Highlights

Symmetry plays a crucial role in understanding fundamental phenomena such as conservation laws, the classification of phases of matter, and their transitions. Recently, researchers have been exploring ways to manipulate symmetries in quantum many-body systems with time-dependent driving protocols and, in particular, engineering new symmetries that do not occur naturally. This enriches the toolbox for quantum simulation and computation significantly, and has led to many exciting discoveries of nonequilibrium phases such as discrete time crystals. However, controlling multiple symmetries—especially in a simple and experimentally friendly way—has remained a challenge. Scientists from the Max Planck Institute for the Physics of Complex Systems have now proposed a novel method to engineer hierarchical symmetries by time-dependent protocols.
By controlling carefully how symmetry-indicating observables evolve over time, the researchers showed how to create a sequence of symmetries that emerge one after another, each with distinct properties. The method relies on a recursive construction that minimizes the effects of symmetry-breaking processes hierarchically. This leads to a corresponding sequence of prethermal steady states with controllable lifetimes, each exhibiting a lower symmetry than the preceding one. The scientists illustrate this protocol with several examples, demonstrating how different types of order can emerge through hierarchical symmetry breaking.
This toolbox of hierarchical symmetries opens a new path to stabilizing quantum states and controlling unwanted symmetry-breaking effects, which can be particularly useful in quantum computing and quantum simulation. The construction applies to classical and quantum, fermionic and bosonic, interacting and noninteracting systems. The underlying mechanism generalizes state-of-the-art dynamical decoupling techniques and is implementable on present-day quantum simulation platforms.

Zhanpeng Fu, Roderich Moessner, Hongzheng Zhao, and Marin Bukov, Phys. Rev. X 14, 041070 (2024)

A few years ago, scientists from the Max Planck Institute for the Physics of Complex Systems
[Saalmann et al., Phys. Rev. Lett. 121, 153203 (2018)] predicted theoretically that the chirp of a short intense laser pulse offers the opportunity to control ionization of atoms with very high efficiency. By means of resonant two-photon processes one can switch between excitation and ionization with a contrast of nearly 100%. This mechanism has now been observed with chirped circularly-polarized laser pulses at the free-electron laser (FEL) FERMI in Trieste (Italy). FERMI belongs to the increasingly popular group of seeded FELs that allow accurate control of the characteristics of the short and intense laser pulses generated. In a large collaboration, led by researchers from the Albert-Ludwigs-Universität Freiburg (Germany), photo-electron spectra from Helium atoms for various chirps—that confirm the theoretical predictions—have been measured. These measurements are a crucial step towards coherent-control experiments making use of spectral-phase shaping in the XUV range of the electromagnetic spectrum.

Fabian Richter, Ulf Saalmann et al., Nature 636, 337 (2024)

Living organisms must sense and process information from their surroundings to survive, but they often cannot directly “listen” to external signals. For example, the internal processes of a red blood cell can only access the external world through the cellular membrane, but how well can this membrane transmit such information? And how can information transmission be achieved with a limited energy budget? Giorgio Nicoletti of EPFL Lausanne and Daniel Busiello of the Max Planck Institute for the Physics of Complex Systems have now shown that, by optimising energy and information at the same time, biological systems can tune themselves to harvest maximal information from the environment. These highly efficient strategies require a price in terms of energy consumption, but this price is far outweighed by the information learned about the hidden, external signals. A particularly relevant case study for these results is red blood cells: The authors quantify how much information about the state of the internal cytoskeleton is transmitted into the flickering of their membrane, unraveling a deep connection between healthy cellular conditions, energy dissipation, and information. In particular, the mechanical stress of the cell affects its efficiency in terms of how well information is transmitted, providing novel and fundamental insights into the functioning of biological systems in complex environments.

Giorgio Nicoletti and Daniel Maria Busiello, Phys. Rev. Lett. 133, 158401 (2024)

Selected for a Viewpoint in Physics.

