*Cambyse Rouzé | Meet the MCQSTians: they are curious about science and passionate about their research, have diverse careers paths, work in inter-disciplinary teams, and are at the forefront of the second quantum revolution. In this series, we regularly feature members of the MCQST community.*

### “Being an independent researcher means facing the challenges that come with freedom.”

Cambyse Rouzé is a 2020 MCQST START Fellow who joined the Department of Mathematical Physics at TU Munich as a postdoctoral researcher in 2019. We invited Cambyse to share the challenges he faces in his day-to-day work, solving difficult problems, and discovering his love for Bavarian food.

**Can you briefly explain your research project? **

My current research lies at the intersection between quantum information theory and quantum many body systems. More precisely, I am interested in the interplay between the behavior of the correlations between apart regions of a spin system at its thermal equilibrium, the time it takes for that system to thermalize in nature, and how hard it is to simulate that system at equilibrium on a classical or quantum computer. These questions have been extensively studied for classical spin systems, for which rigorous equivalences between strongly decaying correlations in the distance between any two regions at equilibrium on the one side, and rapid convergence of the system to its equilibrium on the other, were proven. However, their quantum analogues are much harder to analyze, which is mainly due to the presence of quantum correlations and entanglement.

Another aspect of my research concerns the analysis of bosonic quantum systems and Gaussian channels, for example, collections of electromagnetic modes traveling along an optical fiber, in the context of continuous-variable quantum information theory. This setting, which constitutes the basis of practically all proposed protocols for quantum communication, is of outstanding technological and experimental relevance. Quantum Gaussian channels are the quantum counterpart of Gaussian integral kernels, whose communication capacity under a constraint on the input energy can be estimated by sharp entropic inequalities which go back to the early work of Shannon. The correct quantum analogues of these inequalities, which would provide upper bounds on, for example, the quantum communication capacity of a quantum Gaussian channel, are still missing in their full generality. Once again, this is due to the current lack of good techniques to control the effect of entanglement at the input of such channels.

**What does your day-to-day work look like?**

Being an independent researcher means facing the challenges that come with freedom. On the one hand, you can organize your days at your convenience. However, this means that you need to keep a minimum level of self-discipline and organization to make sure that your projects are advancing. I tend not to strictly organize my days, simply because I hardly ever follow my plans. You never know when you might get the right idea for a project, and it is important to leave a long time to think about each of them until then. For that reason, I tend to have a sketch of what my week should look like. I always leave a day free to read new papers that are connected to my research. I also have weekly meetings with my collaborators, or my PhD students and postdocs here at TUM, to discuss our recent progress and the current challenges that we are facing. COVID has significantly changed these fundamental (and frankly, most exciting) aspects of my work, since all meetings are now held online. A large share of the work also consists of daily tasks, namely answering emails and reading about newly submitted papers, which I try to do while drinking my morning coffee.

**What keeps you excited and makes you want to start work in the morning?**

I think I have a very hard time putting down a problem once I am stuck with it (and this happens relatively often!). So usually, coming back to my old “friend” that I left before going to bed is what makes me want to start the day.

**What inspired you to become a quantum scientist? Did you always know you wanted to be a scientist? **

I would probably be lying if I said so. The quantum education in high school is very minimal, at least in France. On the other hand, I was always attracted by the power that mathematics provides in thinking about a physical problem. However, I directly got excited about quantum science two years into my bachelor, when I learned about how it required a complete rethinking of concepts in physics as basic as the notions of locality and determinism.

##### "A negative result still is a result, and it almost always leads to a better understanding of the system that you are studying."

**What drew you to Munich? **

After my PhD, I wanted to work on more mathematical problems in quantum information theory. I already knew about the groups of Prof. König and Prof. Wolf, whose research interests aligned with mine. I learned only after about the importance of MCQST, and the many great opportunities that it offers.

**What are the biggest challenges that you’ve faced, and how have you overcome them? **

I think the biggest challenge when you work in mathematical physics is being able to overcome the constant frustration that comes with not being able to prove a result that you intuitively know to be true. You very often get stuck (sometimes for a long time) on one seemingly tiny detail, which might turn out to be a “deal breaker”. I think the best way to overcome a situation like this is to embrace it: a negative result still is a result, and it almost always leads to a better understanding of the system that you are studying.

**What was your most memorable moment or proudest achievement? **

Clearly, managing to prove an open problem with the simplest possible method is very exhilarating. This does not happen very often, but when it does, you want to share it! This situation happened at the beginning of the year: there was this conjecture about the exponential entropic convergence of Markovian evolutions towards equilibrium independently of the system’s environment, which several groups had been actively trying to solve for half a decade. My postdoc and I were trying to prove some related (yet different) entropic inequalities when I realized that the methods we were using could also be applied to solve that open problem.

**Outside of science, what do you enjoy doing most? **

With the present COVID situation, we are naturally restricted in the things that we can do. But I was lucky to arrive in Munich in 2019, and hence truly got the chance to explore this beautiful city. As a French citizen, I should probably not say this, but I love Bavarian food, and therefore enjoyed spending some time in beer gardens when they were still open. I also enjoyed visiting the great museums that the city has to offer (and in particular the Deutsches Museum). Nowadays, I mainly try to spend most of my free time exploring the Bavarian countryside. Some of the mountain landscapes are simply breathtaking.

Find out more about our
START Fellowship and
Cambyse's research.