# Highlights

Institute's News

### Call for Distinguished PKS Postdoctoral Fellowship 2021 now open!

Application deadline: 25 November 2020. Distinguished PKS postdoctoral fellows appear personally along with the departments and groups on the main research page of the institute and are expected to have at least one year of postdoctoral experience at an institution other than the one at which their PhD was awarded. Applications for this fellowship directly after completion of the PhD might be considered in exceptional cases. Please click on the link- button to see the full advertisement!
Publication Highlights

### Interaction-Induced Transparency for Strong-Coupling Polaritons

The propagation of light in strongly coupled atomic media takes place through the formation of polaritons—hybrid quasiparticles resulting from a superposition of an atomic and a photonic excitation. Here we consider the propagation under the condition of electromagnetically induced transparency and show that a novel many-body phenomenon can appear due to strong, dissipative interactions between the polaritons. Upon increasing the photon-pump strength, we find a first-order transition between an opaque phase with strongly broadened polaritons and a transparent phase where a long-lived polariton branch with highly tunable occupation emerges. Across this nonequilibrium phase transition, the transparency window is reconstructed via nonlinear interference effects induced by the dissipative polariton interactions. Our predictions are based on a systematic diagrammatic expansion of the nonequilibrium Dyson equations which can be controlled, even in the nonperturbative regime of large single-atom cooperativities, provided the polariton interactions are sufficiently long-ranged. Such a regime can be reached in photonic crystal waveguides thanks to the tunability of interactions, allowing us to observe the interaction-induced transparency transition even at low polariton densities.

J. Lang et al., Phys. Rev. Lett. 125, 133604 (2020)
Awards and Honors

### ERC Starting grant for Christoph Weber

The European Research Council (ERC) has selected 436 early-career top researchers from 40 nationalities across Europe in the 2020 'Starting Grant' competition. The prestigious grants enable the best young researchers in Europe to build their own teams and to conduct pioneering research across all disciplines. Among the awardees is Christoph Weber, research group leader at the MPI-PKS and the Center for Systems Biology Dresden. With his project „FuelledLife”, Christoph and his group want to understand how living cells regulate phase separation and to unveil the role of phase separation for the emergence of life. "Living cells rely on the compartmentalisation of thousands of different molecules and their chemical reactions,” Christoph explains. “Remarkably, many of such compartments form by phase separation of polymers and are controlled by sequence-specific interactions but also cellular fuel driving the system away from thermodynamic equilibrium. If we knew how such polymers evolve in time and compartmentalise in multi-component mixtures, we would better understand the role of phase separation in living cells and how prebiotic cells could have emerged at the origin of life. My team and I will develop a theory for phase separation and chemical reactions in multi-component mixtures that are driven by irreversible, fuel-driven reactions. Our theoretical studies will let us understand how living cells regulate phase separation at the origin of life and determine the prerequisites of a protocell to divide, replicate and undergo selection." Christoph will receive 1.5 million Euros over a period of five years. Congratulations to the Weber group!
Publication Highlights

### Quantum many-body dynamics in two dimensions with artificial neural networks

In the last two decades the field of nonequilibrium quantum many-body systems has seen a rapid development driven, in particular, by the remarkable progress in quantum simulators, which provide access to dynamics in quantum matter with an unprecedented control. However, the efficient numerical simulation of nonequilibrium real-time evolution in isolated quantum matter remains a key challenge for current computational methods especially beyond one spatial dimension. In this work we present a versatile and efficient machine learning inspired approach based on a recently introduced artificial neural network encoding of quantum many-body wave functions. We identify and resolve key challenges for the simulation of real-time evolution, which previously imposed significant limitations on the accurate description of large systems and long-time dynamics. As a concrete example, we study the dynamics of the paradigmatic two-dimensional transverse field Ising model, where we observe collapse and revival oscillations of ferromagnetic order and demonstrate that the reached time scales are comparable to or exceed the capabilities of state-of-the-art tensor network methods.

