Valentin Bruch
I am a PhD candidate in theoretical physics at RWTH Aachen University. My interests lie in the field of open quantum systems, focussing on general analytic insights and semianalytic renormalization group methods.
Research interests
 Unconventional duality mappings
 Applying quantum information to condensed matter problems
 Renormalization group methods for strongly correlated, driven open quantum systems
Publications

V. B., K. Nestmann, J. Schulenborg and M. R. Wegewijs
Fermionic duality: General symmetry of open systems with strong dissipation and memory
, SciPost Phys. 11, 053 (2021)We consider the exact timeevolution of a broad class of fermionic open quantum systems with both strong interactions and strong coupling to wideband reservoirs. We present a nontrivial fermionic duality relation between the evolution of states (Schrödinger) and of observables (Heisenberg). We show how this highly nonintuitive relation can be understood and exploited in analytical calculations within all canonical approaches to quantum dynamics, covering Kraus measurement operators, the ChoiJamiołkowski state, timeconvolution and convolutionless quantum master equations and generalized Lindblad jump operators. We discuss the insights this offers into the divisibility and causal structure of the dynamics and the application to nonperturbative Markov approximations and their initialslip corrections. Our results underscore that predictions for fermionic models are already fixed by fundamental principles to a much greater extent than previously thought. 
K. Nestmann, V. B. and M. R. Wegewijs
How quantum evolution with memory is generated in a timelocal way
, Phys. Rev. X 11, 021041 (2021)Two widely used but distinct approaches to the dynamics of open quantum systems are the NakajimaZwanzig and timeconvolutionless quantum master equation, respectively. Although both describe identical quantum evolutions with strong memory effects, the first uses a timenonlocal memory kernel 𝒦, whereas the second achieves the same using a timelocal generator 𝒢. Here we show that the two are connected by a simple yet general fixedpoint relation: $\mathcal{G}=\widehat{\mathcal{K}}[\mathcal{G}]$. This allows one to extract nontrivial relations between the two completely different ways of computing the timeevolution and combine their strengths. We first discuss the stationary generator, which enables a Markov approximation that is both nonperturbative and completely positive for a large class of evolutions. We show that this generator is not equal to the lowfrequency limit of the memory kernel, but additionally “samples” it at nonzero characteristic frequencies. This clarifies the subtle roles of frequency dependence and semigroup factorization in existing Markov approximation strategies. Second, we prove that the fixedpoint equation sums up the timedomain gradient / Moyal expansion for the timenonlocal quantum master equation, providing nonperturbative insight into the generation of memory effects. Finally, we show that the fixedpoint relation enables a direct iterative numerical computation of both the stationary and the transient generator from a given memory kernel. For the transient generator this produces nonsemigroup approximations which are constrained to be both initially and asymptotically accurate at each iteration step.
CV
 2019–2022 (planned): PhD student in the research training group 1995 with Prof. Dante Kennes (RWTH Aachen University) and Prof. Maarten Wegewijs (FZ Jülich)
 2019: Master of Science, RWTH Aachen University, thesis on
Fermionic duality: dissipative symmetry for open system dynamics beyond weak coupling
in the group of Prof. Maarten Wegewijs  2017: Bachelor of Science, RWTH Aachen University, thesis on
Density oscillations in onedimensional manybody systems with spin
Awards
 2020: SpringorumDenkmünze, awarded by RWTH Aachen University
 2018: Scholarship awarded by RWTH Education Fund, funded by Robert Bosch GmbH and the German Ministry for Education and Science
 2018: Schöneborn Prize
for outstanding results in the Bachelor studies
by the Department of Computer Science at RWTH Aachen University