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Publication Highlights### Towards the realisation of chiral spin liquids and non-Abelian anyons in quantum simulators

Chiral spin liquids are one of the most fascinating phases of matter ever imagined by physicists. These exotic liquids exhibit quasi-particles known as non-Abelian anyons that are neither bosons nor fermions, the manipulation of which could allow for the realisation of a universal quantum computer. Despite intense efforts in condensed matter physics, discovering such a phase in Nature remains an outstanding challenge at the forefront of modern research.

From a theoretical point of view, chiral spin liquids emerge in a simple model that was imagined by Kitaev in 2006, and which allows revealing their properties using analytical tools. Remarkably, recent advances in the design of quantum simulators open a possible path for the first experimental realisation of the original Kitaev model, hence suggesting that chiral spin liquids (including their exotic quasi-particles) can be studied and manipulated in a highly-controlled experimental environment.

Recent work by an international collaboration involving BoYe Sun and Nathan Goldman (ULB, Brussels), Monika Aidelsburger (LMU, Munich), and Marin Bukov (Max Planck Institute for the Physics of Complex Systems and Sofia University) proposes a realistic implementation of the Kitaev model in quantum simulators. Based on a precise pulse sequence, their system is shown to host a chiral spin liquid with non-Abelian anyons. The authors describe practical methods to probe the striking properties of these exotic states. In particular, their methods unambiguously reveal the topological heat current that flows on the edge of the system: a hallmark signature of the non-Abelian anyons that emerge on the edge of chiral spin liquids.

This work paves the way for the quantum simulation of chiral spin liquids, offering an appealing alternative to their experimental investigation in quantum materials.

Bo-Ye Sun, Nathan Goldman, Monika Aidelsburger, and Marin Bukov, Phys. Rev. X Quantum**4**, 020329 (2023).

Read moreFrom a theoretical point of view, chiral spin liquids emerge in a simple model that was imagined by Kitaev in 2006, and which allows revealing their properties using analytical tools. Remarkably, recent advances in the design of quantum simulators open a possible path for the first experimental realisation of the original Kitaev model, hence suggesting that chiral spin liquids (including their exotic quasi-particles) can be studied and manipulated in a highly-controlled experimental environment.

Recent work by an international collaboration involving BoYe Sun and Nathan Goldman (ULB, Brussels), Monika Aidelsburger (LMU, Munich), and Marin Bukov (Max Planck Institute for the Physics of Complex Systems and Sofia University) proposes a realistic implementation of the Kitaev model in quantum simulators. Based on a precise pulse sequence, their system is shown to host a chiral spin liquid with non-Abelian anyons. The authors describe practical methods to probe the striking properties of these exotic states. In particular, their methods unambiguously reveal the topological heat current that flows on the edge of the system: a hallmark signature of the non-Abelian anyons that emerge on the edge of chiral spin liquids.

This work paves the way for the quantum simulation of chiral spin liquids, offering an appealing alternative to their experimental investigation in quantum materials.

Bo-Ye Sun, Nathan Goldman, Monika Aidelsburger, and Marin Bukov, Phys. Rev. X Quantum

Publication Highlights### The tissue collider

Physics has a long tradition of learning about fundamental interactions by making particles collide against each other. Could a similar approach be applied to biological systems? Work by Ricard Alert of the Max Planck Institute for the Physics of Complex Systems and collaborators shows what can be learnt about the mechanics of living tissues by making them collide against each other. In experiments, the researchers placed cell monolayers on a substrate and allowed them to expand and collide. The work first characterised how two tissues change shape upon collision. The researchers discovered that denser tissues, with more cells per unit area, can mechanically displace less dense tissues. Based on their calculations, the team proposes that this displacement arises because denser tissues have a higher pressure, which allows them to push on other tissues. This theory enables measurements of the elastic properties of the tissues just from their displacements upon collision. The researchers then analyzed collisions between three tissues. Surprisingly, they found that these three-tissue events are not simply a superposition of two-tissue collisions. Instead, some cells speed up as if trying to squeeze between the two other tissues. Finally, they used all these collision principles to design and assemble specific tissue patterns, similar to tile patterns, like the diamond pattern in the image. Ultimately, these results might help engineering tissue composites for implants, as well as understanding and controlling tissue interactions during embryonic development or wound healing. In all these situations, tissues grow and collide, either merging into a single tissue or establishing clear boundaries between different tissues and organs.

Matthew A. Heinrich*, Ricard Alert*, Abraham E. Wolf*, Andrej Košmrlj, and Daniel J. Cohen, Nat. Commun.**13**, 4026 (2022).

Read moreMatthew A. Heinrich*, Ricard Alert*, Abraham E. Wolf*, Andrej Košmrlj, and Daniel J. Cohen, Nat. Commun.

