On scientific paradoxes, milestones in quantum mechanics research, and quantum cryptography and teleportation with prof. Marek Żukowski, director of the International Center for Quantum Technology Theory at the University of Gdańsk, speaks dr Beata Czechowska-Derkacz.
Alain Aspect, John F. Clauser and Anton Zeilinger, the leading experimenters, were awarded the Nobel Prize in Physics in 2022 for their most important laboratory observations of entangled photons. You are a close collaborator of Professor Zeilinger and have nominated these scientists for the Nobel Prize several times, but it only succeeded this year.
Anton, Aspect, and Clauser were indeed nominated for the Nobel Prize several times, and I myself nominated them four or five times. This year it finally succeeded. I think the Nobel Committee chose 2022 because it is ideally associated with quantum physics milestones. This happened exactly one hundred years after Niels Bohr, the guru of new quantum physics, received the Nobel Prize for the atomic model, which quickly turned out to be only a good clue and is not acceptable as a fundamental theory. In 1922, the Nobel Prize was also awarded to Albert Einstein, but it was for the year 1921 because he was a controversial figure and members of the Nobel Committee thought long and hard about whether and what to give him the prize for. Einstein, however, introduced so many brilliant ideas to physics that the decision was finally made.
Why quantum mechanics?
After the final form of quantum mechanics was created, there was a great debate about whether it is a complete theory. Some thought it was essentially defective because it does not predict exact measurement results. Einstein believed that quantum mechanics was defective because it only specifies the probabilities of possible results, while Niels Bohr had the opposite opinion - he believed it was a complete theory of our observations of the microcosm. Quantum mechanics is a theory that has explained a lot of things and is one of the most valuable discoveries of humanity. I will give an example that is not so widely known. The quantum theory, invented by physicists, explains what chemical bonding is, which chemistry could not explain. Thanks to quantum mechanics, thousands of other things were later discovered: for example, transistors were built and our entire civilization is based on computers that have integrated circuits built from transistors. This is also the fruit of quantum mechanics.
The Nobel Prize was awarded to three physicists 'for experiments with entangled photons violating Bell inequalities and pioneering quantum computing'. Could Professor explain this difficult formulation to laymen? What is the uniqueness of this research?
The winners of this year's Nobel Prize conducted groundbreaking experiments using so-called entangled quantum states. To approximate these states, let's take maximally entangled states, in particular the 'singlet' of two spins. Spin is a particle's own angular momentum. The total spin of the singlet is zero, so the particles must have opposite spins. If we measure some component, for example, the vertical spin of the first, then the measurement of the same kind in the second must give the opposite result. The distance of these particles is irrelevant. However, for each of the particles separately, the measurement of any spin component will give a completely random result. In summary, we know everything possible about the whole, and nothing about the parts. The work of the laureates paved the way for the development of new technologies in the field of quantum information. But to understand the significance of their discoveries, we must go back to 1935, when Albert Einstein, Boris Podolsky and Nathan Rosen wrote a work that, as Bohr called it, was a 'bolt from the blue'. They claimed that quantum mechanics is missing physical quantities that they called elements of reality. Bohr thought for a month and wrote an answer that physicists then accepted, but in later years they stopped appreciating it - fortunately, I was not among them. It was very deep and showed that Einstein, Podolsky and Rosen's conclusions are not correct. But the situation was not resolved until 1964, when John Bell entered the scene, working at that time in the United States, who was a hobbyist dealing with the basics of quantum mechanics and, interestingly, was on the side of Einstein, Podolsky and Rosen. Bell thought that quantum mechanics 'is rotten'. He managed to derive a very simple mathematical inequality that is satisfied by all theories that are consistent with what Einstein, Podolsky and Rosen postulated. He then checked whether this inequality is satisfied by quantum mechanics - it turned out it was not. Bell posited that certain coincidences should fulfil the fundamental principles of classical physics and the so-called Einsteinian causality, which assumes that interactions cannot propagate at a speed greater than light. This means that distant events that are in coincidence, that is, occurring at the same time, cannot have a direct influence on each other. They can only have a common cause. If at this moment there is an explosion on the Sun that destroys our civilization, we will only learn about it in a few minutes, because the light from this star goes to Earth for a few minutes. If at the same time a meteoroid hits the Moon, we will learn about it in a second, because that is how long it takes the light from the Moon to reach Earth. Bell wrote a paper that resolved the conflict between Bohr and Einstein, showing that Einstein's concepts are inconsistent with quantum mechanics. However, the inequality derived by Bell was imperfect. Five years after Bella's publication, John Clauser enters. Together with Michael Horne, Abner Shimony and Richard Holt, they derived an inequality that did not have the defects of Bella's inequality, which can be used not only to analyze quantum mechanics predictions but also to directly analyze experimental results. Furthermore, their article showed that in such an experiment it is possible to use specific two-photon emissions from a calcium atom.
