Colloquia

Colloquium: The Multiple Roles of Mechanics in Shaping Organisms

Topology in Biology

Active materials such as bacteria, molecular motors and self-propelled colloids are Nature’s engines. They extract energy from their surroundings at a single particle level and use this to do work. Active matter is becoming an increasingly popular area of research because it provides a testing ground for the ideas of non-equilibrium statistical physics, because of its relevance to the collective behaviour of living creatures, from cells to starlings, and because of its potential in designing nanomachines. Dense active matter shows mesoscale turbulence, the emergence of chaotic flow structures characterised by high vorticity and self-propelled topological defects. I will discuss the physics of defects in active materials and describe examples where the concepts of active matter are starting to be used to describe cell sorting, motility and morphogenesis.

Colloquium: Active Droplets

Colloquium: New Types of Interactions, Dynamics, and Emergent States in Active Systems

Colloquium: Space-Time-Matter: Physics based on Finite Projective Geometry

Modern physics is based on the assumption that natural phenomena are consequences of force fields and elementary particles moving in a continuous spacetime, whose dynamics can mathematically be described by differential equations. An alternative to this substance ontology is the assumption that phenomena are processes of elementary events which are causally related so that space, time and matter properties emerge from fundamental process relations. This process ontology - proposed by Alfread North Whitehead - can mathematically be formulated as a finite projective geometry of event points where the dynamics is simply given by local quadratic forms, i.e. by a metric field. The task in physics remains to derive from this geometric structure the dynamical laws which are known to be empirically adaequate. To this end finite projective analogs of classical mechanics (time dependence), electrodynamics (spatial gauge fields) and quantum mechanics (random particle events) are formulated and their equivalence to the standard (analytical) theories is shown in the continuum limit. The origin of such important concepts as Legendre transform, gauge symmetry and commutator relations can be elucidated by basic features of finite projective geometry. Finally, the possibility is outlined of a unified theory of general relativity and quantum field theory of elementary particles, where finite projective geometry is the basic structure instead of a differentiable Riemannian manifold.

Colloquium: Witnessing Many-Body Entanglement in Light-Driven Quantum Materials

The ultrafast light-matter interaction is a powerful tool to control quantum materials’ properties and induce nontrivial dynamical states of matter such as transient superconductivity and Floquet topological phases. Crucial to the identification of these emergent phenomena is the concept of quantum coherence and our ability to distinguish genuine quantum correlations from fluctuations of randomized charge, spin, and orbital degrees of freedom. In certain systems, such as superconductors or certain quantum magnets, we can discriminate between incoherent and coherent processes by measuring specific order parameters. However, this approach cannot be easily generalized to more challenging forms of many-body entangled phases, such as quantum spin liquids. Is there an alternative way to diagnose the presence of quantum correlations in a light-driven material? In this talk, I will discuss how recent developments in the use of quantum metrology operators, called entanglement witnesses, open the possibility to probe many-body entanglement across photoinduced phase transitions. I will outline recent experimental and theoretical progress in quantifying spin entanglement in condensed matter systems and discuss how entanglement witnesses can be used to diagnose entanglement in systems with dynamically tuned electronic interactions. Finally, I will discuss how these methods can be applied to ultrafast optical and resonant inelastic x-ray scattering experiments, thus establishing a pathway to quantitative entanglement spectroscopy in quantum materials.

Exploring the Universe with Gravitational Waves

For centuries, we have explored the Universe and discovered mysterious astrophysical sources and phenomena only through the observation of electromagnetic waves (gamma rays, X-rays, optical, infrared, radio waves). In 2015, we observed the first gravitational wave passing through the Earth produced by the merger of two stellar-mass black holes. Such an event has provided us with a remarkable confirmation of the general theory of relativity by Albert Einstein, and ushered in the era of gravitational-wave astronomy. Since then, about one hundred gravitational waves have been detected, including the gravitational signal produced by the coalescence of two neutron stars, accompanied by a plethora of electromagnetic counterparts observed by numerous telescopes around the world. In this talk I will describe how we decode the gravitational signals to trace the nature of the astrophysical source that emitted them, and discuss how the novel astronomical messengers are already unveiling distinctive properties of the most extreme astrophysical objects in the Universe: black holes and neutron stars. I will also highlight the bright future of gravitational-wave astronomy due to ever more sensitive detectors on the ground and in space, which will open new frequency bandwidths.

Colloquium: Examples of Epithelium Dynamics

I will give examples of symmetry breakìng in epitheliums on substrates with several different topographies.

Colloquium: Control of Attosecond and Strong Field Processes Using Structured Light

Price ceremony and colloquium "Physik-Preis Dresden 2023" - "Failed Theories of Superconductivity"

A historical account of pre-BCS attempts to understand superconductivity, embedded in some personal views on what fascinates me about the topic. It may have too little original research, but it is probably the most entertaining option.

Colloquium: Design and Discover of New Correlated Electron Systems: Exploring New Materials Space

One of the apparent difficulties for New Materials Physics is how to identify promising phase spaces to explore or study. In essence, with hundreds of thousands of possible compounds (known and unknown) to consider, how can promising materials be discovered? The design and discovery of new strongly correlated electron systems or quantum materials is essentially a negotiation with Nature. As with any negotiation, it is important to be as flexible as possible while still holding on to the key points that are vital. In this talk I will try to illustrate how desires can be translated into specific new materials growth efforts. For example, how “wanting a low degeneracy, topological non-trivial compound that we could readily study with ARPES” can lead directly to the identification and study of RhBi2. Another example is how a desire to discover new quantum critical materials can be translated into searches near collapsed tetragonal phase transitions or the study of ternary compounds that include an immiscible pair of elements. NOTE: one of the goals of this talk it to provide my theoretical colleagues some insight into how experimentalists translate complex theoretical ideas into very primitive attempts to shave the dice in this cosmic game of craps.

Colloquium: Exceptional non-Hermitian topology

While topological phases of matter have predominantly been studied for isolated Hermitian systems, a recent shift has been made towards considering these phases in the context of non-Hermitian Hamiltonians. Non-Hermitian topological phenomena reveal an enrichment of the phenomenology of topological phases, and forms a rapidly growing new cross-disciplinary field. In particular, non- Hermiticity plays a central role in both classical and quantum systems. In the classical realm, this comes about due to, e.g., gain and loss processes in optics, while in the quantum realm, non-Hermiticity describes the dynamics of open quantum systems as well as scattering, decay, broadening and resonances due to, e.g., interactions and disorder. Non-Hermitian Hamiltonians may feature many exotic properties, which are radically different from their Hermitian counterparts, such as the generic appearance of exotic exceptional structures, a break down of the famed bulk-boundary correspondence, and the piling up of bulk states at the boundaries known as the non-Hermitian skin effect. In this talk, I will provide an overview of the field focussing on fundamental aspects, experimental realizations and I will briefly touch upon applications.

Colloquium: Computer Simulations of Nuclear Quantum Effects in Molecular and Condensed Matter Systems

Molecular and condensed matter systems are entities composed by interacting electrons and nuclei. An ideal computer simulation needs to address both the electronic and the nuclear systems accurately, as well as their couplings. Compared with the more and more accurate quantum treatments of the interacting electronic systems nowadays, theoretical descriptions of the nuclei still largely remain at the classical level. In this talk, I will explain some of our computer simulation results of nuclear quantum effects in molecular and condensed matter systems, including the quantum nature of hydrogen bonds [1-2], the quantum tunneling of hydrogen and water molecules [3-4], the quantun nature of the nuclei in high-pressure hydrogen [5-6], and the tunneling splitting of water clusters [7]. The basic technique we use is ab initio path-integral molecular dynamics. To accomadate the requirements associated with some specific problems, it was combine it with different computer simulation methods in each case. [1] Xin-Zheng Li, Brent Walker, and Angelos Michaelides, PNAS 108, 6369 (2011) [2] Jing Guo, Jingtao Lü, Yexin Feng, Ji Chen, Jinbo Peng, Xiangzhi Meng, Zhichang Wang, Zeren Lin, Xin-Zheng Li*, Enge Wang*, Ying Jiang*, Science 352, 321 (2016) [3] Wei Fang, Jeremy O. Richardson*, Ji Chen, Xin-Zheng Li*, and Angelos Michaelides*, Phys. Rev. Lett. 119, 126001 (2017) [4] Wei Fang, Ji Chen, Philipp Pedevilla, Xin-Zheng Li*, Jeremy O. Richardson*, and Angelos Michaelides*, Nat. Commun. 11, 1689 (2020) [5] Ji Chen, Xin-Zheng Li*, Qianfan Zhang, Matthew I. J. Probert, Chris J. Pickard, Richard J. Needs, Angelos Michaelides, and Enge Wang*, Nat. Commun. 4, 2064 (2013) [6] Xiaowei Zhang, Enge Wang*, and Xin-Zheng Li*, Phys. Rev. B 98, 134110 (2018) [7] Yu-Cheng Zhu, Shuo Yang, Jia-Xi Zeng, Wei Fang, Ling Jiang, Dong H. Zhang,* and Xin-Zheng Li*, J. Am. Chem. Soc. 144, 21356 (2022)

