# Highlights

Publication Highlights

### "Census" in the zebrafish's brain

Dresden scientists explore newborn, regenerated neurons The zebrafish is a master of regeneration: If brain cells are lost due to injury or disease, it can simply reproduce them - contrary to humans where this only happens in the fetal stage. However, the zebrafish is evolutionarily related to humans and, thus, possesses the same brain cell types as humans. Can a hidden regeneration potential also be activated in humans? Are therapies for stroke, craniocerebral trauma and presently incurable diseases such as Alzheimer's and Parkinson's possible?
Dresden scientists have now succeeded in determining the number and type of newly formed neurons in zebrafish; practically conducting a “census” in their brains. Following an injury, zebrafish form new neurons in high numbers and integrate them into the nervous system, which is the reason for their amazing brain regeneration ability. The study is a true collaboration project “made in Dresden”: Scientists from the Center for Regenerative Therapies TU Dresden (CRTD) combined their expertise in stem cell biology with complex bio-informatic analyses from the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) and the Center for Systems Biology Dresden (CSBD) and with the latest sequencing methods from the DRESDEN-concept Genome Center.
For their study now published in Development, the team led by Christian Lange and Michael Brand from the CRTD used adult transgenic zebrafish in whose forebrain they were able to identify the newborn neurons. The forebrain of the zebrafish is the equivalent to the human cerebral cortex, the largest and functionally most important part of the brain. Together with the Steffen Rulands group at the MPI-PKS and the CSBD, the interdisciplinary research team investigated the newborn and mature neurons as well as brain stem cells using single cell sequencing. Thus, they discovered specific markers for newborn neurons and were able to comprehensively analyze which types of neurons are newly formed in the adult brain of the zebrafish. Together, researchers also investigated the data obtained from brain cells of mice and found that zebrafish and mice have the same cell types. This also makes these results highly relevant for humans.
"On the basis of this study, we will further investigate the regeneration processes that take place in zebrafish. In particular, we will study the formation of new neurons after traumatic brain damage and their integration," explains Michael Brand, CRTD Director and senior author of the study. "We hope to gain insights that are relevant for possible therapies helping people after injuries and strokes or suffering from neurodegenerative diseases. We already know that a certain regenerative ability is also present in humans and we are working on awakening this potential. The results of our study are also important for understanding the conditions under which transplanted neurons can network with the existing ones and thus could let humans re-gain their former mental performance.”
Original Publication: Christian Lange, Fabian Rost, Anja Machate, Susanne Reinhardt, Matthias Lesche, Anke Weber, Veronika Kuscha, Andreas Dahl, Steffen Rulands and Michael Brand: „Single cell sequencing of radial glia progeny reveals diversity of newborn neurons in the adult zebrafish brain”
Development 20 147, published 9 January 2020, doi: 10.1242/dev.185595
Publication Highlights

### Phasonic Spectroscopy of a Quantum Gas in a Quasicrystalline Lattice

Phasonic degrees of freedom are unique to quasiperiodic structures and play a central role in poorly understood properties of quasicrystals from excitation spectra to wave function statistics to electronic transport. However, phasons are challenging to access dynamically in the solid state due to their complex long-range character and the effects of disorder and strain. We report phasonic spectroscopy of a quantum gas in a one-dimensional quasicrystalline optical lattice. We observe that strong phasonic driving produces a nonperturbative high-harmonic plateau strikingly different from the effects of standard dipolar driving. Tuning the potential from crystalline to quasicrystalline, we identify spectroscopic signatures of quasiperiodicity and interactions and map the emergence of a multifractal energy spectrum, opening a path to direct imaging of the Hofstadter butterfly.

S.V. Rajagopal et al., Phys. Rev. Lett. 123, 223201 (2019)
Publication Highlights

### Emergent Quasicrystalline Symmetry in Light-Induced Quantum Phase Transitions

The discovery of quasicrystals with crystallographically forbidden rotational symmetries has changed the notion of the ordering in materials, yet little is known about the dynamical emergence of such exotic forms of order. Here we theoretically study a nonequilibrium cavity-QED setup realizing a zero-temperature quantum phase transition from a homogeneous Bose-Einstein condensate to a quasicrystalline phase via collective superradiant light scattering. Across the superradiant phase transition, collective light scattering creates a dynamical, quasicrystalline optical potential for the atoms. Remarkably, the quasicrystalline potential is “emergent” as its eightfold rotational symmetry is not present in the Hamiltonian of the system, rather appears solely in the low-energy states. For sufficiently strong two-body contact interactions between atoms, a quasicrystalline order is stabilized in the system, while for weakly interacting atoms the condensate is localized in one or few of the deepest minima of the quasicrystalline potential.

