Highlights

Quantum excited states underpin new states of matter, support biological processes such as vision, and determine opto-electronic properties of photovoltaic devices. Yet, while ground-state properties can be determined by rather accurate computational methods, there remains a need for theoretical and computational developments to target excited states efficiently. Inspired by the duplication of the Hilbert space used to study black-hole entanglement and the electronic pairing of conventional superconductivity, researchers from the Max Planck Institute for the Physics of Complex Systems and their collaborators from Munich and Modena have developed a new scheme to compute the full spectrum of a quantum many-body Hamiltonian, rather than only its ground or lowest-excited states. An important feature of their proposed scheme is that these spectra can be computed in a one-shot calculation. The scheme thus provides a novel variational platform to excited-state physics. It is also suitable for efficient implementation on quantum computers, so has the potential to enable unprecedented calculations of excited-state processes of quantum many-body systems.

C. L. Benavides-Riveros et al., Phys. Rev. Lett. 129, 066401 (2022), Editors' Suggestion

Much like animals can trace odors, cells also move toward certain chemicals. In fact, cells often do this in groups, which can be up to millions of individuals strong. But how do these cell populations manage to move together as a cohesive unit while following chemical cues? New work by Ricard Alert of the Max Planck Institute for the Physics of Complex Systems and collaborators shows that the answer lies in limitations in the ability of cells to sense chemicals at high concentrations. Thus, the work bridges scales by connecting the sensing of tiny molecules by individual cells to the shape and motion of an entire cell population, which can be centimeters or even larger in size. The work is important because it reveals a potentially general principle: Sensing—a distinguishing feature of living systems—governs the ability of cells to migrate in groups. This principle could operate in many other examples of collective migration, as cells and other living creatures can sense and follow a variety of stimuli, such as electric fields, temperature, and light intensity. Finally, the new results open a tantalizing question for future work: Has evolution pushed the sensing limitations of cells to ensure that they can follow chemical cues as a cohesive group? (Image credit: Mariona Esquerda Ciutat.)

R. Alert, A. Martínez-Calvo, and S. S. Datta, Phys. Rev. Lett. 128, 148101 (2022)

The search for departures from standard hydrodynamics in many-body systems has yielded a number of promising leads, especially in low dimension. Researchers at the Max Planck Institute for the Physics of Complex Systems studied one of the simplest classical interacting lattice models, the nearest-neighbour Heisenberg chain, with temperature as tuning parameter. Their numerics expose strikingly different spin dynamics between the antiferromagnet, where it is largely diffusive, and the ferromagnet, where they observe strong evidence either of spin superdiffusion or an extremely slow crossover to diffusion. At low temperatures in the ferromagnet, they observe an extremely long-lived regime of remarkably clean Kardar-Parisi-Zhang (KPZ) scaling (see figure). The anomalous behaviour also governs the equilibration after a quench, and, remarkably, is apparent even at very high temperatures.

A. J. McRoberts, T. Bilitewski, M. Haque, and R. Moessner, Phys. Rev. B 105, L100403 (2022)

An important but challenging task is to treat mesoscopic systems that are coupled to a complex environment at finite temperature. Alexander Eisfeld of the Max Planck Institute for the Physics of Complex Systems and his collaborators have derived a stochastic hierarchy of matrix product states (HOMPS) for non-Markovian dynamics, which is numerically exact and efficient. In this way the exponential complexity of the problem can be reduced to scale polynomially with the number of particles and modes of the environment. An additional feature caused by the stochastic noise is that individual trajectories stay well localized. The validity and efficiency of HOMPS is demonstrated for the spin-boson model and long chains where each site is coupled to a structured, strongly non-Markovian environment.

X. Gao, J. Ren, A. Eisfeld, and Z. Shuai, Phys. Rev. A 105, L030202 (2022)

A warm welcome to Pieter Claeys! Coming to our institute from the University of Cambridge, Pieter establishes the research group "Dynamics of quantum information". The group’s research lies at the interface of condensed matter physics and quantum information, using a variety of theoretical and numerical approaches to study the dynamics of quantum many-body systems. Research topics include the dynamics of entanglement, quantum chaos and thermalization, unitary circuit models, and general aspects of non-equilibrium quantum dynamics. The group will also focus on bridging recent advances in the dynamics of quantum systems and quantum computation.