Colloquium: Extreme Waves in Science and Engineering

Breather solutions of the nonlinear Schrödinger equation, also referred to as solitons on finite and continuous background, have been known since the late 70s. Some of these pulsating localized structures describe the nonlinear stage of the modulation instability in a variety of nonlinear dispersive media and are prized to model and control rogue wave events in such wave systems. The talk will elaborate on the latest groundbreaking breather observations in optics, hydrodynamics, plasma, and Bose-Einstein condensates. A particular focus will be devoted to applications, for instance in marine engineering as well as ocean rogue wave modeling and prediction. Moreover, perspectives on implementations and limitations of breather modeling for extreme sea waves will also be discussed.

Colloquium: t.b.a.

Colloquium: Exploring Human Creativity and AI for Engineered Quantum Matter

In research labs worldwide, quantum physics is making unprecedented strides. The realization of robust quantum systems holds tremendous promise for applications in secure communication and computing. Yet, as physicists, our most exciting pursuit lies in experimentally testing quantum phenomena predicted over the past century within highly controlled environments. In this colloquium, I will explore two distinct approaches to engineering quantum matter: one fueled by human creativity and the other driven by artificial intelligence. Throughout the colloquium, we will uncover how these approaches can be effectively deployed in contemporary quantum experiments, paving the way for advancements in our understanding and control of quantum phenomena.

Colloquium: Interactions and Topology in Materials with Flat Bands: Superconductivity, Magnetism, and Fractional Chern Insulators

We will review remarkable recent progress in both experimental and theoretical research on flat bands materials. When a system exhibits flat bands despite having sizable hopping integrals/electron itineracy, the flat band is likely to be topological. In stoichiometric materials, one now has flat band catalogues of materials exhibiting such electronic states at the Fermi level. I will review the interacting and topological physics of such bands, first with simple one-body models and then adding interactions to obtain Kondo physics, magnets, charge density waves, and superconductors. In engineered moire materials, where the Fermi level can be gate-tuned, a new degree of precise controllability allows for the development of flat bands exhibiting Chern insulators, Superconductivity, and - most recently - Fractional Chern insulators. We will show that the physics of most of these bands holds a close resemblance to heavy Fermion physics.

Colloquium: t.b.a.

Colloquium: Modelling Critical Thresholds in Ice Sheets

Colloquium: t.b.a.

ATOM23 Colloquium: Attosecond Physics: 2023 Nobel Prize Achievements and Unraveling Quantum Insights

In my talk, I will start by giving an overview of this year’s Nobel Prize in Physics awarded to Anne L'Huillier, Ferenz Krausz and Pierre Agostini “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”. Following their pioneering works, we have witnessed a revolutionary development of attosecond light sources, and methods to conduct attosecond time-resolved measurements. These now allow researchers to explore electron dynamics at this timescale, including charge migration in molecules and ultrafast processes in semiconductors and dielectrics. While experimental techniques have progressed rapidly, theory has not developed at the same pace. Insights that can be derived from present-day theory formulated in terms of single-electron and classical modeling are reaching their barrier. This situation impedes a deeper understanding of the quantum motion of electrons in matter and the quantum properties of light. Therefore, in the second part of my talk, I will discuss the challenges faced by current theoretical models and discuss ongoing efforts to enhance them. Three key areas of focus include (i) Electron-electron correlations: Exploring ways to incorporate electron-electron correlation in theoretical frameworks and identify their effect, (ii) Relativistic corrections: accounting for relativistic corrections is needed to provide a more accurate description of electron motion (iii) Beyond classical treatment of light: Going beyond a classical description of the electromagnetic fields to better capture the quantum nature of light, and explore the merging of quantum optics and strong-field laser physics.

Gutzwiller Colloquium: Hydration in Bio-Nano Complex Matter

Water is a crucial element for life. However, there is still debate on why it is so important. This is because the mechanisms that control the effect of water on biological and nanoscopic solutes still need to be fully understood. Additionally, the challenges of renewable energy sources and water pollution are two major environmental concerns that need addressing: hydrated nano-interfaces could offer new solutions for applications such as desalination, decontamination, energy storage, and heterogeneous catalysis for green hydrogen production. This Colloquium will provide a broad introduction to these topics, connecting them with the unique properties of water [1, 2, 3]. Using Statistical Physics to create phenomenological models and multiscale computation can fill the knowledge gap and guide us toward new applications. For instance, it is possible to understand how hydration water modulates the folding, condensation, aggregation, and adsorption of proteins in solutions onto nanomaterials [4, 5]. This is a crucial element in designing new drugs or drug-delivery nanomedical devices. Another example is how this approach provides a way to explore the feasibility of using graphene nano-sponges and slit-pores for the physical filtration of water from pollutants [6, 7]. [1] Coronas, L. E., Vilanova, O., Bianco, V., de los Santos, F., and Franzese, G., The Franzese-Stanley Coarse Grained Model for Hydration Water, in 'Properties of Water from Numerical and Experimental Perspectives', Martelli, F. ed. (CRC press, Boca Raton 2022). [2] Skarmoutsos, I., Franzese, G., and Guardia, E., Using Car-Parrinello simulations and microscopic order descriptors to reveal two locally favored structures with distinct molecular dipole moments and dynamics in ambient liquid water, Journal of Molecular Liquids 364, 119936 (2022). [3] Abella, D., Franzese, G., and Hernández-Rojas, J., Many-Body Contributions in Water Nanoclusters, ACS Nano 17, 1959 (2023). [4] March, D., Bianco, V., and Franzese, G., Protein Unfolding and Ag- gregation near a Hydrophobic Interface, Polymers 13, 156 (2021). [5] Dura-Fauli, B., Bianco, V., and Franzese, G., Hydrophobic Homopolymer’s Coil–Globule Transition and Adsorption onto a Hydrophobic Surface under Different Conditions, The Journal of Physical Chemistry B 127 5541-5552 (2023). [6] Leoni, F., Calero, C., and Franzese, G., Nanoconfined Fluids: Uniqueness of Water Compared to Other Liquids, ACS Nano 15, 19864 (2021). [7] Bellido-Peralta, R., Leoni, F., Calero, C., and Franzese, G., Exploring optimal graphene slit-pore width for the physical separation of water-methanol mixture, The Journal of Molecular Liquids 391, 123356 (2023)

Colloquium: Open Floquet Systems

Floquet engineering is the idea to use time-periodic driving to coherently control quantum systems, so that they acquire novel properties that are hard or even impossible to achieve in non-driven systems. In isolated systems, this is achieved by engineering drives to shape the one-cycle evolution operator describing the stroboscopic evolution of the system in steps of the driving period. Often (unless the system is integrable or localized) the desired properties can only be achieved approximately, on a prethermal time scale, before the system eventually heats up to infinite temperature. Dissipative Floquet systems, which are coupled to an environment, instead approach non-equilibrium steady state (NESS) in the long-time limit. The properties of these states are determined (or controlled) by the interplay (and engineering) of driving and dissipation, opening the door for a robust form of dissipative Floquet engineering. I will give a basic introduction to these concepts and provide some examples of how steady (and also transient) states of open Floquet systems can be engineered.

