From thought experiment to satellite link: the evolution of Quantum Key Distribution

The story begins earlier than most people expect. In the late 1960s, a graduate student named Stephen Wiesner proposed an idea so far ahead of its time that no journal would publish it. He imagined “quantum money” banknotes protected by photons in quantum states that could not be copied without disturbing them. His paper, “Conjugate Coding,” remained unpublished for over a decade, finally appearing in 1983.

 

BB84: the birth of QKD

Wiesner’s idea reached the right minds through Charles Bennett, a physicist at IBM, and Gilles Brassard, a computer scientist at the University of Montreal. In 1984, the two combined quantum conjugate coding with a cryptographic key-exchange scheme, producing what became known as the BB84 protocol.

The premise was clear: encode information in the polarization of single photons, using two incompatible bases. Because quantum mechanics forbids measuring a photon in an unknown basis without disturbing it, any eavesdropper attempting to intercept the key leaves detectable traces. Two parties could therefore generate a shared secret key and know, with mathematical certainty, whether anyone had listened in. At the time, BB84 was a theoretical curiosity. The first working demonstration didn’t arrive until 1989, when Bennett and Brassard built a rudimentary system at IBM that transmitted photons across about 30cm of open air.

 

From lab bench to optical fiber

Through the 1990s and 2000s, researchers pushed QKD out of the laboratory. Fiber-optic experiments extended transmission distances from meters to tens, then hundreds of kilometers. New protocols emerged increase security and improve efficiency: Ekert’s 1991 entanglement-based scheme offered an alternative route to the same guarantee, while measurement-device-independent QKD later addressed a class of hacking attacks that exploited imperfect detectors rather than the quantum theory itself.

The 2000s also saw QKD’s first steps toward real infrastructure. In 2004, a bank transfer in Vienna was secured using quantum cryptography. Between 2008 and 2010, Europe’s SECOQC project linked multiple QKD nodes into a metropolitan network, while China and Japan built their own metropolitan-scale test networks. Commercial vendors began selling QKD hardware, moving the technology from university laboratories to a new-born industry.

 

Breaking the distance barrier: satellites and repeaters

Optical fiber has a hard limit: photon loss grows exponentially with distance, capping practical fiber-based QKD at a few hundred kilometers without intermediate nodes. The most dramatic leap past that barrier came in 2016, when China launched the world’s first quantum satellite. The following year, they distributed entangled photon pairs to ground stations 1,200 kilometers apart and enabled an intercontinental quantum-secured video call between Beijing and Vienna, a milestone that showed QKD could scale globally. Since then, progress has continued on multiple fronts. In Europe, projects like QSNP are working towards developing and deploying the QKD network necessary for the future European Quantum Communications Infrastructure (EuroQCI). Some partners in the project are testing real-world set ups between European cities and in metropolitan areas.

QKD has also become a business. Half of the partners in QSNP are companies preparing to be part of the QKD supply chain: from RTOs developing components to QKD vendors and network providers. Within the project we have witnessed collaboration among the different partners that have demonstrated the capabilities of this technology, fostering the European quantum ecosystem and proving that the quantum future is quickly approaching.

 

Where it stands today

Six decades after Wiesner’s unpublished idea, QKD occupies an unusual position: proven in principle, demonstrated at continental scale, yet still constrained by cost, hardware vulnerabilities and the practical difficulty of building a true “quantum internet” with repeaters. Whether QKD becomes a mainstream layer of global security or remains a specialized tool for governments, banks, and defense networks is still an open question. But its trajectory: from a rejected physics paper to satellites relaying secret keys across continents, remains one of the most striking journeys in the history of applied science.