Publication Highlights### From Dual Unitarity to Generic Quantum Operator Spreading

Chaotic quantum many-body systems hide information non-locally in many degrees of freedom via a dynamical process called scrambling. Understanding the capacity of quantum systems to scramble information is a crucial requirement for the design and control of quantum computing platforms as well as being intimately related to questions of thermalisation and quantum chaos.
In recent years, dual-unitary circuits have emerged as minimal models for quantum many-body dynamics in which the dynamics are chaotic yet analytically tractable. However, despite their chaoticity these circuits display behaviour that is in many respects non-generic. Michael Rampp, Roderich Moessner, and Pieter Claeys from the Max Planck Institute for the Physics of Complex Systems have investigated the effect of weakly broken dual-unitarity on the spreading of local operators, a particular probe of information scrambling. They recover two universal features of ergodic quantum spin chains absent in dual-unitary circuit dynamics: a butterfly velocity smaller than the light-cone velocity and a diffusively broadening operator front. They present a physical picture for these effects through a discrete path-integral formalism, allowing for a quantitative connection between the microscopic properties of these gates and the macroscopic butterfly velocity and diffusion constant.

M. A. Rampp, R. Moessner, P. W. Claeys, Phys. Rev. Lett.**130**, 130402 (2023).

Read moreM. A. Rampp, R. Moessner, P. W. Claeys, Phys. Rev. Lett.

Publication Highlights### Cancer cells move to stiff environments as living droplets

Recent work by Ricard Alert of the Max Planck Institute for the Physics of Complex Systems and his collaborators uncovered a similarity between liquid droplets and cell groups, revealing that surface tension helps cells to migrate towards stiffer environments. The new work proposes that this process, called durotaxis and which was defined in the field of cell biology, can be accounted for quite precisely with the physics of wetting. This new insight could help us to understand how cancer cells disseminate across tissues with different rigidity in our body.

M. E. Pallarès*, I. Pi-Jaumà*, I. C. Fortunato, V. Grazu, M. Gómez-González, P. Roca-Cusachs, J. M. de la Fuente, R. Alert, R. Sunyer, J. Casademunt, and X. Trepat. Nat. Phys.**19**, 279 (2023).

Read moreM. E. Pallarès*, I. Pi-Jaumà*, I. C. Fortunato, V. Grazu, M. Gómez-González, P. Roca-Cusachs, J. M. de la Fuente, R. Alert, R. Sunyer, J. Casademunt, and X. Trepat. Nat. Phys.

Publication Highlights### Non-Fermi-Liquid Behavior from Cavity Electromagnetic Vacuum Fluctuations

In a number of different behaviour, so-called non-Fermi-liquid behaviour appears due to strong correlations between electrons. The standard theoretical scenario relies on emergent collective bosonic modes with strong critical fluctuations that destroy the electronic quasiparticles. Due to the complexity of the actual material, it is difficult to determine the microscopic origin of the relevant bosonic modes systematically. Peng Rao and Francesco Piazza of the Max Planck institute for the Physics of Complex Systems have now shown that cavity quantum electrodynamics within two-dimensional materials is ideal to implement non-Fermi-liquid behaviour. The emergent bosonic modes belong here to the vacuum electromagnetic field, a microscopic degree of freedom of which the dynamics and coupling with electrons can be controlled by cavity engineering.

P. Rao and F. Piazza, Phys. Rev. Lett.**130**, 083603 (2023)

Read moreP. Rao and F. Piazza, Phys. Rev. Lett.

Publication Highlights### Symmetry-induced decoherence-free subspaces

Preservation of coherence is a fundamental, yet subtle, phenomenon in open systems. We uncover its relation to symmetries respected by the system Hamiltonian and its coupling to the environment. We discriminate between local and global classes of decoherence-free subspaces for many-body systems through the introduction of “ghost variables”. The latter are orthogonal to the symmetry and the coupling to the environment depends solely on them. Constructing them is facilitated in classical phase space and can be transferred to quantum mechanics through the equivalent role that Poisson and Lie algebras play for symmetries in classical and quantum mechanics, respectively. Examples are given for an interacting spin system.

J. Dubois, U.Saalmann, and J.M. Rost, Phys. Rev. Research**5**, L012003 (2023)

Read moreJ. Dubois, U.Saalmann, and J.M. Rost, Phys. Rev. Research

Publication Highlights### Discrete time crystal created by two-frequency external driving

Time crystals are a freshly discovered nonequilibrium phase of matter without an equilibrium counterpart, stabilized by external periodic drives and characterized by broken spatiotemporal symmetry. Scientists from the Nonequilibrium Quantum Dynamics group at the Max Planck Institute for the Physics of Complex Systems, together with collaborators at the KTH Royal Institute of Technology and at UC Berkeley, created a critical time crystal in a system of long-range interacting nuclear spins. Designing a novel two-frequency external driving protocol allowed the scientists to monitor the time-crystalline behavior continuously (avoiding the wave function collapse), and take real-time movies displaying the formation, lifetime, and meltdown of this exotic phase of matter. The experimental platform used offers unprecedented clarity and measurement throughput, which turned out fundamental for determining the boundaries of the time-crystalline phase, and investigating in detail the melting dynamics of the time crystal as it gradually heats up.