Colloquium: Climate Meets Complex Systems: Exploring Predictability of Extreme Climate Events via a Complex Network Approach

The Earth system is a very complex and dynamical one basing on various feedbacks. This makes predictions and risk analysis even of very strong (sometime extreme) events as floods, landslides, or heatwaves a challenging task. After introducing physical models for weather forecast already in 1922 by L.F. Richardson, a fundamental open problem has been the understanding of basic physical mechanisms and exploring anthropogenic influences on climate. In 2021 Hasselmann and Manabe got the Physics Nobel Price for their pioneering works on this. I will shortly review their main seminal contributions. Next, I will discuss a recently developed approach via complex networks mainly to analyze strong climate events. This leads to an inverse problem: Is there a backbone-like structure underlying the climate system? To treat this problem, we have proposed a method to reconstruct and analyze a complex network from spatio-temporal data. This approach enables us to uncover relations to global and regional circulation patterns in oceans and atmosphere, which leads to construct substantially better predictions of extreme climate events, in particular for the Indian Summer Monsoon, extreme rainfall in South America, the Indian Ocean Dipole, El Nino and tropical cyclones.

Colloquium: A Mind Over Matter: The Life and Science of Philip W. Anderson

This colloquium presents a biographical survey of the life and science of the Nobel laureate Philip W. Anderson, arguably the most influential physicist of the second half of the twentieth century. I discuss his upbringing during the American Great Depression, his education at Harvard University, his service during World War II, and his subsequent career as a condensed matter physicist at Bell Laboratories, Cambridge University, and Princeton University. I highlight some of his best-known scientific achievements and also some of his forays into national and scientific politics. A few remarks about his activities as a public intellectual and his personal life round out the talk.

Colloquium: Disorder-robust Phase Crystal in High-temperature Cuprate Superconductors from Topology and Strong Correlations

The simultaneous interplay of strong electron-electron correlations, topological zero-energy states, and disorder is still a largely unexplored territory but of considerable interest due to their inevitable presence in many materials. Copper oxide high-temperature superconductors (cuprates) with pair breaking edges host a flat band of topological zero-energy states, making them an ideal playground where strong correlations, topology, and disorder are strongly intertwined. Here we perform fully self-consistent calculations of the superconducting state in cuprate superconductors and find that strong correlations stabilize a fully gapped phase crystal state along the pair breaking [110] edge. The phase crystal breaks both translational and time reversal invariance and is characterized by a nanoscale modulation of the phase of the d-wave order parameter. In particular, we first show how strong correlations increase the number of zero-energy states for any uniform superconducting phase, contradicting simple topological arguments. Then, when allowing for a non-uniform solution, we find a cascade of phase transitions occurring at different temperatures: d-wave superconductivity occurs below a transition temperature $T_c$, the phase crystal appears further below at temperature $T^* \sim 0.2 T_c$, and, finally, an additional extended s-wave order, with the same spatial modulations as the phase crystal, is generated below $T_s$, producing a full energy gap. Taken together, these phase transitions explain a set of so far seemingly contradictory experimental results. Furthermore, we find that the phase crystal state is unexpectedly very robust to disorder, but notably only in the presence of strong correlations. Our results show that the combined effects of strong correlations and topology lead to the emergence of novel phases of matter that survives strong disorder in a highly non-intuitive manner.

Colloquium: Relativistic-like Electron Hydrodynamics in Dirac Semimetals

Electron quasiparticles in Dirac semimetals may exhibit a hydrodynamic regime under certain conditions. Since the corresponding quasiparticle fluid is relativistic-like, anomalous, and electrically charged, it has a range of unusual properties. Anomalous physics affects properties of low-energy collective modes, relativistic-like effects show up in novel instabilities, and the charged nature of the electron fluid is responsible for the strong suppression of convection. In this talk, I will review some of these unusual properties of the electron fluid in Dirac semimetals.

Colloquium: Chimeras in Physics and Biology: Synchronization and Desynchronization of Rhythms

Rhythms influence our life in various ways, e.g., through heart beat and respiration, oscillating brain currents, life cycles and seasons, clocks and metronomes, pulsating lasers, transmission of data packets, and many others. The physics of complex nonlinear systems has developed methods to describe and analyze self-sustained oscillations and their synchronization in complex networks, which are composed of many components. Synchronized oscillations as well as completely asynchronous chaotic oscillations play a major role in many networks in nature and technology. For instance, the synchronous firing of all neurons in the brain represents a pathological state, like in epilepsy or Parkinson's disease, and should be suppressed, as well as the synchronous mechanical vibration of bridges. On the other hand, synchronization is desirable for the stable operation of power grids or in encrypted communication with chaotic signals. In networks composed of identical components, intriguing hybrid states ("chimeras") may form spontaneously, which consist of spatially coexisting synchronized and desynchronized domains, i.e., seemingly incongruous parts. This might be of relevance in inducing and terminating epileptic seizures, or in unihemispheric sleep which is found in certain migratory birds and mammals, or in cascading failures of the power grid.

Colloquium: Aspects of topological magnons in Kitaev and non-Kitaev honeycomb magnets

We discuss various examples of topological magnons in quantum magnets on honeycomb lattice. These include topological magnons in generalized Kitaev models on two-dimensional honeycomb lattice and the corresponding thermal Hall transport, higher order magnon topology and hinge states in stacked honeycomb magnets, and Weyl/nodal magnons in Kitaev magnets on three-dimensional hyperhoneycomb lattice.

Colloquium: Self-organizing Reconfigurable Robot Collectives at the Micron-Scale

Mobile microrobots, which can navigate, sense, and interact with their environment, could potentially revolutionize medicine and environmental remediation. Many self-organizing microrobotic collectives have been developed to overcome inherent limits in actuation, sensing, and manipulation of individual microrobots; however, reconfigurable self-organized collectives with robust transitions between behaviors are rare. Such systems that perform multiple functions are advantageous to operate in complex environments. Here, we present a versatile self-organizing robotic collective system at the microscale capable of on-demand reconfiguration to adapt to and utilize their environments to perform various functions at the air-water interface. Each microrobot is disc-shaped with a magnetic film coating to be oscillated and pulled using applied external magnetic field waveforms and gradients, respectively. Microrobots have magnetic, capillary and hydrodynamic interactions inducing various nonlinear coupled dynamics and dynamic self-assembly modes. Our system exhibits diverse modes ranging from isotropic to anisotropic behaviors and transitions between a globally driven and a novel self-propelling behavior. We show the transition between different modes in experiments and simulations, and demonstrate various functions, using the reconfigurability of our system to navigate, explore, and interact with the environment. Such versatile microrobot collectives with globally driven and self-propelled behaviors have significant potential in future medical and environmental applications.

Colloquium: Tracking Lymphocyte Clones through Tissues, Individuals and Time

The DNA sequences of antibody and T cell receptor gene rearrangements can be used to trace clonally-related lymphocytes through different tissues, in different individuals and over time. In this presentation, I will describe methods for inferring clonal relatedness from adaptive immune repertoire profiling data. I will describe networks of clones in different tissues of human organ donors and show how features of gene rearrangements yield insights into tissue compartmentalization of immune responses. I will also discuss some of our recent data on antibody responses to SARS-CoV-2 vaccines, highlighting how clonal lineage analysis can be used to understand how antibody responses evolve over time. Finally, I will describe some of our recent efforts to study shared repertoire features between individuals.

Colloquium: Researching Living Systems by Considering Modes of Change and Not Endpoints

Quantum simulation with ultracold atoms – from Hubbard models to gauge theories

Well-controlled synthetic quantum systems, such as ultracold atoms in optical lattices, offer intriguing possibilities to study complex many-body problems in regimes that are beyond reach using state-of-the-art classical computations. The basic idea is to construct and use a well-controlled quantum many-body system in order to study its in- and out-of-equilibrium properties and potentially use it to develop more efficient tailored numerical methods that can then be applied to other systems that are not directly accessible with the simulator. An important future quest concerns the development of novel experimental techniques that allow us to expand the range of models that can be accessed. I will demonstrate this using the example of topological lattice models, which in general do not naturally appear in cold-atom experiments. I will show how the technique of periodic driving, also known as Floquet engineering, facilitates their realization and show how charge-neutral atoms in lattices can mimic the behavior of charged particles in the presence of an external magnetic field. A key ingredient for quantum simulation is the degree of control one has over the individual particles and the microscopic parameters of the model. We have recently succeeded to not only use the technique of periodic driving to emulate physical systems that we know exist in nature, but to take this idea one step further and realize completely new topological regimes that do not have any static analog. Moreover, we are currently developing a novel hybrid optical lattice platform, where tightly focused optical tweezers are used to locally control the motion of the atoms in the lattice, paving the way towards quantum simulation of simplified lattice gauge theories, which play a fundamental role in a variety of research areas including high-energy physics and topological quantum computation.

