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Three decades ago, a paper published by the founder of QUTE.sk helped shape the modern understanding of quantum information. Its central question was deceptively simple: how well can an unknown quantum state be copied?

On 1 September 1996, Vladimír Bužek and Mark Hillery published Quantum copying: Beyond the no-cloning theorem in Physical Review A. The paper introduced what is now known as the Bužek–Hillery quantum cloning machine – an optimal universal quantum cloner that established a fundamental limit on how accurately an unknown quantum state can be copied.

The result was both surprising and profound. While the laws of quantum mechanics prohibit perfect copying of an arbitrary unknown quantum state, Bužek and Hillery showed that imperfect copying is possible – and that two copies can achieve a fidelity of 5/6, or 83.33%, with the original state. Importantly, this limit is universal: it applies equally to all possible input states.

At the time, the broader significance of this result was still emerging. Over the following decades, quantum cloning became deeply connected with some of the most important concepts in quantum information science. The no-cloning principle is now recognized as one of the fundamental features distinguishing quantum information from its classical counterpart. It also plays a central role in understanding the security of quantum cryptography: an eavesdropper cannot simply make a perfect copy of an unknown quantum state without disturbing it.

The Bužek–Hillery result therefore represents more than an elegant solution to a theoretical problem. The optimal quantum cloner provides a precise way of understanding the boundary between what quantum mechanics forbids and what it permits. It tells us not only that perfect copying is impossible, but also exactly how close we can get when we try our best.

A lasting scientific contribution

Today, quantum technologies are attracting unprecedented attention. With that attention comes a growing number of voices claiming expertise in a field that is developing at remarkable speed. Expertise, however, is not built overnight. It is built through years of research, original results, peer-reviewed work and contributions that stand the test of time.

The Bužek–Hillery paper is a particularly tangible example of this. Thirty years after its publication, its results remain relevant, its concepts continue to be taught and developed, and the paper remains one of the most cited works associated with Slovak quantum physics with more than a thousand citations. Its impact is also reflected in the literature that followed. The subject of quantum cloning has been extensively developed in subsequent reviews and research, including the influential 2005 Reviews of Modern Physics article Quantum Cloning by Scarani, Iblisdir, Gisin and Acín.

For us, this anniversary is therefore about more than looking back at a remarkable paper. It is a reminder of what genuine expertise means: not simply being part of the latest wave, but having contributed to the field long before it became a global trend – and having the scientific results to prove it.

The people behind QUTE.sk have been part of this journey for more than three decades and we believe that in quantum science, as in science more broadly, credibility and competence are earned through sustained work and tangible results. And perhaps there is no better way to mark this journey than with a result that still speaks for itself: a contribution that cannot be cloned – and has stood the test of time.