What are the implications of living in a quantum world? When can we expect a first full-fledged quantum computer? Why our everyday logic gives us different results than quantum logic? Turns out, those are not easy questions to answer even by the most educated scholars. We sat down Borivoje Dakić from the University of Vienna, whose research group tackles the most important and difficult questions in quantum physics, and talked about the complexities of quantum foundations. 

Prof. Borivoje Dakić came to the University of Gdańsk within the Visiting Professors programme but his visit is also a part of a longer tradition of collaboration between UG and the University of Vienna, started by prof. Anton Zeilinger and prof. Marek Żukowski. 

Marcel Jakubowski: - At the beginning of the 20th century, a few scientists such as Bohr, Heisenberg, and Schrödinger began to realise that we do not live in a mechanical world, but in a quantum one. What are the implications of this change of paradigm? 

Prof. Borivoje Dakić: - The development of quantum mechanics has a long history. Scientists such as Schrödinger, Heisenberg, and Dirac developed the formalism of quantum mechanics, which was very strange to their contemporaries. It introduced some notions that were completely orthogonal to anything we had in physics before, such as Hilbert spaces, unitarities, or wave functions. Although this formalism was extremely successful and triggered a revolution in technology, there was this bitterness and lack of understanding of the meaning of quantum formalities. John Bell developed a theorem that showed incompatibility between quantum theory and our classical perception of the world. At the time, Bell's theorem was not recognised as a really important discovery, but it started a new discussion among curious physicists. Before, there was still hope that the strange quantum formalism could be reinterpreted into standard concepts of physics, but after Bell's theorem, it became clear that this was not possible. So, scientists have come up with many interpretations of quantum mechanics, such as the many-worlds interpretation. Some would say that quantum theory is just a non-local concept, others that probabilities are ontological. We don't have a global consensus on this. People are still arguing about it. However, thanks to this process, a new field has emerged - quantum information. It was born out of the study of quantum foundations. The critical turning point of this study was Bell's theorem.

- So if I understand correctly, it wasn't easy to combine quantum theory with the mechanical rules that had already been established?

- It's not just the mechanical world. It's our logic. A beautiful example of this is the formulation of Bell's theorem as a game played by two players, Alice and Bob, with a referee. Both players can share quantum resources. Using our everyday rules of logic, we would see that they will win a certain number of times, with a threshold of 75% success. Quantum mechanically speaking, if they share an entanglement state, they reach an 85% success threshold. The mathematics of quantum theory gives you this result, but what does it mean? To cut a long story short, I don't know the answer to your question. I've been studying quantum mechanics for more than 15 years, and I have to say that I haven't found a satisfactory answer.

- So it's not surprising that the whole quantum theory, quantum mechanics, has inspired a lot of philosophers, right?

- Very much so. It's clear how you apply the formal rules to calculated things, but the meaning and the ontology of this process is unclear. Some philosophers would say that quantum theory is not ontologically complete. That's why we have different interpretations of quantum mechanics, ranging from things like many worlds to quantum being non-local.

- Are you interested in the philosophical aspect of quantum mechanics?

- Sure. I come from the Vienna Group, which has been working on these questions for years.

My PhD supervisor was Caslav Brukner, who was a PhD student of Anton Zeilinger. We represent the same school of thought. The main driving force in research is fundamental questions, which are very much related to quantum information. This way, by studying concepts, we also discover protocols for quantum information. There is a strong fundamental side, which goes hand-in-hand with the applied side of such research.

- So your work is mostly theoretical, but you once said that your research is almost immediately verified by the parallel research team at the University of Vienna. How does this symbiosis work? What are the advantages?

- The University of Vienna is one of the very few places in the world where you can do this kind of research as a theoretician. I think if you're a good physicist, you need to know both theory and experiment. Physics needs empirical confirmation, so the final word on whether your theory is correct or not belongs to the experiment. We have a phenomenal synergy between our theory and experimental departments. We talk to each other, meet regularly at seminars, and exchange ideas. There is really a huge interest from the experimental side to test our latest developments. It's an amazing opportunity for both sides.

- Does current technology allow you to test all your theoretical developments?

- I would say that most of my research can be tested, but there are other groups at our university that deal with more futuristic areas like gravitational quantum physics. These are the theories that we might be able to test in 10 or 20 years' time.

- Where do you see the most useful applications of quantum technology?

- Nobody really knows at the moment. We can make some educated guesses. Cybersecurity is certainly one of them, because quantum key distribution is probably the most mature quantum technology. It already exists. It's very simple. It does not require large quantum systems or huge entanglement, so secure quantum communication probably has the most potential. Especially if you can find specific applications for it in the real world. As for other quantum technologies, like quantum computing, there are a lot of big players developing it. Many companies working on this technology won't publish their results until they've developed a full-fledged quantum computer. We don't know what's happening in China, we don't really know what the US military is doing and so on. So it's hard to assess our current progress. We don't know whether we will achieve real-world applications of quantum technologies. However, there is a general consensus in the scientific community that we are reaching the moment of sufficient complexity for quantum systems. We have ion traps that can load hundreds of qubits, and other systems that can go up to thousands. These are huge quantum systems, so there must be something useful in them. The question is, how do you identify those advantages? It would require a complete optimisation of every single component and trying to identify something useful in it. Scientists agree that something will emerge, but what exactly? At this moment, we do not know.

- How did prof. Anton Zeilinger's Nobel Prize influence your university and the physics department?

- In concrete terms, not much has changed, but the perception of basic research has changed. Before, some conventional physicists would consider quantum foundations to be pointless research. But now you can show them that this field has been awarded a Nobel Prize. It was a worldwide recognition, the Nobel committee showed people that today's foundations are the technology of the future, although it might be 100 years away. This is one side. On the other hand, we're pushing the boundaries of human knowledge, regardless if useful or not.

- Your cooperation with the University of Gdańsk is part of a longer tradition that began with prof. Marek Żukowski and prof. Anton Zeilinger. Do you see your visit as part of this heritage or as something new?

- A bit of both. When I was a PhD student of Caslav Brukner, Marek used to come to Vienna quite often. We worked together and I learned a lot from him. I also published one of my first papers with Tomasz Paterek from the University of Gdańsk, who was a post-doc at the time. Marcin Pawłowski also did part of his PhD in Vienna. There is this long tradition established by Anton Zeilinger and Marek Żukowski, but we are a new generation. We want to continue and expand this collaboration because it's proven good for both our institutions. There is a new dimension to this relationship based on the application of quantum technology. For example, Marcin Pawłowski is trying to commercialise his ideas with his start-up company Sequre Quantum. We want to continue this tradition of foundational research, but as the next generation, we want to add new elements, such as applications.