Colloquium: Quantum Transport and Dissipation of Superfluid Fermi Gases in Structured Optical Potentials

I will report on the realization of supercurrents in homogeneous, tunable fermionic rings. We gain exquisite, rapid control over quantized persistent currents in all regimes of the BCS-BEC crossover through a universal phase-imprinting technique. High-fidelity read-out of the superfluid circulation state is achieved by exploiting an interferometric protocol, which also yields local information about the superfluid phase around the ring. In the absence of externally introduced perturbations, we find the induced metastable supercurrents to be as long-lived as the atomic sample. We trigger and inspect the supercurrent decay by inserting a single small obstacle within the ring. For circulations higher than a critical value, the quantized currents dissipate via the emission of vortices. Our results demonstrate fast and accurate control of quantized collective excitations in a macroscopic quantum system, and establish strongly interacting fermionic superfluids as excellent candidates for atomtronic applications.

Colloquium: The Softest Crystals

Usually, crystals have three-dimensional periodicity. Smectic liquid crystals, however, have one-dimensional order, even in three-dimensional samples. These systems, as simple as they might seem, connect the physics of biomembranes, superconductivity, and even special relativity. I will provide an introduction for non-specialists and show how this diverse set of ideas comes together in these very, very soft systems.

Colloquium: The Observable Fingerprints of Network Dynamics

Colloquium: Trapped and Measured

Atoms made of a particle and an antiparticle are unstable, usually surviving less than a microsecond. Antihydrogen, the bound state of an antiproton and a positron, is made entirely of antiparticles and is believed to be stable. It is this longevity that holds the promise of precision studies of matter-antimatter symmetry. I will give an overview of the ALPHA experiment (with an emphasis on the physical processes involved in the measurements) which has succeeded in trapping antihydrogen in a cryogenic Penning trap for times up to 15 minutes and has successfully performed precision measurements of several properties of antihydrogen. For example, we have measured the 1S-2S frequency to about one part in a trillion. I will conclude with prospects for future precision measurements.

Colloquium: Bacteria as Multicellular Organisms - History and Perspectives

Chromatin at Small and Large Scales

DNA molecules with a total length of two meters contain the genetic information in every cell in our body. To control access to the genes and to organize its spatial structure in the nucleus the cell makes use of sophisticated physical mechanisms. Base pair sequences multiplex various layers of information, chromatin remodelers mobilize nucleosomes via twist defects, and biomolecular condensates concentrate proteins and nucleic acids in specialized membraneless compartments. In this talk I discuss our current state of understanding of some of these physical mechanisms that influence the organization of the genetic material in space and time.

Microfluidic Island Biogeography — Jam and Conquer

Microbes often colonize spatially-constrained habitats, such as pores in the skin or crypts in the colon. The resulting micro-communities can be very stable and contribute to the long-term function of our microbiomes. Due to a lack of spatio-temporal observations, it is however unclear how these communities and their ecological functions arise. Using microfluidic devices to systematically vary ecological correlation lengths, we uncover sharp transitions between different colonization states with different evolutionary properties. Our results show that density-dependent passive diffusion can drive reproducing populations to a jamming threshold, which entails supreme resilience against invaders at the cost of a total loss of mixing and intra-species competition. These results elucidate how cell proliferation can drive unique non-equilibrium phase transitions (different from MIPS). The emerging sensitivity to scale, foreshadowed in the field of island biogeography, underscores the need to control for scale in microbial eco-evolutionary experiments.

Colloquium: A Case for Carbon Dioxide Removal from Air

As a global society we have been burning fossil fuels to meet our energy and transportation needs since the start of the industrial revolution. This has resulted in atmospheric CO2 concentrations much greater than at any other time during the last 650,000 years. That concentration reached a record 415 parts per million in May 2019. The replacement of fossil fuels with renewables, advances in energy efficiency, and carbon capture and storage are among the key strategies required to prevent warming beyond 2°C within this century. But they will not be enough. We need to ramp up our efforts in reducing CO2 emissions, and then we need to do even more. The Earth’s natural systems, such as forests and oceans, are capable of removing roughly half of global CO2 emissions each year, while the rest steadily accumulates in the atmosphere. Until now, our best approach to avoiding the worst impacts of climate change was simply to avoid such emissions in the first place. But because of our failure to act quickly and at a large enough scale, we are now faced with the need to go beyond that strategy—to actually start removing CO2 directly from the air. Trees and oceans already do this, but these systems are overwhelmed. Manufactured or synthetic removal systems are designed to pull CO2 from the atmosphere, and at a much faster rate than natural systems. This talk will review both the promise and pitfalls of this approach.

Catching and reversing a quantum jump mid-flight

Measurements in quantum physics, unlike their classical physics counterparts, can fundamentally yield discrete and random results. Historically, Niels Bohr was the first to hypothesize that quantum jumps occurred between two discrete energy levels of an atom. Experimentally, quantum jumps were directly observed many decades later in an atomic ion driven by a weak deterministic force under strong continuous energy measurement. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Despite the non-deterministic character of quantum physics, is it possible to know if a quantum jump is about to occur? Our work1 provides a positive answer to this question: we experimentally show that the jump from the ground state to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable “flight” by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the evolution of the jump — once completed — is continuous, coherent, and deterministic. Based on these insights and aided by real-time monitoring and feedback, we then pinpoint and reverse one such quantum jump “mid-flight”, thus deterministically preventing its completion. Our findings, which agree with theoretical predictions essentially without adjustable parameters, lend support to the modern formulation of quantum trajectory theory; most importantly, they may provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as the early detection of error syndromes. 1. Z. Minev et al., Nature 570, 200–204 (2019)

Colloquium: tba

Colloquium: Exoplanets and the Search for Atmospheric Biosignature Gases

Thousands of exoplanets are known to orbit nearby stars and small rocky planets are established to be common. Driving the field is the study of exoplanet atmospheres, with the goal of detecting a gas that might be indicative of life. A suitable “biosignature gas” is not just one that might be produced by life, but one that: can accumulate in an atmosphere against atmospheric radicals and other sinks; has strong atmospheric spectral features; and has limited abiological false positives. Which gases might be potential biosignature gases in an as yet unknown range of exoplanetary environments? New computer simulations and next-generation telescopes coming online means the ambitious goal of searching for “biosignature gases” in a rocky exoplanet atmosphere is within reach.

Colloquium: Leaving the physical corner: Tensor network methods augmented by mode transformations in molecular quantum chemistry and condensed matter physics

Tensor network methods have become workhorses in the study of strongly correlated systems in condensed matter physics and increasingly also in the context of molecular systems. While their variational character provides results of remarkable accuracy, the capture quantum states featuring low entanglement well, a set of states often referred to as the 'physical corner'. While this physical describes ground states of local Hamiltonian problems from the condensed matter context well, the same cannot be said for problems involving molecules in quantum chemistry or systems undergoing time evolution. In this talk I will provide an overview over tensor network methods augmented by fermionic mode transformations in the context of molecular problems [1], condensed-matter simulations [2], and time evolution [3]. If time allows, I might briefly mention two new applications of tensor networks in being a design principle for building quantum devices [4] and in machine learning [5,6]. [1] Fermionic orbital optimisation in tensor network states (C. Krumnow, L. Veis, Ö. Legeza, J. Eisert, Phys. Rev. Lett. 117, 210402 (2016)) [2] Towards overcoming the entanglement barrier when simulating long-time evolution (C. Krumnow, J. Eisert, Ö. Legeza, arXiv:1904.11999) [3] Dimension reduction with mode transformations: Simulating two-dimensional fermionic condensed matter systems (C. Krumnow, L. Veis, J. Eisert, Ö. Legeza, arXiv:1906.00205) [4] Simulating topological tensor networks with Majorana qubits (C. Wille, R. Egger, J. Eisert, A. Altland, Phys. Rev. B 99, 115117 (2019)) [5] Expressive power of tensor-network factorizations for probabilistic modeling, with applications from hidden Markov models to quantum machine learning, I. Glasser, R. Sweke, N. Pancotti, J. Eisert, J. I. Cirac, arXiv:1907.03741, NeurIPS (2019)) [6] Tensor network approaches for learning non-linear dynamical laws (A. Goessmann, M. Goette, I. Roth, G. Kutyniok, J. Eisert, R. Sweke, Submitted to ICML2020 (2020))

