InstituteQ has selected Professor Sergey Kubatkin of Chalmers University of Technology as the InstituteQ Chair of Excellence 2024-26. Read his Q&A interview below. Professor Kubatkin's research group has two main research directions the forms of the basis of his involvement as InstituteQ Chair of Excellence:

- Studying the sources of decoherence in superconducting devices, aiming at the material side of application of superconducting qubits in quantum technology.
- Working on quantum transport in two-dimensional materials; the material of choice in our group is an epitaxial graphene grown on silicon carbide.

I got a PhD at Kapitza Institute in Moscow. I was studying low temperature physics at that time, and it was not possible to move abroad – we met Jukka when the team of Helsinki University of Technology visited Kapitza Institute when we got to know each other somewhere around 1985. When I came to Chalmers after ‘perestrojka,’ I was Interested in the research and physics of very small electronic systems at low temperatures. Then I continued to work in this direction, studying the physics of low dimensional materials.

Jukka was also doing superconducting electronics at low temperatures, so we followed the progress of each other and in principle I knew what he was doing. The physics of low temperatures was not as wide as it is now, so it was a close-knit community.

About 10 years ago, we started a Marie Curie project for the education of students and postdocs. Then it was possible to visit each other's labs, and I got to know that Jukka was interested in energy exchange and thermalization of quantum systems. And at that time in my group, we were working with this new material, graphene.

We exchanged ideas and it turned out that maybe we can use graphene to detect a single photon interacting with quantum systems, which Jukka’s research was focused on. And then we started a project that was supported by quantum efforts in Sweden at the Wallenberg Centre for Quantum Technology and Nano Area of Advance at Chalmers University. About half a year ago, Jukka told me that there is this opportunity to apply for the InstituteQ Chair of Excellence position here in Finland to continue this research.

There are local programs throughout the Nordic countries. In Denmark, we were collaborating with the Danes already for a long time, but not on quantum information. We were doing molecular electronics with them at the Niels Bohr Institute. But then I got to know that Charlie Marcus was there and he was starting down this line.

Of course, here in Finland, we know that here is also a huge effort in quantum science and technology, and in in Sweden there is that Wallenberg Centre for Quantum Technology.

There is an effort to combine activities on quantum materials to strengthen the numerous common research strands and bring together key experts in Nordic region. I think it was last December, we had a meeting, and this effort is initiated by Professor Alexander Balatsky and supported by Professor Mikael Fogelström. So, under the NORDITA umbrella, they want to combine all these things together involving also Baltic parties. And there will be another meeting in August already at Chalmers to see what we can do. Also, I know that Nordic Quantum community has been working towards stronger collaboration in quantum science and technology on a broader scope for several years now.

It's interesting to see different approaches to the same problems. We all know what the problems are of this industry of quantum science and technology research and development. It's clear that one group cannot solve these problems. Maybe even one country cannot solve these problems, but there are different approaches to find the solutions. It was interesting to see the approaches in Finland compared to Sweden and where I can contribute here.

For example, in universities it's very difficult to do research in this field, because it happens that you educate a person, and then they move elsewhere. There is already collaboration between Sweden and Finland, so this Wallenberg project is funding postdocs which will be affiliated both on Finnish side and on Swedish side.

With Jukka and his group, we are trying to understand how quantum systems are affected by the environment. This is one of the key problems in the field of quantum technology. If you have an isolated quantum system, it is governed by the rules of quantum mechanics. But as you add influence from the environment, it becomes more classical. How to understand this transition is a very important question. One of the things that we were trying to do at Chalmers in my group is also study the coherence of quantum systems which are material related.

Now the technology is such that you can design the system and you can design the strength of the interaction of this system with the environment. For me, this is very interesting and I would like to contribute to the solution of this problem.

In the real world, for example, you make your quantum device on a substrate, and this substrate has defects, and these defects can “steal” quantum information from your system, so it's important to design the material which will be more-or-less calm so it will not affect the system.