M. Schmitt and M. Heyl, Phys. Rev. Lett. 125, 100503 (2020)
Awards and Honors

### European funding to unravel scientific mysteries

Steffen Rulands receives ERC Starting Grant The prestigious ERC starting grants allow the best young researchers in Europe to build their own teams and to conduct pioneering research across all disciplines. Among this year’s awardees is Steffen Rulands, research group leader at the Max Planck Institute for the Physics of Complex Systems, and the Center for Systems Biology Dresden! With the interdisciplinary project “AHH-OMICS” Steffen and his group aim to apply theoretical methods originally developed in the physics of solids to understand the mechanisms underlying the behaviour of cells during development, regeneration and ageing. Steffen explains, “The recent breakthroughs of single-cell sequencing technologies for the first time give us the opportunity to probe the inner life of cells with unprecedented molecular detail. Biological function, however, relies on how many molecules work together in space and time, which up to now cannot be inferred from these experiments.” The theoretical physicist continues, “It’s like a car, where from detailed knowledge of all car parts we still cannot understand how an engine works. We need methods from statistical physics to do that! The same holds true for the cell – from detailed measurements of molecules we don’t learn about biological function. The theoretical tools that will be developed in my project will bridge this gap.” The ERC project will combine the novel single-cell technologies with methods from statistical and solid-state physics to understand collective processes regulating cellular behaviour. With this conceptually new approach the Rulands group will overcome existing limitations in an emerging technology and pioneer the application of methods from statistical physics to single-cell genomics. Steffen Rulands will receive 1.5 million Euros from the ERC over a period of five years. The European Research Council (ERC) has selected 436 early-career top researchers from 40 nationalities across Europe in their 2020 'Starting Grant' competition. The funding, worth in total €677 million, is part of the EU’s Research and Innovation programme, Horizon 2020. Congratulations to the Rulands group!
Institute's News

### New Research Group at the Center for Systems Biology Dresden starts in January 2021

Starting in January 2021, Pierre Haas will lead the research group 'Self-organization of Multicellular Systems' at the Center for Systems Biology Dresden. The research of the group will focus on the mechanics of cells and tissues, with a particular interest in deriving the continuum theories that represent the rich mechanical behaviour of tissues during development, and to understand how robust development is compatible with mechanical constraints and variability. While the research of the group is theoretical, it will work in close collaboration with experimental groups at MPI-CBG and beyond.
Awards and Honors

### Anne E. B. Nielsen receives the H.C. Ørsted Research Talent Prize 2020

The H.C. Ørsted Prize and the two H.C. Ørsted Research Talent Prizes are awarded annually to celebrate the Danish physicist and chemist Hans Christian Ørsted's influence on culture, art, thinkers, and scientists all over the world. The prizes are awarded by the H.C. Ørsted Association and Langeland municipality with support from the energy company Ørsted. H.C. Ørsted discovered in 1820 that an electric current produces a magnetic field, and to celebrate the 200 years anniversary of this influential discovery, the prizes are this year awarded to scientists, whose research is related to electromagnetism and its applications in a broad sense. Anne E. B. Nielsen receives the H.C. Ørsted Research Talent Prize, which is donated with a diploma and 10,000 Danish kroner, for her many innovative contributions to the fields of anyon research and quantum light, where she investigates possibilities that arise by combining quantum mechanics and electromagnetism. The prize ceremony took place on H.C. Ørsted's birthday on August 14 in Rudkøbing on the Danish island Langeland, where H.C. Ørsted grew up.
Publication Highlights

### Realization of an anomalous Floquet topological system with ultracold atoms

Coherent control via periodic modulation, also known as Floquet engineering, has emerged as a powerful experimental method for the realization of novel quantum systems with exotic properties. In particular, it has been employed to study topological phenomena in a variety of different platforms. In driven systems, the topological properties of the quasienergy bands can often be determined by standard topological invariants, such as Chern numbers, which are commonly used in static systems. However, due to the periodic nature of the quasienergy spectrum, this topological description is incomplete and new invariants are required to fully capture the topological properties of these driven settings. Most prominently, there are two-dimensional anomalous Floquet systems that exhibit robust chiral edge modes, despite all Chern numbers being equal to zero. Here we realize such a system with bosonic atoms in a periodically driven honeycomb lattice and infer the complete set of topological invariants from energy gap measurements and local Hall deflections.

Wintersperger et al., Nature Physics (2020)
Publication Highlights

### Excitonic Laughlin states in ideal topological insulator flat bands and their possible presence in moiré superlattice materials

We investigate few- and many-body states in half-filled ideal topological insulator flat bands realized by two degenerate Landau levels which experience opposite magnetic fields. This serves as a toy model of flat bands in moiré materials in which valleys have Chern numbers $C=\pm 1$. We argue that although the spontaneously polarized Ising Chern magnet is a natural ground state for repulsive Coulomb interactions, it can be in reasonable energetic competition with correlated Laughlin states of excitons when short-distance corrections to interactions are included. This is because charge neutral excitons in these bands behave effectively as charged particles in ordinary Landau levels. In particular, the Ising Chern magnet is no longer the ground state once the strength of a short-range intravalley repulsion is about 30% larger than the intervalley repulsion. Remarkably, these excitonic Laughlin states feature valley number fractionalization but no charge fractionalization and a quantized charge Hall conductivity identical to the Ising magnet, $\sigma_{xy}=±e2/h$, and thus cannot be distinguished from it by ordinary charge transport measurements. The Laughlin state with the highest density of excitons that can be constructed in these bands is an analog of $\nu=1/4$ bosonic Laughlin state and has no valley polarization even though it spontaneously breaks time reversal symmetry.

N. Stefanidis and I. Sodemann, Phys. Rev. B 102, 035158 (2020)