Colloquium: Maxwell in Chains

In standard text books on electrodynamics, Maxwell's laws are often illustrated using field lines -- a notion that lacks relativistic covariance and obscures the superposition principle (see e.g. R. Feynman in: The Feynman Lectures on Physics, Vol II, Chapter 1-5). In this colloquium, an improved variant of the field line picture is presented: the discrete approximation of the electromagnetic field by chains (B and D are modeled as 1-chains, E and H as 2-chains). The language of chains offers a simple, direct and intuitive approach to Maxwell's electrodynamics, without compromising physical or mathematical correctness. Fundamental aspects such as parity invariance or the transcription of the theory to curved space-time are especially transparent in this approach. The talk illustrates the chain picture at a number of examples including vortex motion in superconductors, spin-orbit coupling, equivalence between problems of magneto- and electrostatics, emission of an electromagnetic signal by the discharge of a capacitor, quantum Hall-Ohm law. The colloquium is an outgrowth of the speaker's 30 years of experience teaching electrodynamics to physics students.

WS Colloquium: Unconventional superconductivity

Superconductivity comes in a wide variety of incarnations including conventional s-wave, high-Tc d-wave, and topological p-wave. While these states are well defined theoretically, they are extremely difficult to distinguish experimentally: all superconductors have Cooper pairs, a Meissner effect, and a zero-resistance state, while any differences between states are subtle. I will give an overview of the experimental situation with a particular focus on how to find a bulk, three-dimensional topological superconductor, drawing on my own group's research on Sr2RuO4 and UTe2.

Colloquium talk: What to do with a near-term quantum computer?

In 2019, Google researchers carried out an experiment where they achieved "quantum supremacy". This is defined as when a quantum computer performs *any* task in a way that a classical computer takes impractical amounts of time to simulate its outcome. This is a huge milestone for the field, but several milestones lie ahead. In this talk, I will describe a potential algorithmic road to "quantum advantage", ie. the moment when a quantum computer carries out a *practical* task on a system that is unreachable by current classical computers. To reach quantum advantage presumably many years of hardware development and algorithmic development lie ahead of us. The current era of quantum computing, coined near-term intermediate-scale quantum computing (NISQ) by John Preskill presents many challenges and opportunities. I will discuss this algorithmic road in the context of algorithms for the simulation of molecules and materials, as well as will briefly mention quantum machine learning applications.

Colloquium: May's Instability in Firm Networks

Will a large economy be stable? Building on Sir Robert May’s original argument for large ecosystems, we conjecture that evolutionary and behavioural forces conspire to drive the economy towards marginal stability. We study networks of firms in which inputs for production are not easily substitutable, as in several real-world supply chains. We argue that such networks generically become dysfunctional when their size increases, when the heterogeneity between firms becomes too strong, or when substitutability of their production inputs is reduced. At marginal stability and for large heterogeneities, we find that the distribution of firm sizes develops a power-law tail, as observed empirically. Crises can be triggered by small idiosyncratic shocks, which lead to “avalanches” of defaults characterized by a power-law distribution of total output losses. This scenario would naturally explain the well-known “small shocks, large business cycles” puzzle, as anticipated long ago by Bak, Chen, Scheinkman, and Woodford.

Gutzwiller Colloquium: Topological electrostatics

Two-dimensional electron gases under a strong magnetic field have tremendously expanded our understanding of many-body physics, with the discovery of integer and fractional quantum Hall effects, together with chiral edge states, fractional excitations, anyons. Another striking effect is the strong coupling between charge and spin/valley degrees of freedom, which takes place near integer filling of the magnetic Landau levels. More precisely, because of the large energy gap associated to cyclotron motion, any slow spatial variation of the spin background induces a variation of the electronic density proportional to the topological density of the spin background. Minimizing Coulomb energy leads to an exotic class of two-dimensional crystals, which exhibit a periodic non-collinear spin texture called a Skyrmion lattice. I will review the history of these concepts, with an emphasis on the notions of Berry phases and Berry curvature, which play a prominent role in all aspects of topological condensed matter physics. A main feature of these Skyrmion crystals is that their theoretical description involves effective theories with local and possibly non-Abelian gauge symmetries. It is therefore a theoretical challenge to identify the actual physical degrees of freedom in such systems. I will show how this may be achieved using some ideas from complex geometry. The main outcome of such analysis is the existence of two regimes depending on whether the topological charge per unit cell is smaller (unfrustrated case) or larger (frustrated case) than the number of internal states (spin/valley) accessible to electrons.

Virtual Coll.-Talk: Ductile and brittle yielding in sheared amorphous materials

We discuss recent theoretical progress in understanding the nature of yielding in amorphous materials. Within an athermal elastoplastic model in quasistatic shear, we find that yielding is brittle in the limit of large system size, whatever the level of annealing/ageing of the sample prior to shear. For small systems, in contrast, we find a crossover from apparently ductile to brittle yielding behaviour with increasing degree of sample annealing prior to shear. Differences between studies in one and two spatial dimensions will be discussed. We will also compare the nature of ductile/brittle yielding in athermal materials with that in thermal materials.

Prize Colloquium "Physik-Preis Dresden 2021"

Cells and tissues are highly dynamic but at the same time need to withstand large mechanical loads. This paradoxical mechanical behavior is governed by fibrous protein scaffolds known as the cytoskeleton and the extracellular matrix. Fibrous networks have many advantageous mechanical properties: fibers can form space-filling elastic networks at low volume fractions and they reversibly stress-stiffen, which provides protection from damage. However, it is still poorly understood how biopolymer networks can combine these features with the ability to dynamically adapt their structure and mechanics. This question has guided our research over the past years. I will provide an overview of the most important things we have learned by performing quantitative measurements on reconstituted cytoskeletal and extracellular matrix networks. We perform these measurements from the macroscopic scale (using rheology) all the way down to the molecular scale (using optical tweezers, in situ small-angle-X-ray scattering, and atomic force microscopy). I will furthermore mention connections to applications in bottom-up synthetic biology and for tissue (re)generation.

Colloquium talk: Colliders, Higgs and the strong interaction

Particle colliders are our main laboratory tool to study the smallest distance scales accessible to humankind. Recent years have seen major advances, notably the discovery of long-hypothesised, but qualitatively new interactions in the Higgs sector, which are essential for a universe as we know it. Central to the progress of collider particle physics is our understanding of the strong interaction and its theoretical formulation, quantum chromodynamics. This rich theory operates across many orders of magnitude in distance and momentum, producing hundreds of particles in each collision. One of today's major challenges is to learn how to maximally and reliably exploit the resulting information.

Colloquium talk: A statistical physics view of cells under strong perturbations

Hybrid Colloq-talk: Many-body chaos and order, through the lens of OTOCs

The concepts of order and chaos are central to our understanding of the statistical mechanics of dynamical systems. Starting from the fabled butterfly effect, this talk presents a brief survey of general properties of chaotic many-body dynamics, in particular showing how out-of-time-ordered correlators (OTOCs) appear naturally there. It addresses such dynamics in simple and familiar model systems exhibiting different degrees of order, and concomitant descriptions with and without quasiparticle excitations. It finally demonstrates how OTOCs can be used in practise to mitigate errors in a noisy quantum computing platform. This is used to extract the intrinsic signal of a new type of spatiotemporal eigenstate order realised in a many-body localised discrete time crystal.