It seems that we are entering an even more scientifically complex area...
Because this is an unfinished story. In quantum physics, the sequence of events, discoveries, and published works is important. Without going into detailed definitions, let us just recall that sunlight is not polarized. Polarized light is that in which the direction of the waves describing it is ordered. Light polarization is used, for example, in 3D cinematography. In his experiment, Clauser used so-called photon cascades in calcium atoms. He carried out this experiment in 1972, together with Stuart Freedman, in order to refute quantum mechanics, but he obtained measurements indicating that Bell's inequalities are not satisfied. Quantum mechanics thus triumphed. He was very surprised by this. This is a kind of scientific paradox. Bell believed that Einstein, Podolsky and Rosen were right, but showed that they were not. Clauser believed that Bell's inequalities would be satisfied and quantum mechanics would collapse, but it turned out that it survived his experiment. At this point, Alain Aspect appears, who carried out essentially the same experiment testing the local hidden parameter theory, but without many of the shortcomings of Clauser's experiment. In the early 80s, he set up a small research group in Paris and together they performed experiments, inspired by Clauser's results, but using more modern tools. The key was to create two sets of photon detectors, now most commonly called Alice and Bob. One station caught the first photon from the emitted pair, the other caught the second and they operated independently. This was a huge step forward. Thanks to these experiments, certain scientists at the University of Gdansk began to slowly take an interest in these issues, and a few years later conduct their own research. The greatest enthusiast was Jarek Pykacz. My dear friend and colleague at the time, co-author of my first work on entangled photon experiments, who died this year.
Did the cooperation between the University of Gdańsk and Professor Anton Zeilinger begin then?
Professor Anton Zeilinger noticed us in 1988 after our publication. He sent us a postcard, in which he wrote that our article is very interesting and asked for a copy. It was exactly written... 'very interesting!!!'. We were of course thrilled with this fact. Anton, I can say this because he is my friend, appeared in Gdańsk in 1989 for a conference organized by us. Jarek, who was not a quantum optics, said: 'Take care of Zeilinger because there will definitely be a discussion of experiments, and I am a mathematical physicist'. Anton wanted to see the old Gdańsk. During the tour, during our endless discussion, we concluded that the most beautiful thing in quantum physics is what people are most irritated by it, so we should just study these areas and show them. This was an important moment in my scientific development. But it was not a popular research trend at the time, rather the sense of it was still questioned.
Must a scientist be rebellious and go against the grain?
Anton Zeilinger began working on photonic entanglement when these studies began to enter the mainstream. This happened thanks to the great quantum optics Leonard Mandel who began to make more efficient experiments with entanglement and perfected the techniques of using 'nonlinear' crystals in such experiments. But scientists dealing with entanglement still had a very difficult time. I remember a seminar in the 90s when I tried to give a lecture on my new work on these issues when my fellow physicists shouted me down. It was only at the third seminar, when they were tired of shouting, that they allowed me to speak. This shows the scale of distrust in this area of research. At that time, Anton Zeilinger was already thinking about ways to use the aforementioned crystals to improve experiments with entangled pairs of photons. He proposed, along with Horne and Abner Shimony, a better experiment than the one Jarek and I had developed. He also published a paper with Daniel Greenberger and Horne, which showed the consequences of three-particle correlations. When I heard about this, a year later, I completely abandoned the previous scientific field I was working in and decided that I would only deal with the fundamentals of quantum physics and multi-photon interferometry. Bell's considerations about two-particle correlations were very interesting, but three- and more-particle opened up limitless, surprising possibilities and revealed new, paradoxical quantum phenomena.