F. Mivehvar et al., Phys. Rev. Lett. 123, 210604 (2019)
Publication Highlights

### Quantifying and Controlling Prethermal Nonergodicity in Interacting Floquet Matter

Periodic driving has become a key element in the experimental efforts to synthesize interesting many-body quantum states. Importantly, this depends crucially on the existence of a prethermal regime, which exhibits drive-tunable properties while forestalling the effects of undesirable heating. This dependence motivates the search for direct experimental probes of the underlying localized nonergodic nature of the wave function in this metastable regime. We report experiments on a many-body Floquet system consisting of atoms in an optical lattice subjected to ultrastrong sign-changing amplitude modulation. In this work, we measure an inverse participation ratio quantifying the degree of prethermal localization of the many-body wave function as a function of tunable drive parameters and interactions. We obtain a complete prethermal map of the drive-dependent properties of Floquet matter spanning four square decades of parameter space. Following the full time evolution, we observe sequential formation of two prethermal plateaux, interaction-driven ergodicity, and strongly frequency-dependent dynamics of long-time thermalization. The quantitative characterization of the prethermal Floquet matter realized in these experiments, along with the demonstration of control of its properties by variation of drive parameters and interactions, opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering.

Singh et al., Phys. Rev. X 9, 041021 (2019)
Publication Highlights

### Cavity-Quantum-Electrodynamical Toolbox for Quantum Magnetism

The recent experimental observation of spinor self-ordering of ultracold atoms in optical resonators has set the stage for the exploration of emergent magnetic orders in quantum-gas-cavity systems. Based on this platform, we introduce a generic scheme for the implementation of long-range quantum spin Hamiltonians composed of various types of couplings, including Heisenberg and Dzyaloshinskii-Moriya interactions. Our model is composed of an effective two-component Bose-Einstein condensate, driven by two classical pump lasers and coupled to a single dynamic mode of a linear cavity in a double $\Lambda$ scheme. Cavity photons mediate the long-range spin-spin interactions with spatially modulated coupling coefficients, where the latter ones can be tuned by modifying spatial profiles of the pump lasers. As experimentally relevant examples, we demonstrate that by properly choosing the spatial profiles of the pump lasers achiral domain-wall antiferromagnetic and chiral spin-spiral orders emerge beyond critical laser strengths. The transition between these two phases can be observed in a single experimental setup by tuning the reflectivity of a mirror. We also discuss extensions of our scheme for the implementation of other classes of spin Hamiltonians.

F. Mivehvar et al., Phys. Rev. Lett. 122, 113603 (2019)
Publication Highlights

### Genes lost during the transition from land to water in cetaceans highlight genomic changes associated with aquatic adaptations

While whales and dolphins spend their entire life in the ocean, these air-breathing mammals actually evolved from terrestrial species. The transition from land to water in the ancestors of modern whales and dolphins about 50 million years ago was accompanied by profound anatomical, physiological, and behavioral adaptations that facilitated life in water. However, which changes in the DNA underlie these adaptations remains incompletely understood. To reveal them, researchers at the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG), the MPI for the Physics of Complex Systems (MPI-PKS), and the Center for Systems Biology Dresden (CSBD) systematically searched for genes that were lost in the ancestor of today’s whales and dolphins. The research team discovered 85 gene losses, some of which likely helped whales to thrive in their new habitat.

M. Huelsmann et al., Science Advances 5, eaaw6671 (2019)
Publication Highlights

### Dynamical Structure Factor of the Three-Dimensional Quantum Spin Liquid Candidate NaCaNi$_2$F$_7$

We study the dynamical structure factor of the spin-1 pyrochlore material NaCaNi$_2$F$_7$, which is well described by a weakly perturbed nearest-neighbour Heisenberg Hamiltonian, Our three approaches—molecular dynamics simulations, stochastic dynamical theory, and linear spin wave theory—reproduce remarkably well the momentum dependence of the experimental inelastic neutron scattering intensity as well as its energy dependence with the exception of the lowest energies. We discuss two surprising aspects and their implications for quantum spin liquids in general: the complete lack of sharp quasiparticle excitations in momentum space and the success of the linear spin wave theory in a regime where it would be expected to fail for several reasons.

S. Zhang et al., Phys. Rev. Lett. 122, 167203 (2019)
Awards and Honors

### Markus Heyl receives ERC starting grant

Congratulations to Markus on receiving a starting grant of the European Research Council (ERC)! The starting grants can be awarded to scientists who conduct their research at an EU-based research organisation and who have two to seven years of experience since completion of their PhD. The grants are awarded on an annual basis and valued at up to 1.5 million euros each.
Publication Highlights

### Bath-Induced Decay of Stark Many-Body Localization

Recently, Stark-localized systems of interacting fermions in tilted lattices were predicted to show signatures of many-body localization similar to Anderson-localized interacting fermions in disorder potentials. Investigating the decay of such Stark many-body localization induced by the coupling to a dephasing bath (as it can be engineered for ultracold atoms in optical lattices), we find qualitative differences in comparison to disordered systems. For instance, the bath-induced growth of the total von Neumann entropy is not found to be logarithmically slow. Our results can directly be tested in systems of ultracold atoms in optical lattices.

L. Wu et al., Phys. Rev. Lett. 123, 030602 (2019)