretly acquire knowledge of these results. Any eavesdropping leaves traces that clearly indicate the intrusion. In short: legitimate parties can interact with each other in ways that, thanks to quantum theory, are fundamentally beyond the control of any adversary. In classical cryptography, an equivalent security guarantee is clearly impossible.

Over the years, it has been realized that QKD schemes based on the ideas introduced by Ekert can have another remarkable advantage: users only need to make very general assumptions about the devices used in the process. In contrast, earlier forms of QKD based on other basic principles require detailed knowledge of the inner workings of the devices used. The new form of QKD is now generally known as “device-independent QKD” (DIQKD), and an experimental implementation of it has become a major goal in the field. Hence the enthusiasm aroused by such a revolutionary experience.

**The culmination of years of work**

The scale of the challenge is reflected in the scale of the team, which combines leading experts in theory and experience. The experiment involved two single ions – one for the transmitter and one for the receiver – confined in separate traps connected by a fiber optic link. In this basic quantum network, the entanglement between ions was generated with record fidelity over millions of runs. Without such a sustained source of high quality entanglement, the protocol could not have been performed in a practically meaningful way. It was equally important to certify that the entanglement is suitably exploited, which is done by showing that conditions known as Bell’s inequalities are violated. Additionally, for data analysis and efficient cryptographic key extraction, significant advances in theory were required.

In the experiment, the “legitimate parts” – the ions – were located in one and the same laboratory. But there is a clear path to extending the distance between them to miles and beyond. With this in mind, along with new recent advancements in related experiments in Germany and China, there is now a real prospect of turning Ekert’s theoretical concept into practical technology.

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Breakthrough in experimental quantum cryptography – News Quantum Physics and Computing

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