Periodically driven systems have emerged as a useful technique to engineer the properties of quantum systems, and are in the process of being developed into a standard toolbox for quantum simulation. An outstanding challenge that leaves this toolbox incomplete is the manipulation of the states dressed by strong periodic drives. To achieve fast control of nonequilibrium quantum matter, Paul Schindler and Marin Bukov of the Max Planck Institute for the Physics of Complex Systems have now generalised the notion of variational counterdiabatic driving away from equilibrium. The researchers discuss applications to two-level, Floquet band, and interacting periodically driven models.

Paul M. Schindler and Marin Bukov, Phys. Rev. Lett. 133, 123402 (2024)

In biological processes such as embryonic development, wound healing, and cancer invasion, cells move in cohesive groups. These groups are often led by a so-called leader cell, which is thought to pull and direct the followers. New work by Ricard Alert of the Max Planck Institute for the Physics of Complex Systems and his experimental collaborators in the group of Xavier Trepat at the Institute for Bioengineering of Catalonia (IBEC) now shows that the local action of a leader is not enough to guide the migration of cell groups. Instead, the researchers show that mechanical coordination across the entire cell group is needed for the group to move. To test whether leader cells can pull others, the scientists used genetically-modified cells that turn into leaders when illuminated with blue light. In this way, the team could create leader cells on demand. The researchers then studied whether leader cells could act as the locomotive of small cell trains, up to four cells long. They found that a leader cell can robustly drag one follower but not longer cell trains. Leader cells therefore need the contribution of followers to guide the group. Ricard Alert then developed a physical model that shows how the motion of cell trains arises from asymmetries in cellular traction forces across the entire cell train, in agreement with the team’s experimental measurements. The new work therefore challenges the notion of autonomous leader cells, and it shows that cells need to coordinate their forces to move in groups.

More details can be found in the press release (PDF).

Leone Rossetti, Steffen Grosser, Juan Francisco Abenza, Léo Valon, Pere Roca-Cusachs, Ricard Alert, and Xavier Trepat. Optogenetic generation of leader cells reveals a force–velocity relation for collective cell migration, Nat. Phys. (2024)

Topological protection provides unprecedented robustness of physical phenomena against all kinds of perturbations; but in doing so, it exercises topological censorship by hiding all kinds of interesting and important microscopic information. Recent experiments have collected microscopic information precisely of the kind hidden by such Topological Censorship. The work by Douçot, Kovrizhin and Moessner provides a detailed microscopic theory which goes beyond such topological censorship. It not only identifies an unexpected phenomenon – the meandering edge state carrying topologically quantised current – at variance with common expectations; but also identifies mechanisms which allow for tuning between qualitatively different microscopic implementations corresponding to one and the same topologically protected global quantity.

More details can be found in the press release (PDF).

Benoit Douçot, Dmitry Kovrizhin, and Roderich Moessner, Proc. Natl. Acad. Sci. USA, 121, e2410703121 (2024)

Self-propelled agents such as birds, cells, and active colloidal particles often move collectively in flocks. In the paradigmatic Vicsek model, flocking emerges due to alignment interactions between the active agents, which align much in the same way as spins do. Suchismita Das, Matteo Ciarchi, Ricard Alert of the Max Planck Institute for the Physics of Complex Systems and their collaborators have now discovered that flocking can emerge even if the agents turn away from each other.
The researchers made this surprising discovery in experiments with self-propelled colloidal particles that repel more strongly in their front half than in their rear half, in such a way that they turn away from each other. They then used simulations and two types of kinetic theory to explain how these particles end up flocking. Their theory revealed that repulsion between the particles is key: When two particles interact, repulsion pushes them apart before they can turn away too much, thus producing effective alignment, as shown in the figure. This crucial role of repulsion is surprising as repulsion is not even an ingredient in the paradigmatic models of flocking, such as the Vicsek model, where collective motion emerges just from alignment interactions between particle orientations. The new work also showed that, via repulsion, the particles can form flocking crystals, which are active counterparts of Wigner crystals formed through electrostatic repulsion in electron gases.
In conclusion, these active particles move in the same direction as a compromise between turning away from left and right neighbors. This mechanism of flocking could potentially be relevant for certain cells, which also turn away from each other upon collision via a process known as contact inhibition of locomotion. Whether these findings can explain how cells flock remains an open question for future work.