During the announcement of the winners of this year's Nobel Prize in Physics, it was emphasized that Aspect, Clauser, and Zeilinger paved the way for new technologies based on discoveries in the field of quantum information. Have we moved from theory to specific tools that allow us to use the discoveries of quantum mechanics?
Of course! Using improved tools and long series of experiments, Anton Zeilinger began to use entangled states of photons in prototype applications of quantum information theory. His research group was the first to implement a procedure called quantum teleportation, which is the transfer of a quantum state from one particle to another – a remote one. The effects of quantum mechanics are therefore beginning to find direct applications. Currently, there is an extensive field of research covering the theory and experimental attempts of elementary quantum computing, quantum communication networks, and quantum-encrypted secure information transfer.
Unbreakable cyphers?
Exactly! The forerunner of quantum cryptography is Stephen Wiesner, who came up with the theory of quantum notes that are impossible to counterfeit. Inspired by the above, Charles H. Bennett and Gilles Brassard in 1984 published a paper on quantum cryptography, based on Bohr's complementarity. In 1991, when Anton Zeilinger invited me to Innsbruck as a visiting professor, there was a paper by Arthur Ekert in which he developed Bennett's and Brassard's ideas concerning also the possibility of using entanglement in quantum cryptography. The aim of our research in Innsbruck was initially to develop a theory of experiments that would allow the laboratory observation of the three-particle correlations of Greenberger, Horne and Zeilinger. The available efficient sources of entangled photons only give us two-particle emissions. We had to develop a new method, which we did after two years, during which we also followed many wrong paths. Twenty-nine years ago, we published a paper showing the so-called entanglement exchange, allowing quantum entanglement of particles from two different sources. On the last page of this paper, we mention that this is also the way to obtain the mentioned three-particle correlations and that our method is likely to be a big step towards the final experimental test of Bell's inequality, which would be perfect and have no drawbacks, and we stated that the presented method would allow to perform quantum teleportation between photons. Because Anton, as co-author of this paper and subsequent improved versions of the method, knew 'how it was done' earlier than others, he was at an advantage when it came to the experimental realisation of these processes. A milestone in the development of quantum technology was the paper by Anton and colleagues in Nature reporting the performance of the first quantum teleportation, which demonstrated the new possibilities of quantum engineering. Earlier, in 1994, US scientist Peter Shor developed the first useful quantum algorithm, demonstrating the possibilities of future computational methods based on quantum information theory, which involves processing quantum states and allows very large numbers to be factorised into prime factors. Factorisation, an action based on division that is much more difficult than multiplication, is used in classical, i.e. non-quantum, encryption algorithms. Now I will ask you a question: is the number 9973 a prime number?
I confess straight away that I have no idea.
Well, that's right, and a quantum computer, if it existed, would 'tell' us right away. Normal computers would take a little longer. But when we get down to the really big numbers, the quantum advantage increases incredibly quickly. If a quantum computer is created, then all cyphers based on factorisation, and therefore part of the secrets, can be decrypted.
But at the same time, part of the secrets can be safely encrypted.
Yes, using quantum cryptography.
Is this a very distant future?
Quantum cryptography is already in use, although still in imperfect versions, but the quantum computer is still in its infancy.
You mentioned quantum teleportation, which to the layman is associated with science fiction films...
This was best put by Asher Peres, one of the authors of the first theoretical paper on quantum teleportation, a great person, unfortunately, he is no longer alive. He said that you don't teleport your body, you teleport your soul. Anton did teleportation between photons, not photon teleportation. He teleported a state of polarity.
You talk with great passion about physics, quantum mechanics and quantum engineering. Was it out of this passion that the International Centre for Theory of Quantum Technologies, of which you are the head, was born at the University of Gdańsk?