Coll.-Talk: Vacuum fluctuations in cavities: breakdown of the topological protection in the integer Quantum Hall effect

When a collection of electronic excitations are strongly coupled to a single mode cavity, mixed light-matter excitations called polaritons are created. The situation is especially interesting when the strength of the light-matter coupling WR is such that the coupling energy becomes close to the one of the bare matter resonance w0. For this value of parameters, the system enters the so-called ultra-strong coupling regime, in which a number of very interesting physical effects were predicted caused by the counter-rotating and diamagnetic terms of the Hamiltonian. In a microcavity, the strength of the electric field caused by the vacuum fluctuations, to which the strength of the light-matter coupling WR is proportional, scales inversely with the cavity volume. One very interesting feature of the circuitbased metamaterials is the fact that this volume can be scaled down to deep subwavelength values in all three dimension of space. Using metamaterial coupled to two-dimensional electron gases under a strong applied magnetic field [1], we have now explored to which extend this volume can be scaled down and reached a regime where the stability of the polariton is limited by diffraction into a continuum of plasmon modes [2]. We have also used transport to probe the ultra-strong light-matter coupling [3], and show now that the latter can induce a breakdown of the integer quantum Hall effect. [1] G. Scalari et al., "Ultrastrong Coupling of the Cyclotron Transition of a 2D Electron Gas to a THz Metamaterial," (in English), Science, vol. 335, no. 6074, pp. 1323-1326, Apr 15 2012, doi: 10.1126/science.1216022. [2] S. Rajabali, E. Cortese, M. Beck, S. D. Liberato, J. Faist, and G. Scalari, "Polaritonic nonlocality in light–matter interaction," Nature Photonics, vol. 15, no. 9, pp. 690-695, 2021, doi: 10.1038/s41566-021-00854-3. [3] G. L. Paravicini-Bagliani et al., "Magneto-transport controlled by Landau polariton states," (in English), Nature Physics, OriginalPaper vol. 15, no. 2, pp. 186-+, Feb 2019, doi: 10.1038/s41567-018-0346-y

Colloquium: Probabilistic cellular automata for quantum fermions

Fermions are often considered as somewhat strange quantum objects due to their anti-commuting properties. Cellular automata describe deterministic changes for bit-configurations, as for classical computing. At first sight these two issues do not seem to be much related. We show that probabilistic cellular automata, for which initial conditions are given by a probability distribution, can describe certain quantum many body systems of interacting fermions. The notions of wave functions, non-commuting operators for observables and quantum rules for expectation values arise in a natural way from the classical statistics for the probabilistic cellular automata. This constitutes an example how quantum mechanics can emerge from a classical statistical system. The famous particle-wave duality combines the discreteness of the bit observables or fermionic occupation numbers with the continuity of the probabilistic information and its evolution in time.

Rydberg spin liquids and beyond: Creating highly entangled quantum states in synthetic matter

In sharp contrast to ordered states such as crystals and magnets, entirely new patterns of quantum entanglement emerge in states with intrinsic topological order (ITO) such as fractional quantum Hall states and spin liquids. Their unique properties hold promise for a range of applications - most notably in the quest for fault-tolerant quantum computing. However, ITOs are notoriously hard to realize experimentally. For example, quantum spin liquids were proposed by Anderson in 1973 but remain to be clearly identified in any magnetic material. Key obstacles include the propensity towards order and the lack of clear signatures associated with ITO. Synthetic platforms, such as Rydberg atom arrays, with their high tunability and access to non-local `string' correlations can circumvent these barriers. In this talk, I will describe our theoretical proposal to create ITO in Rydberg atom arrays, methods to characterize their subtle signatures, and the experimental progress towards these goals. Finally, I will describe our ongoing exploration of new approaches utilizing measurement to efficiently realize many kinds of ITOs, including fractons, in synthetic matter.

Gutzwiller Colloquium: Birth, Death, and Flight: the hydrodynamics of Malthusian flocks

I'll present the hydrodynamic theory of ``Malthusian Flocks": moving aggregates of self-propelled entities (e.g., organisms, cytoskeletal actin, microtubules in mitotic spindles) that reproduce and die. Long-ranged order (i.e., the existence of a non-zero average velocity $\langle \bf{v} (\bf{r,t}) \rangle \neq \bf{0}$) is possible in these systems, even in spatial dimension $d=2$. Their spatiotemporal scaling structure can be determined exactly in $d=2$; furthermore, they lack both the longitudinal sound waves and the giant number fluctuations found in immortal flocks. Number fluctuations are very ${\it persistent}$, and propagate along the direction of flock motion, but at a different speed. I'll also present recent results for the three dimensional version of this problem, which required the first full blown dynamical renormalization treatment of a flocking system in its ordered phase.

Colloquium: Quantum Spacetime from Causal Dynamical Triangulations

Causal Dynamical Triangulations (CDT) is a candidate theory for quantum gravity, formulated nonperturbatively as the scaling limit of a lattice theory in terms of piecewise flat (triangulated) spacetimes. From a technical and conceptual perspective, an important feature of this approach is its elegant resolution of the problem of how to treat the diffeomorphism symmetry of the classical theory in the full, background-free quantum theory. This has enabled the concrete computation of geometric observables in a highly nonperturbative, Planckian regime, an important step in putting quantum gravity on a quantitative footing, and understanding the structure of quantum spacetime. While the need to find quantum observables describing this regime is common to all approaches, CDT quantum gravity provides a concrete testing ground for implementation and measurements. In particular, a new notion of quantum Ricci curvature has opened a new window on some of the counterintuitive properties of quantum geometry.

Colloquium: Ca$_2$RuO$_4$: correlated electron physics in a nutshell

The set of compounds (Ca/Sr)$_2$RuO$_4$ exhibit a rich array of properties, ranging from fermi liquid/Hunds metal physics and nontrivial superconductivity to metal-insulator transition that is driven by the interplay of correlated electron and lattice physics and is highly controllable by strain and current. Following a brief review of the material family, this talk will focus on the end-member Ca$_2$RuO$_4$, showing how attempts to understand the properties via multiple theoretical methods are leading to new insights into the techniques of many-body physics, how the metal insulator transition has provided new insights into the role of the lattice in correlated electron physics, and how the current-driven transition is providing new insights into nonequilibrium properties of correlated systems.

Colloquium: Synthesizing “Toy Model” Quantum Materials

Connecting theoretical models for exotic quantum states to real physical systems is a key goal in the study of quantum materials. Among such theoretical models, a “toy model” is one made deliberately simplistic in order to demonstrate new physical concepts and their underlying mechanisms. Such models have proven to be tremendously successful in offering insight in to new condensed matter phenomena including those involving electronic topology and correlation. We describe here our recent progress in experimentally realizing “toy model” quantum materials which, in analogy to their theoretical counterparts, are designed to capture simple model systems by lattice and superlattice design. We detail developments in synthesizing and studying magnetic and superconducting materials that allow for new connections to long-standing predictions for unusual topological electronic phases. We close with a perspective for realizing further toy model systems in complex material structures.

Colloquium: The scales that limit: biological scaling and the physical boundaries of evolution

Organisms are subject to the laws of physics, so the process of evolution is constrained by these fundamental laws. Classic and recent studies of the biophysical limits facing organisms have shown how fundamental physical constraints can be used to predict broad-scale relationships between body size and organismal biomechanics and physiology. In this talk I will provide a broad framework for the connection between physical constraints, evolution, and allometric physiology. I will focus on an under-appreciated aspect of allometry: ultimate limits that occur at distinct scales for broad categories of organisms. I will connect these limits to higher-order evolutionary phenomena such as major evolutionary transitions and constraints on populations of organisms. In discussing these concepts I will cover case-studies ranging from bacterial physiology to mammalian population dynamics, and I will outline a broad physics-based evolutionary theory.

VIRTUAL Colloquium: Artificial intelligence: success and limits

Artificial Intelligence is about to have a dramatic impact on many sectors of human activity. In the last ten years, thanks to the development of machine learning in “deep networks”, we have experienced spectacular breakthroughs in diverse applications such as automatic interpretation of images, speech recognition, consumer profiling, or go and chess playing. Algorithms are now competing with the best professionals at analyzing skin cancer symptoms or detecting specific anomalies in radiology; and much more is to come. Worrisome perspectives are frequently raised, from massive job destruction to autonomous decision-making “warrior” robots. In this talk, we shall open the black box of deep networks and explore how they are programmed to learn from data by themselves, using a statistical physics perspective. This will allow us to understand their limits, to underline the lack of theoretical understanding, to question whether their achievements have anything to do with “intelligence”, and to reflect on the foundations of scientific intelligence.

VIRTUAL Colloquium: Computational Epidemiology at the time of COVID-19

The data science revolution is finally enabling the development of large-scale data-driven models that provide scenarios, forecasts and risk analysis for infectious disease threats. These models also provide rationales and quantitative analysis to support policy making decisions and intervention plans. At the same time, the non-incremental advance of the field presents a broad range challenges: algorithmic (multiscale constitutive equations, scalability, parallelization), real time integration of novel digital data stream (social networks, participatory platform, human mobility etc.). I will review and discuss recent results and challenges in the area, and focus on ongoing work aimed at responding to the COVID-19 pandemic.

