Highlights

Quantum phase transitions are central to our understanding of why matter at very low temperatures can exhibit starkly different properties upon small changes of microscopic parameters. Accurately locating those transitions is challenging experimentally and theoretically. Here, we show that the antithetic strategy of forcing systems out of equilibrium via sudden quenches provides a route to locate quantum phase transitions. Specifically, we show that such transitions imprint distinctive features in the intermediate-time dynamics, and results after equilibration, of local observables in quantum chaotic spin chains. Furthermore, we show that the effective temperature in the expected thermal-like states after equilibration can exhibit minima in the vicinity of the quantum critical points. We discuss how to test our results in experiments with Rydberg atoms and explore nonequilibrium signatures of quantum critical points in models with topological transitions.

A. Haldar et al. Phys. Rev. X 11, 031062 (2021)

Quantum spin liquids provide paradigmatic examples of highly entangled quantum states of matter. Frustration is the key mechanism to favor spin liquids over more conventional magnetically ordered states. Here we propose to engineer frustration by exploiting the coupling of quantum magnets to the quantized light of an optical cavity. The interplay between the quantum fluctuations of the electro-magnetic field and the strongly correlated electrons results in a tunable long-range interaction between localized spins. This cavity-induced frustration robustly stabilizes spin liquid states, which occupy an extensive region in the phase diagram spanned by the range and strength of the tailored interaction. This occurs even in originally unfrustrated systems, as we showcase for the Heisenberg model on the square lattice.

A. Chiocchetta et al. Nat. Commun. 12, 5901 (2021)

Understanding the normal-metal state transport in twisted bilayer graphene near magic angle is of fundamental importance as it provides insights into the mechanisms responsible for the observed strongly correlated insulating and superconducting phases. Here we provide a rigorous theory for phonon-dominated transport in twisted bilayer graphene describing its unusual signatures in the resistivity (including the variation with electron density, temperature, and twist angle) showing good quantitative agreement with recent experiments. We contrast this with the alternative Planckian dissipation mechanism that we show is incompatible with available experimental data. An accurate treatment of the electron-phonon scattering requires us to go well beyond the usual treatment, including both intraband and interband processes, considering the finite-temperature dynamical screening of the electron-phonon matrix element, and going beyond the linear Dirac dispersion. In addition to explaining the observations in currently available experimental data, we make concrete predictions that can be tested in ongoing experiments.

G. Sharma et al. Nature Commun. 12, 5737 (2021)

Application deadline: 24 November 2021. 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!

We cordially welcome Ricard Alert at the institute! Ricard joins MPI-PKS from Princeton University and establishes the research group "The Physics of Living Matter". The group aims at uncovering physical principles of living matter. In particular, it will develop the physics of active matter to understand collective behaviors in cells and tissues. The group's research topics include collective cell migration, self-organization in bacterial colonies, active turbulence, mechanochemical patterns in tissues, mechanically-regulated tissue growth, and active fluctuations in cells. Ricard is also a member of the Center for Systems Biology Dresden and strengthens the collaboration of MPI-PKS with experimental groups at MPI-CBG.

Digital quantum simulation on quantum computers provides the potential to simulate the unitary evolution of any many-body Hamiltonian with bounded spectrum by discretizing the time evolution operator through a sequence of elementary quantum gates. A fundamental challenge in this context originates from experimental imperfections, which critically limits the number of attainable gates within a reasonable accuracy and therefore the achievable system sizes and simulation times. In this work, we introduce a reinforcement learning algorithm to systematically build optimized quantum circuits for digital quantum simulation upon imposing a strong constraint on the number of quantum gates. With this we consistently obtain quantum circuits that reproduce physical observables with as little as three entangling gates for long times and large system sizes up to 16 qubits. As concrete examples we apply our formalism to a long-range Ising chain and the lattice Schwinger model. Our method demonstrates that digital quantum simulation on noisy intermediate scale quantum devices can be pushed to much larger scale within the current experimental technology by a suitable engineering of quantum circuits using reinforcement learning.

A. Bolens et al., Phys. Rev. Lett. 127, 110502 (2021).

The one-body reduced density matrix $\gamma$ plays a fundamental role in describing and predicting quantum features of bosonic systems, such as Bose-Einstein condensation. The recently proposed reduced density matrix functional theory for bosonic ground states establishes the existence of a universal functional $F[\gamma]$ that recovers quantum correlations exactly. Based on a decomposition of $\gamma$, we have developed a method to design reliable approximations for such universal functionals: Our results suggest that for translational invariant systems the constrained search approach of functional theories can be transformed into an unconstrained problem through a parametrization of a Euclidian space. This simplification of the search approach allows us to use standard machine learning methods to perform a quite efficient computation of both $F[\gamma]$ and its functional derivative. For the Bose-Hubbard model, we present a comparison between our approach and the quantum Monte Carlo method.

J. Schmidt et al., Phys. Rev. Res. 3, L032063 (2021).

We combine quantum renormalization group approaches with deep artificial neural networks for the description of the real-time evolution in strongly disordered quantum matter. We find that this allows us to accurately compute the long-time coherent dynamics of large many-body localized systems in nonperturbative regimes including the effects of many-body resonances. Concretely, we use this approach to describe the spatiotemporal buildup of many-body localized spin-glass order in random Ising chains. We observe a fundamental difference to a noninteracting Anderson insulating Ising chain, where the order only develops over a finite spatial range. We further apply the approach to strongly disordered two-dimensional Ising models, highlighting that our method can be used also for the description of the real-time dynamics of nonergodic quantum matter in a general context.