Suchismita Das, Matteo Ciarchi, Ziqi Zhou, Jing Yan, Jie Zhang, and Ricard Alert, Phys. Rev. X 14, 031008 (2024)

The framework of the quantum Hall effect has been extended to a framework of a quantum skyrmion Hall effect by Ashley Cook of the Max Planck Institute for the Physics of Complex Systems and the Max Planck Institute for Chemical Physics of Solids, by generalizing the notion of a particle to include compactified p-dimensional charged objects. This is consistent with three sets of topologically non-trivial phases of matter previously discovered by Cook and collaborators: the topological skyrmion phases of matter, the multiplicative topological phases of matter, and the finite-size topological phases of matter. These findings indicate that topological states of D-dimensions can persist after compactification and yield previously unidentified generalizations of particles, a finding of relevance to many areas of physics, and particularly string theory, with great potential for rapid experimental confirmation.

Ashley Cook, Phys. Rev. B 109, 155123 (2024)

Quantum electrodynamics (QED) is the fundamental theory that describes the interactions between electrons and photons. Its success has led some to wonder whether quantum field theories, like QED, can describe quasiparticles in a solid. These collective excitations include phonons, which describe lattice vibrations, and magnons, which are waves in a magnetic material, but might also be of a more exotic nature. In a recent study, Alexander Wietek of the Max Planck Institute for the Physics of Complex Systems and his collaborators show that QED in two spatial dimensions can be observed in frustrated antiferromagnets.
An antiferromagnet is a material in which neighbouring electron spins in the crystal lattice would like to point in opposite directions. However, in certain geometries, such as a triangular lattice, it is impossible to have all neighbouring spins align in precisely the opposite way. This is called geometric frustration and can lead to strong disorder in the system. This disorder is not featureless, however. In fact, it is shown that the quasiparticles of such a spin soup, known as a quantum spin liquid, are related one-to-one to excitations of QED. Importantly, even the elusive magnetic monopoles, among a wide variety of other particle-hole excitations, are observed.
The precise understanding of the spin-liquid state with magnetic monopoles as elementary excitations is a key step to discovering these exotic quasiparticles in antiferromagnetic materials. It is unlikely that the founders of QED would have predicted such a surprising emergence in condensed matter.

Alexander Wietek, Sylvain Capponi, and Andreas M. Läuchli, Phys. Rev. X 14, 021010 (2024)

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MicroRNAs (miRNAs) are short strands of genetic material that regulate various cellular functions and developmental processes. One of the regulatory functions of miRNAs is noise control that confers robustness in gene expression. The interaction with their target messenger RNA (mRNA) requires a specific binding sequence of 6-8 nucleotide pairs in length. There are a variety of open questions about the evolution of miRNA regulation regarding their functional efficiency and binding specificity.

Efe Ilker of the Max Planck Institute for the Physics of Complex Systems and Michael Hinczewski (Case Western Reserve University) show that this regulation incurs a steep energetic price, so that natural selection may have driven such systems towards greater energy efficiency. This involves tuning the interaction strength between miRNAs and their target messenger RNAs, which is controlled by the length of a miRNA seed region that pairs with a complementary region on the target. They show for the first time that microRNAs lie in an evolutionary sweet spot that may explain why 7 nucleotide pair interactions are prevalent: sequences that are much longer or shorter would not have the right binding properties to reduce noise optimally. To achieve this, they develop a stochastic model of miRNA noise regulation, coupled with a detailed analysis of the associated metabolic costs and binding free energies for a wide range of miRNA seeds. Moreover, the behaviour of the optimal miRNA network mimicks the best possible linear noise filter, a classic concept in engineered communication systems. These results illustrate how selective pressure toward metabolic efficiency has potentially shaped a crucial regulatory pathway in eukaryotes.

Efe Ilker and Michael Hinczewski, Proc. Natl. Acad. Sci. USA 121, e2308796121 (2024)