In the series “Behind Optologic”, we interview each of the projects partners. Meet MBI.

In any project, all of the members bring different expertise to the table. In a project where the goal is to construct quantum devices that can dissipate the less energy possible and save up on energy storage, how do we start?

We know that in physics, the evolution and validation of a theory always happen following the scientific process, you pose a hypothesis and you test it experimentally in the lab. If the experiment results coincide with your hypothesis, you can consider your hypothesis a possible theory, always taking into account that such theory needs to be tested and validated by independent groups of scientists before naming it as such.

The pandemic has shown us that we can be connected globally without having to move from home. Zoom, Teams, Skype, you name it. So, we took advantage of this and asked Misha Ivanov, Olga Smirnova and Alvaro Jiménez-Galán, from MBI Berlin, to join us in a call, and explain what is the theory lying behind this project, what they expect to achieve and what unknowns do they foresee to encounter.

Optologic intends to use topological materials for developing energy dissipation fewer devices. How did the group come up with this idea?

Álvaro Jiménez-Galán: It began because we started exploring the topological materials using attosecond science. We asked ourselves, is it possible to induce the topological phase transition with light in non-topological materials? We want to create a switch with ultrafast pulses of light that allows us to control or change the topological properties of a material.

Olga Smirnova. Misha and Maciej (Lewenstein, ICFO) know each other for many years. They published a famous paper together, on the foundations of high-harmonic generation, which is basically the theory that is at the core of many nonlinear optical quantum effects in materials. Because they have very similar interests, they decided almost simultaneously to move to topological materials. Jens (Biegert) and Maciej, both from ICFO, have been thinking about the project, and then Maciej thought about including us.

Misha Ivanov: Alvaro had the idea of whether we could modify the structure of the material using light. People have been doing it for some time, but with long pulses, so we decided to try if we could do it using ultrafast pulses. And so far, it seems to work, so we are confident, at least, in the theoretical aspect of it all.

How do you relate to Maciej’s group? Do you bring expertise to the field? 

Misha: Maciej and I have been collaborating together for many decades now. We both came into the field of topological materials just a couple of years ago, and into strong field theory at the turn of the 1990s. Recently, we wanted to understand how to probe the dynamics of materials with strong laser fields and see what physics could be extracted from it from a quantum physics point of view… But there is a huge contribution from Alvaro and Rui in this idea. Alvaro, go ahead, tell them.

Álvaro: I’m a theorist by training and the idea that we had in our group, together with Rui (then at the MBI, now at the University Autonoma de Madrid), was to search for ways to modify the structure of the material with light. This was first achieved around 2009 with Floquet topological insulators, but using long laser pulses. The challenge we face now is to do this with short laser pulses, to study the dynamics and induce new unusual states in topological materials… and do this in the attosecond science regime!

What are exactly these topological materials?

Olga: In the micro world, electrons are moving and running around certain surfaces, and the structure and geometry of these surfaces really matter. Topological materials are characterized by their different topologies, by how they look: for example if it’s a ring with one hole or two holes. Topological materials are popular because people think that their properties can be used to create better devices. And what we realized is that the topology can be probed also by looking at the ultrafast response of electrons, on the time-scale of a millionth of a billionth of a second. Is like looking at a stadium full of runners running in the dark, looking at where they run, you can conclude the shape of the stadium.

Misha: And sometimes you can’t see the stadium, because it is in the dark. So what we are looking at is the light. Each runner is carrying a flashlight; you can look at the traces of light they leave. This is not an exaggeration — we are literally looking at the light emitted by the electrons. Light is something you detect relatively easily; it is a beautiful probe. Also, you can measure its amplitude, phase, intensity, and those are very sensitive parameters that tell us a lot about the material.

Alvaro: We are using simple 2D materials, so we can theoretically model them. We are using 2d materials, such as hexagonal boron nitrate (hBN), which is our favourite one, because it has a huge bandgap. This is important for what we do because we require big bandgaps, and this material, in particular, can be described relatively well with just 2 bands, which is the simplest case to study. And it also gives a lot of information, because we tried the light-induced topology and saw it was working. So the next step is to use 3D topological insulators, the paradigm is bismuth selenite (Bi2(SeO3)3), but it is a more complex material, so it will be a challenge for us.

How are these topological materials used for qubits or logic operations?

Alvaro: The main challenge for us right now is to include correlation effects.

Misha: Yes, excitons, that is what we need to include. We always had the belief, maybe unjustified, that because our laser fields are normally strong, excitons would be killed. We still have not checked it, but our experience from atomic physics suggests that weakly bound states might be surprisingly robust against strong laser fields. If they survive, that’s good news, because maybe we can do something interesting with them. And if they die, that’s also good news, because we can keep going without them.

Why did you decide to join this field of research?

Alvaro: To be honest, for me it was the Review of modern physics written by Misha. After I graduated in physics, I tried to be a musician for one and a half years. But I still read scientific papers, and I read Misha’s review, so I decided to go and work in this field and ended working in Fernando Martín’s group.

Olga: I was at the Moscow State University and I wanted to do quantum statistics and quantum field theory because I thought it was the most challenging field for theorists. But when I entered that department, they told me I better not do it and better not consider that field, because I am a woman, so one day I would marry, then become a mother, and then have no time… they didn’t want to work with me. So I found a supervisor that really wanted to work with me. It was no longer about choosing a topic, but about choosing a person willing to work with female scientists. And fortunately, I found the perfect person to work with. For me, it was really accidental, I didn’t get to choose the field. But I don’t regret it!

Misha: I’ll start from the most important thing. When I was a kid, I lived in a high-rise building, on the 15^th floor. On that same floor, there lived a beautiful woman, that was a great friend of my mother’s. She divorced and then remarried, and her new husband was Nicolai Delone. Delone is the father of strong field and multiphoton science physics in the Soviet Union. He had an incredible influence on me as a child, I was 12 years old when I met him. He was (and still is) my role model, my mentor and my teacher for many years. One day I told him “I cannot choose whether I should study literature or mathematics”, and he said, “None of the above, you should do theoretical physics, it’s almost exactly the same as mathematics, but it is easier to find a job”. So I became a theoretical physicist. The reason I started to do what I’m doing is that Delone forged a beautiful family of people in the field of strong-field physics, all over the Soviet Union, creating a great atmosphere. Both Olga and I have been a part of this family, we treasure this memory, and we’re trying to continue Delone’s legacy with what we do.

Do you think that working with a team that you like is more important than the topic itself?

Olga: It doesn’t matter where you start, you will always find things that are interesting for you. It is very important that the environment is creative, that it is dynamic.

Misha: What is really important is that there is good chemistry and a good atmosphere. If you have people that you enjoy working with, this makes the whole process a lot more fun, and if it’s fun, then you get good results. It’s very hard to create new ideas if you don’t have fun with them, and the team is very important.

Alvaro: If the question is, would I do high energy physics with Misha, Olga and Rui? The answer is, of course! We would have fun anyways.