Colloquium: Scaling down the laws of thermodynamics

Thermodynamics provides a robust conceptual framework and set of laws that govern the exchange of energy and matter. Although these laws were originally articulated for macroscopic objects, nanoscale systems also exhibit “thermodynamic¬-like” behavior – for instance, biomolecular motors convert chemical fuel into mechanical work, and single molecules exhibit hysteresis when manipulated using optical tweezers. To what extent can the laws of thermodynamics be scaled down to apply to individual microscopic systems, and what new features emerge at the nanoscale? I will describe some of the challenges and recent progress – both theoretical and experimental – associated with addressing these questions. Along the way, my talk will touch on non-equilibrium fluctuations, “violations” of the second law, the thermodynamic arrow of time, nanoscale feedback control, strong system-environment coupling, and quantum thermodynamics.

Colloquium: Dynamical Phase Transitions in Cold Atomic Gases

Non-equilibrium quantum many-body systems can display fascinating phenomena relevant for various fields in science ranging from physics, to chemistry, and ultimately, for the broadest possible scope, life itself. The challenge with these systems, however, is that the powerful formalism of statistical physics, which have allowed a classification of quantum phases of matter at equilibrium does not apply. Therefore, using controllable cold atomic systems to shed light on the organizing principles and universal behaviors of dynamical quantum matter is highly appealing. One emerging paradigm is the dynamical phase transition (DPT) characterized by the existence of a long-time-average order parameter that distinguishes two non-equilibrium phases. I will report the observation of a DPT in two different but complementary systems: a trapped quantum degenerate Fermi gas and long lived arrays of atoms in an optical cavity. I will show how these systems can be used to simulate iconic models of quantum magnetism with tunable parameters and to probe the dependence of their associated dynamical phases on a broad parameter space. Besides advancing quantum simulation our studies pave the ground for the generation of metrologically useful entangled states which can enable real metrological gains via quantum enhancement.

Colloquium: 3D Metamaterials: Rationally Designed Artificial Crystals

Metamaterials are rationally designed three-dimensional composites made of one or more bulk ingredient materials allowing for effective-medium properties going beyond (“meta”) those of the ingredient materials, qualitatively and/or quantitatively. In this talk, I focus on three-dimensional crystalline metamaterials. After an introduction into the concept, I discuss recent examples. This includes artificial cubic chainmail-like crystals leading to a sign reversal of the effective isotropic Hall coefficient, anisotropic crystals exhibiting the parallel Hall effect, chiral cubic crystals showing twist effects and acoustical activity forbidden in Cauchy elasticity, and cubic crystals exhibiting a sign reversal of the isotropic thermal expansion coefficient and a negative static effective compressibility, respectively.

Colloquium: The Sound of Fermions

Transport is the defining property of states of matter, but often the most difficult to understand. Strongly interacting Fermi gases are especially challenging, despite their ubiquitous presence across many fields of physics, from atomic nuclei to high-temperature superconductors and neutron stars. Experiments on ultracold fermionic atoms allow the direct measurement of transport properties in ideal model systems where the Hamiltonian is precisely known, while transport properties are difficult to calculate theoretically. In this talk I will present our experiments on two systems: The unitary Fermi gas, where interactions are tuned to be as strong as quantum mechanics allows, and the Fermi-Hubbard lattice gas, believed to hold the key for our understanding of high-temperature superconductivity. Locally resolved sound and heat transport can distinguish superfluid and normal flow regimes in the unitary gas. Our observation of spin transport in the Fermi-Hubbard system reveals the mechanism governing quantum magnetism.

Colloquium: Landscape and generalisation in deep learning

Deep learning is very powerful at a variety of tasks, including self-driving cars and playing go beyond human level. Despite these engineering successes, why deep learning works remains unclear; a question with many facets. I will discuss two of them: (i) Deep learning is a fitting procedure, achieved by defining a loss function which is high when data are poorly fitted. Learning corresponds to a descent in the loss landscape. Why isn’t it stuck in bad local minima, as occurs when cooling glassy systems in physics? What is the geometry of the loss landscape? (ii) in recent years it has been realised that deep learning works best in the over-parametrised regime, where the number of fitting parameters is much larger than the number of data to be fitted, contrarily to intuition and to usual views in statistics. I will propose a resolution of these two problems, based on both an analogy with the energy landscape of repulsive particles and an analysis of asymptotically wide nets.

Colloquium: Half-integer charge injection by a Josephson junction without excess noise

A Josephson junction in a topological superconductor can inject a charge e/2 into a normal-metal contact, carried by chiral Majorana edge modes. We address the question whether this half-integer charge is a sharp observable, without quantum fluctuations. Because the Majorana modes are gapless, they support charge fluctuations in equilibrium at zero temperature. But we find that the excess noise introduced out of equilibrium by the e/2 charge transfer vanishes. We discuss a strategy to reduce the equilibrium fluctuations, by means of a heavy-tailed time-dependent detection efficiency, to achieve a nearly noiseless half-integer charge transfer.

Colloquium: From Entangled Quantum Puzzles to Quantum Information

Entangled photons have become an essential workhorse for experiments on the foundations of quantum mechanics. Tests of Bell’s Inequality (cf. the Einstein-Podolsky-Rosen Paradox) have with increasing sophistication ruled out more and more alternative views to quantum mechanics. Besides that, they emerged to be also essential for a number of future quantum communication tasks. In most experiments, entanglement of two qubits, that is, of two photons in two states, e.f. polarization are employed. Going beyond two particles, and beyond two dimensions interesting novel phenomena arise. One of them is the GHZ contradiction, where a conflict between an objective local (i.e. classical) view in quantum mechanics arises even for non-statistical predictions of the theory. A most fascinating subject is entanglement swapping, where photons can be entangled which share no common past. In delayed-choice entanglement swapping, the decision whether two photons are entangled or not can be done at a time after they have already been detected and their measurement results (e.g. their polarization) has been registered. Going beyond qubits, states of orbital angular momentum (OAM) of photons provide an in principle unlimited discrete state space where the photons can also be entangled in many different ways. That way, entanglement has been confirmed for quantum numbers above 10.000 and between two photons, each carrying more than 100-dimensional states. A Cosmic Bell Test employed very distant quasars. Also, quantum communication was established between Europe and China using the Chinese satellite Micius. I will conclude with the Big Bell Test, an experiment to test quantum entanglement employing human free will with twelve laboratories on five continents and a hundred million human participants.

Colloquium: Strongly Interacting QCD Matter at Finite Temperature and Density

Quantum Chromodynamics (QCD) is the fundamental theory for elementary particles (quarks and gluons) which constitutes the proton, the neutron and the atomic nuclei. Because the quarks and gluons are strongly interacting, their arise characteristic phases at finite temperature and baryon density: The hadronic phase at low temperature and density with the non-perturbative phenomena such as confinement and chiral symmetry breaking, the quark-gluon plasma phase at high temperature and low density, and the superconducting quark matter at low temperature and high density. In this colloquium, I discuss the characteristic properties of these phases on the basis of the many-body approach to QCD as well as their relevance to the physics of early universe, accelerator experiments, neutron stars and gravitational waves.

Colloquium: Symmetry, topology and electronic phases of matter

Symmetry and topology are two of the conceptual pillars that underlie our understanding of matter. While both ideas are old, over the past several years a new appreciation of their interplay has led to dramatic progress in our understanding of topological electronic phases. A paradigm that has emerged is that insulating electronic states with an energy gap fall into distinct topological classes. Interfaces between different topological phases exhibit gapless conducting states that are protected and are impossible to get rid of. In this talk we will discuss the application of this idea to the quantum Hall effect, topological insulators, topological semimetals and topological superconductors. The latter case has led to the quest for observing Majorana fermions in condensed matter, which opens the door to proposals for topological quantum computation. We will close by surveying the frontier of topological phases in the presence of strong interactions.