W. Beatrez*et al.*, Nat. Phys. (2023)

Read moreW. Beatrez

Publication Highlights### Dynamical fractal discovered in clean magnetic crystal

A new type of fractal has been discovered in a class of materials called spin ices—famous, among other reasons, for their emergent magnetic monopole excitations. Spin ice materials are some of the most researched and best understood topological magnets. Nevertheless, the unusual dynamical properties of spin ice have been puzzling scientists for almost two decades. An international research team, including Jonathan N. Hallén and Roderich Moessner of the Max Planck Institute for the Physics of Complex Systems, has now shown that the dynamical rules governing the motion of the magnetic monopoles constrain these to move on fractal structures. By hosting the monopole motion, the fractals cause the peculiar dynamical behaviours observed in spin ice materials.

The discovery was surprising because the fractals were seen in a clean three-dimensional crystal, where they would not be expected conventionally. Even more remarkably, the fractals are visible in dynamical properties of the crystal, and hidden in static ones. The capacity of spin ice to exhibit such striking phenomena makes the team hopeful that spin ice will allow further surprising discoveries in the cooperative dynamics of even simple topological many-body systems. More details can be found in a press release (PDF).

J. N. Hallén*et al.*, Science **378**, 1218 (2022)

See also the related*Science Perspective* article by F. Flicker.

Read moreThe discovery was surprising because the fractals were seen in a clean three-dimensional crystal, where they would not be expected conventionally. Even more remarkably, the fractals are visible in dynamical properties of the crystal, and hidden in static ones. The capacity of spin ice to exhibit such striking phenomena makes the team hopeful that spin ice will allow further surprising discoveries in the cooperative dynamics of even simple topological many-body systems. More details can be found in a press release (PDF).

J. N. Hallén

See also the related

Publication Highlights### Recipe for a spin-orbital liquid

An international team of scientists including Roderich Moessner of the Max Planck Institute for the Physics of Complex Systems has observed an exotic quantum state of matter: a spin-orbital liquid formed on the pyrochlore oxide Pr_{2}Zr_{2}O_{7}. Here, both spin and orbital degrees of freedom remain dynamic down to extremely low temperature. It is known from the long history of condensed matter physics that suppressing orbital order down to low temperatures is extremely difficult, a precondition for the next tricky step of obtaining a spin-orbital liquid state. Pr_{2}Zr_{2}O_{7} serves as a rare counterexample in which spins and orbitals are interlocked, so that fluctuation of one necessitates fluctuation of the other. More details can be found in a press release (PDF).

N. Tang, Y. Grisenko, K. Kimura*et al.*, Nat. Phys. (2022)

See also the related*Nat. Phys. News & Views* article by V. S. Zapf, M. Lee, and P. F. S. Rosa.

Read moreN. Tang, Y. Grisenko, K. Kimura

See also the related

Publication Highlights### Scaling Description of Creep Flow in Amorphous Solids

Amorphous solids, which include colloidal glasses, dense emulsions, foams, and granular materials, are ubiquitous and important in both engineering and industry. When subjected to a suddenly imposed stress, they can exhibit a transient flow known as creep during which the flow rate decays as a power law over time. This power law is characterized by a quantity called the creep exponent. If the stress inducing the creep flow is low, the material eventually stops moving. But if this stress is sufficiently high, the power-law decay can be followed by sudden fluidization. Together with colleagues from École polytechnique fédérale de Lausanne (EPFL) and Université Paris-Saclay, Marko Popović of the Max Planck Institute for Physics of Complex Systems developed a theory of creep flow that can predict both the creep exponent and the time at which sudden fluidization occurs, as well as the temperature dependence of these two quantities. These predictions have been tested in numerical simulations and are consistent with previously published experimental observations. The key ingredient of the proposed theory is the new concept of a transient yield stress, which reflects the dynamics of the maximal stress that the material could sustain without flowing while it undergoes creep flow. Remarkably, the scaling of the creep exponent and the time of fluidization then follow from generic properties of the transient yield stress for both athermal and thermal systems. The success of the transient yield stress concept opens new exciting questions: What is the origin of the transient yield stress and what controls its dynamics? Can the concept of a transient yield stress be employed to describe other characteristic behaviours associated with the yielding of amorphous solids, such as shear banding instabilities?

Marko Popović,*et al.*, Phys. Rev. Lett. **129**, 208001 (2022), Editors' Suggestion

Selected for a*Synopsis in Physics*

Read moreMarko Popović,

Selected for a