I see myself not as a General of science, but as a field soldier of science – my experiments are driven by curiosity, rather than a shiny ‘strategic’ plot. So I see something interesting, and I go there and try to seek out more information. In this respect, technological progress in hardware, motivated by an effort in quantum computation, is very encouraging for an experimentalist – now we can look for experimental answers to fundamental questions, formulated in the textbooks on quantum mechanics – the non-demolition measurement of a quantum state and the quantum error correction are exemplary exciting directions here.

Quantum computation has stimulated a lot of efforts in material science, I have already mentioned the issue of defects, affecting coherence of a quantum system, and the efforts to eliminate their detrimental effects. We are also looking at 2D materials, but since there are many combinations of things that you can do, some theoretical guidelines are needed to narrow down the experimental search. Maybe topological considerations provide a good approach to select materials, but promising topology must be combined with the material stability and technological compatibility with the processing used in fabrication of quantum hardware. I fear that the journey along this road may be long and not necessarily successful. It took us 10 years to master the growth and processing of graphene. Now we know everything about it. But we knew that it is worth going there.

We were working with the Niels Bohr Institute at Copenhagen, and the subject was molecular electronics. It was a very attractive idea that everything is getting smaller and smaller, so eventually it can go to a single molecule which will work as a transistor. We started with graphene as an ideal electrode for contacting single molecule – a big challenge at that time. The requirement of scalability brought me to epitaxial graphene on silicon carbide, which is a single crystal on a wafer scale, and then we found that it shows very robust quantum Hall effect, beating in performance all materials used for over 30 years of quantum Hall metrology. Then it was deviation from molecular electronics into quantum Hall resistance study, and now most of metrological institutions are using our devices. This was an example of a challenging task, which brought an unexpected and useful outcome; I hope that similar things will happen when developing quantum technology.

I hope that my position here at InstituteQ will help me in following two challenging directions, both of which are related to the interaction of quantum system with its environment.

I am interested in the improvement of coherence in solid-state quantum devices by active interference with solid-state defects based on an understanding the microscopic mechanisms of thermalization and the energy exchange of two-level systems they are hosting. Another fundamental question is what are the parameters controlling a transition between quantum-mechanical and thermodynamical description of a system – while many research groups are trying to demonstrate quantum advantage of calculation with increasing number of qubits in the quantum processor, practice tells us that a large enough system on a long time scale is governed by thermodynamics. Understanding the mechanism of such a transition, based on detailed study of the energy exchange between the quantum system and its environment is therefore very important both from the fundamental and practical points of view: we may also learn whether scaling up a quantum processor in terms of the number of qubits will lead to decoherence and eventually classical thermodynamic behavior on a given technological platform.

I believe that my experience with low-dimensional systems, in particular, 2D materials will help addressing both problems. For example graphene, grown and treated in my lab, can be doped to extremely low charge carrier densities, allowing fabrication of a ‘click’ photon detector to measure energy exchange at the level of individual microwave quanta. Experiments with these detectors will shed light on the fundamental questions of the role of quantum entanglement in thermodynamics and the thermalization of simple quantum systems, allowing its applications to broader quantum technologies such as sensing, computation, and communication.

The effort in quantum computing has triggered a boost in development of quantum technology, which has a value on its own. I see a historical analogy with the development of classical computers, which has lead to huge advances in microelectronics – and now electronic devices are a part of our everyday life.

I see that most immediate outcome of advances in quantum technology will be in quantum sensing. Here an advantage of quantum approach is more achievable, from my point of view. What I want to get from this collaboration is advances in quantum sensing; a ‘click’ microwave photon counter, based on graphene is one example here.

In general, I am looking forward to establishing synergetic interaction with the PIs of InstituteQ: we are already familiar with the work and scientific interests of each other. For example, my former postdoc is working now in the group of Professor Pertti Hakonen. The parametric amplifier from Visa Vesterinen at VTT is now being installed in our cryostat, which came about after my postdoc visited VTT to learn how to handle it. We have discussed lines of collaboration with Sorin Paraoanu, and I also expect collaboration in quantum metrology with the group of Antti Manninen at VTT. It would be nice to turn these expectations in reality – the whole field is exciting.