On May 24, 2022, Prof. Tomaž Prosen from the University of Ljubljana in Slovenia received the "Physik-Preis Dresden" (Dresden Physics Prize), jointly awarded by the TU Dresden and the Max Planck Institute for the Physics of Complex Systems (MPI-PKS). The theoretical physicist receives the award for his outstanding work on quantum mechanical many-body systems, nonequilibrium statistical physics, quantum information, classical chaos and quantum chaos. Tomaž Prosen has authored over 200 publications on this broad range of topics, which have over 7000 citations and are recognized in expert circles worldwide. In 2016, he received a prestigious ERC Advanced Grant. The award ceremony took place at a festive colloquium in the Recknagel Building of the TU Dresden, preceded by a reception. Prof. Carsten Timm, Dean of the Faculty of Physics, gave the welcome address, and Prof. Roderich Moessner of MPI-PKS delivered the laudation. The Physik-Preis Dresden was endowed in 2015 by Dresden physicist Prof. Peter Fulde, the founding director of the MPI-PKS. The prize winners are determined by a joint commission of the TU Dresden and the MPI-PKS. In addition to the central criterion of scientific excellence, it is particularly important for the decision that the work of the award winners is of special significance for the cooperation between the two DRESDEN-concept partners MPI-PKS and TU Dresden and that their connection has been further strengthened in the long term. The 2022 awardee, Prof. Tomaž Prosen, has a wide range of connections to the professorships at the Institute for Theoretical Physics at TU Dresden and at MPI-PKS due to his broad scientific orientation.

Bilateral symmetry of the vertebrate musculoskeletal system is necessary for proper function and its defects are associated with debilitating conditions, such as scoliosis. In a collaboration with biologists from the École polytechnique fédérale de Lausanne (EPFL) in Switzerland, Marko Popović of the Max Planck Institute for Physics of Complex Systems has studied the early stage of body segmentation using zebrafish as a model organism. They discovered that biochemical signalling and transcriptional processes that drive segmentation are not sufficient to explain the precision and symmetry of tissue shapes and sizes. However, they found that surface tension forces of the newly formed segments are a crucial component of the mechanism that is responsible for recovery and maintenance of the symmetric body plan. This discovery highlights the importance of physical interactions for precise and robust development of living beings.
S. R. Naganthan, M. Popovic, and A. C. Oates, Nature 605, 516-521 (2022)

We cordially welcome Christina Kurzthaler at the institute! Christina joins MPI-PKS from Princeton University and establishes the research group "Transport and flows in complex environments“. The group aims to unravel physical phenomena arising in soft and active matter, with emphasis on the role of transport and flows for biological systems and microfluidics. Its research topics range from the hydrodynamics of swimming bacteria and their interactions with their environments to the statistical physics of active transport in porous materials to the motion of colloidal suspensions in microfluidic settings. While the group's work is theoretical, it seeks to establish collaborations with experimentalists of the Dresden research community.

Attractive forces are ubiquitous in nature: they glue very different objects ranging from atomic nuclei, droplets of water, to stars, galaxies, and black holes. Peter Karpov and Francesco Piazza of the Max Planck Institute for the Physics of Complex Systems have now demonstrated that highly tuneable attractive interactions can be engineered artificially using optical cavities, leading to various novel phases of quantum droplets of ultracold atoms. Upon tuning the cavity-mediated interactions it is possible to switch between superfluid droplets and incompressible water-like droplets, as well as to realise crystalline and even more exotic supersolid droplets combining superfluid and solid properties.
P. Karpov and F. Piazza, Phys. Rev. Lett. 128, 103201 (2022)

A fundamental question in biology is how complexes of several molecules can assemble reliably. Tyler Harmon and Frank Jülicher of the Max Planck Institute for the Physics of Complex Systems have now shown that a molecular assembly line can be self-organized by active droplets where it can form spontaneously. This assembly line arranges different assembly steps spatially so that a specific order of assembly is achieved and incorrect assembly is suppressed. They have shown how assembly bands are positioned and controlled and discuss the rate and fidelity of assembly as compared to other assembly scenarios.
T. S. Harmon and F. Jülicher, Phys. Rev. Lett. 128, 108102 (2022)