Of course, but it didn't happen right away. In the early 1990s, my colleagues considered me to be a kind of safe madman. When I got a professorship in Innsbruck, they started to change their minds and some people recognised that entanglement research perhaps made sense. In 1993, I was the first Gdańsk citizen to publish any scientific paper in the prestigious Physical Review Letters. It turned out that something could be achieved in this field. Ryszard Horodecki, together with his two sons, who were later joined by a third, Karol, started to produce fundamental papers on entanglement theory in Gdańsk a few years later. They too were pilloried in the beginning, and what a pillory! but their incredible publication success meant that they were finally recognised. They are leading scientists in our Centre, which we were able to establish in 2018 thanks to the generosity of the Foundation for Polish Science and European Union funding. We were awarded a five-year grant worth then ten million dollars in a competition announced by the FNP to establish an international scientific centre for quantum research. Today, it is a team that is winning further grants.
Do you assume concrete results from this research?
Even a great deal. The head of the Quantum Cyber Security Team, operating within the Centre, is my former PhD student, Marcin Pawłowski, now a postdoctoral fellow and professor at the University of Gdańsk. The team is winning new grants and developing patents related to quantum cryptography. Marcin is a co-founder of SeQure SpA, a company that manufactures random number generators. His team is also researching satellite technologies: he calculates how to perform corrections to sources of entanglement when they are installed on a satellite.
Again, this sounds a bit like fantasy. What are the chances of success for such research?
They are already bearing fruit. It is the patents mentioned, the start-ups. Industry-business-science consortia are being formed, such as the Polish Q Cluster. We are conducting research into the practical aspects of developing quantum communication networks, even with partners as distant as South Korea and Chile. An important development is the inclusion of quantum research in the document defining the National Science Policy, and not as a mention, but with extensive discussion indicating that it is to be one of the priorities. But for me at the moment, the priority is to get the tailcoat, which is the necessary attire for a guest of the official celebrations in Stockholm, which is just around the corner, on December 10.
Thank you for the interview.
dr Beata Czechowska-Derkacz
Research Promotion Officer
Institute of Media, Journalism and Social Communication
Prof. dr hab. Marek Żukowski
Professor of theoretical physics at the University of Gdańsk. Deals with fundamental aspects of quantum mechanics and quantum information. A graduate of the Faculty of Mathematics, Physics and Chemistry at the University of Gdańsk, he has been affiliated with our university since the beginning of his scientific career. He was repeatedly a visiting professor at the University of Innsbruck and the University of Vienna thanks to invitations from Anton Zeilinger, as well as Tsinghua University in Beijing and the Chinese Academy of Sciences (USTC). His best-known paper appeared in 'Physical Review Letters' (M. Zhukowski, A. Zeilinger, M. Horne, A. Ekert, 'Event-ready detectors' Bell experiment via entanglement swapping, 71, 4287 (1993)). This work and subsequent ones provided the theoretical operational basis for experiments showing multiphoton interference carried out in the 1990s by Zeilinger's group, such as experimental quantum teleportation, highlighted in the official description of the Nobel Prize in Physics for 2022 (https://www.nobelprize.org/uploads/2022/10/advanced-physicsprize2022-2...).
He has won several prestigious awards, including the Foundation for Polish Science Prize, commonly referred to as the 'Polish Nobel', for his research on multiphoton entangled states, which led to the formulation of information causality as a principle of physics (2013), the Maria Skłodowska-Curie Science Prize of the Polish Academy of Sciences (2013) and the Polish-German Science Prize 'Nicolaus Copernicus' (together with Harald Weinfurter, 2014). In 2016, he was awarded the 2015 Jan Hevelius Science Prize of the City of Gdańsk in the category of natural and exact sciences for formulating the theoretical basis of experimental multiphoton interferometry of entangled states and investigating the consequences of non-classical phenomena for our understanding of the foundations of quantum theory and the applicability of these phenomena in quantum communication.
He has held a number of positions: director of the Institute of Theoretical Physics and Astrophysics at the University of Gdańsk (2005-2018), member of the first and subsequent Boards of the National Science Center (2010-2018), editor in the American Physical Society journal 'Physical Review A' (2016-2019). He has been a Corresponding Member of the Polish Academy of Sciences since 2020 and Director of the UG International Centre for Theory of Quantum Technologies (ICTQT) since 2018, funded by the MAB programme of the FNP with funds from the European Union.