H. Burau et al., Phys. Rev. Lett. 127, 050601 (2021)

Understanding how individual proteins work together to perform complex cellular processes such as transcription, DNA replication, and repair represents a crucial goal in cell biology. Transcription is a process in the nucleus where protein complexes work together to generate transcripts of RNA from genes. For proper transcriptional regulation, enhancers—short strips of DNA—must be brought into close proximity of the gene’s promoter. Given that enhancers and promoters are often located far apart within the genome, the question then arises: how do proteins bring these enhancers and promoters together in space and time? And what are the physics behind it?

Answering these questions would provide deep insights into the proper regulation of transcription in the cell nucleus. However, extracting such information is far from trivial. But recent work from the research group of Jan Brugués at the Max Planck Institute of Molecular Cell Biology and Genetics in collaboration with Frank Jülicher at the MPI for the Physics of Complex Systems has revealed an important clue: Forces. Jan’s lab is also located at the MPI for the Physics of Complex Systems and is affiliated with the Center for Systems Biology Dresden.

Interactions between liquids and solids have long been known to generate forces, such as those maintaining the tension of a spider web or those that allow insects to walk on water. However, whether such forces play a role inside the cell has remained unclear. With the development of precise biophysical methods and advanced imaging techniques, we are getting closer to not only observing such forces but also measuring them.

The Brugués lab imaged the interactions between single molecules of DNA and the transcription factor FoxA1, a protein responsible for determining cell fate in many species. They discovered that FoxA1 molecules brought distant regions of DNA together, generating forces that condensed the DNA. When the single molecule of DNA was stretched tightly — like a tightened elastic band — FoxA1 molecules could not bring DNA together. However, when the DNA molecule was floppy, FoxA1 molecules worked together to condense the DNA, overcoming the DNA’s intrinsic tension. This new information helps paint a clearer picture of the interactions between transcriptional regulators and the surface of the DNA.

Remarkably, the physics underlying these FoxA1-DNA interactions are reminiscent of the forces that maintain the tension of a spider web. Similar to how liquid droplets on a spider web generate forces that reel in broken strands of silk, FoxA1 acts as the liquid phase that condenses DNA and brings it together.

This study demonstrated how proteins work together to generate forces in the cell nucleus. Such a result opens an exciting research direction to understanding other complex processes in the cell. Thomas Quail, post-doctoral researcher in the Brugués lab says: “Our findings set forth a novel mechanism that the cell nucleus may use to organize its chromatin and DNA. It’s possible that these condensation forces generated between solid and liquid surfaces could also be relevant for other cellular bodies such as the mitotic spindle and membranes.”

T. Quail et al., Nature Physics (2021)

Am 6. Juli 2021 wurde der „Physik-Preis Dresden“ der TU Dresden und des Max-Planck-Instituts für Physik komplexer Systeme (MPI-PKS) zum fünften Mal verliehen. Der Physik-Preis Dresden 2021 geht an Professor Gijsje Koenderink von der Technischen Universität Delft. Gijsje Koenderink ist eine herausragende experimentelle Biophysikerin mit einer Reihe von bahnbrechenden Arbeiten zur Zellmechanik und zellulären Krafterzeugung. In Anerkennung ihrer hervorragenden Beiträge zur Physik der Zellen erhält Gijsje Koenderink den Dresdner Physikpreis 2021, der gemeinsam vom Max-Planck-Institut für Physik komplexer Systeme und der TU Dresden verliehen wird. Die Forschung von Gijsje Koenderink ist von großem Interesse für eine Reihe von Forschungsgruppen in Dresden, insbesondere im Rahmen des Exzellenzclusters "Physik des Lebens". Die Verleihung des Physik-Preis Dresden 2021 an Professor Koenderink schafft eine wertvolle Verbindung zwischen ihrer Forschungsgruppe und der Forschung auf den Gebieten Polymerphysik, weicher kondensierter Materie, Biophysik und Zellbiologie in Dresden. Gastgeber des Abends, Prof. Dr. Frank Jülicher, Direktor am Max-Planck-Institut für Physik komplexer Systeme, war überaus erfreut, dass mit Gijsje Koenderink eine so bedeutende internationale Forscherpersönlichkeit geehrt wurde. Der Physik-Preis Dresden wurde 2015 von dem Dresdner Physiker Prof. Peter Fulde, dem Gründungsdirektor des MPI-PKS gestiftet. Die Preisträger werden von einer gemeinsamen Kommission der TU Dresden und des MPI-PKS bestimmt. Neben dem zentralen Kriterium der wissenschaftlichen Exzellenz ist für die Entscheidung vor allem wichtig, dass die Arbeiten der Preisträger für die Zusammenarbeit zwischen beiden Dresden-concept Partnern MPI-PKS und TU Dresden von besonderer Bedeutung sind und deren Verbindung langfristig weiter gestärkt wurde.