Colloquium: Simulating Condensed matter physics with light

When confining photons in semiconductor lattices, it is possible deeply modifying their physical properties. Photons can behave as finite or even infinite mass particles, photons inherit topological properties and propagate along edge states without back scattering, photons can become superfluid and behave as interacting particles. These are just a few examples of properties that can be imprinted into fluids of light in semiconductor lattices. Such manipulation of light present not only potential for applications in photonics, but great promise for fundamental studies. During the talk, I will illustrate the variety of physical systems we can emulate with fluids of light by presenting a few recent experiments: a photonic benzene molecule that emits helical photons, a photonic 1D lattice with topological edge states and photonic graphene with exotic Dirac cones. Perspectives in terms of quantum simulation will be discussed.

Colloquium: Emergent configurational quantum matter created out of equilibrium in layered chalcogenides

The idea that many body systems in complex materials may self-organize under highly non-equilibrium conditions suggests that entirely new emergent states with new and unexpected functionalities might be created in this way. The route for creation of such states is not limited to symmetry-breaking phase transitions, and entirely novel mechanisms may come into play. I will discuss recent experimental findings of mesoscopically textured topologically protected metastable states created through density-driven non-equilibrium transitions. Combining ultrafast optical pulse excitation with scanning tunneling microscopy under various non-equilibrium conditions we find various intertwined electronic orders and metastable amorphous electronic states stabilized in real space by non-trivial topological defects. The relaxation of these states reveals a novel state of configurational quantum electronic matter that may find its uses in ultrafast memory devices.

Colloquium: Fluctuations of Energy-Relaxation Times in Superconducting Qubits

Superconducting qubits are an attractive platform for quantum computing since they have demonstrated high-fidelity quantum gates and extensibility to modest system sizes. Nonetheless, an outstanding challenge is stabilizing their energy-relaxation times, which can fluctuate unpredictably in frequency and time. Here, we use qubits as spectral and temporal probes of individual two-level-system defects to provide direct evidence that they are responsible for the largest fluctuations. This research lays the foundation for stabilizing qubit performance through calibration, design, and fabrication.

Colloquium: Non-hermitian topological photonics

The recent development of topological insulator states for photons1–3 has led to an exciting new field of “topological photonics,” which has as a central goal the development of extremely robust photonic devices4. Topological insulator physics has been studied extensively in condensed matter5 and cold atoms6. However, photonic systems offer the possibility of non-Hermitian effects through the engineering of optical gain and loss, which opens an entire new field of physics. In this talk, we will discuss our recent experimental work on non-Hermitian topological phenomena in coupled lattice systems, in particular the first demonstration of non-hermitian PT-symmetric topological interface state. References [1] Haldane, F. D. M. & Raghu, S., Phys. Rev. Lett. 100, 013904 (2008) [2] Wang, Z. et al., Nature 461, 772–775 (2009) [3] Rechtsman, M. C. et al., Nature 496, 196–200 (2013) [4] Hafezi et al., Nature Physics 7, 907-912 (2011) [5] Hasan, M. Z. & Kane, C. L., Rev. Mod. Phys. 82, 3045–3067 (2010) [6] Jotzu, G. et al., Nature 515, 237–240 (2014)

Colloquium: Impurity states in superconductors and Dirac Materials

Impurity engineered states are the key component of modern electronics. We expect impurity engineered states will continue to be an important ingredient in quantum technologies beyond Si. New platforms that take advantage of impurity engineered bands such as superconductors and Dirac Materials are emerging. In this talk I will review the status and future perspectives of the field of impurity bound states in superconductors and will discuss striking similarities and important differences between defect states in superconductors and Dirac materials. I will highlight the recent progress made in our understanding of Yu-Shiba-Rusinov states in superconductors and topological states and illustrate the proposed induction of novel odd frequency Berezinskii pairing by YSR impurities. I will also discuss the universal nature of impurity resonances in unconventional SC and Dirac Materials and the implications these resonances have on functional properties of doped Dirac materials. Work supported by ERC, KAW and Villum Center for Dirac Materials.

Colloquium: Phase transitions and the principle of detailed balance in living systems

The mechanics of cells and tissues are largely governed by scaffolds of filamentous proteins that make up the cytoskeleton, as well as extracellular matrices. Evidence is emerging that such networks can exhibit rich mechanical phase behavior. A classic example of a mechanical phase transition was identified by Maxwell for macroscopic engineering structures: networks of struts or springs exhibit a continuous, second-order phase transition at the isostatic point, where the number of constraints imposed by connectivity just equals the number of mechanical degrees of freedom. We will present recent theoretical predictions and experimental evidence for mechanical phase transitions in both synthetic and biopolymer networks. Living systems typically operate far from thermodynamic equilibrium, which affects both their dynamics and mechanical response. As a result of enzymatic activity at the molecular scale, living systems characteristically violate detailed balance, a fundamental principle of equilibrium statistical mechanics. We discuss fundamental non-equilibrium signatures of living systems, including violations of detailed balance at the meso-scale of whole cells.

Colloquium: Interplay between superconductivity and non-Fermi liquid above a quantum critical point in a metal

I discuss the interplay between non-Fermi liquid behavior and superconductivity near a quantum-critical point (QCP) in a metal. It is widely thought that the tendency towards superconductivity and towards non-Fermi liquid behavior compete with each other, and if the pairing interaction is reduced below a certain threshold, the system displays a naked non-Fermi liquid QC behavior. I show that the situation is more complex. First, there is a difference between spin-triple and spin-singlet superconductivity. For spin-triplet pairing, thermal fluctuations are crucial and make superconducting transition first order. For spin-singlet pairing, they are essentially irrelevant, and the transition is second order. Second, I show that for spin-singlet pairing, there are multiple solutions with the same gap symmetry. For all solutions, except one, Tc vanishes when the pairing interaction drops below the threshold. For the one special solution, Tc remains finite even when the pairing interaction is arbitrary small, despite that there is no Cooper logarithm. I argue that superconductivity between this Tc and a lower T, when other solutions appear, is special, as it is entirely induced by fermions with the first Matsubara frequency. I show that this has specific implications for the observable quantities, such as the density of states and the spectral function.

Colloquium: Many-body optical excitations in solid-state systems

Two dimensional materials provide new avenues for synthesizing compound quantum systems. Monolayers with vastly different electric, magnetic or optical properties can be combined in van der Waals heterostructures which ensure the emergence of new functionalities; arguably, the most spectacular example to date is the observation of strong correlations and low electron density superconductivity in Moire superlattices obtained by stacking two monolayers with a finite twist angle. Optically active monolayers such as molybdenum diselenide provide a different "twist" as they allow for investigation of nonequilibrium dynamics in van der Waals heterostructures by means of femtosecond pump-probe measurements. Moreover, interactions between electrons and the elementary optical excitations such as excitons or polaritons, provide an ideal platform for investigation of quantum impurity physics, with possibilities to probe both Fermi- and Bose-polaron physics as well as mixtures with tunable density of degenerate fermions and bosons. After introducing the framework we use to describe many-body optical excitations in van der Waals heterostructures, I will describe two recent developments in the field. The first experiment uses pump-probe measurements to demonstrate how exciton-electron interactions beyond the non-self-consistent T-matrix approximation lead to optical gain by stimulated cooling of exciton-polaron-polaritons. The second experiment shows that a tri-layer system, consisting of two semiconducting monolayers separated by an insulating layer, could lead to hybridization of intra- and inter-layer excitons. The latter has potential applications ranging from strongly interacting polaritons to reaching Feshbach resonance condition in exciton-electron scattering.

Colloquium: Axial fields in phonic crystals

The elastic properties of materials are determined by a few material constants such as the Young’s modulus. Using super-structures one can effectively change these “constants”. In this way we obtain functionalities such as wave-guiding, acoustic lensing or programmable failure. I will show how topological band theory, known from the description of electrons in solids, provides us with a powerful design-principle for such mechanical metamaterials. Moreover, mechanical metamaterials offer a powerful platform for the study of fundamentally new phenomena that are hard to observe in other arenas. Here, I will highlight the first measurement of a quadrupole topological insulator in a silicon based metamaterial and the implementation of an axial gauge field in an acoustic Weyl system.

Colloquium: Quantum Memories

In this talk I introduce a phenomenon of enhancement of quantum memory storage capacity that is taking place in a wide class of systems with high occupancy of cold bosons at criticality. The S-matrix formulation shows that black holes are prominent members of this category. From particle physicists perspective a black hole is describable as a critical state of maximal memory storage capacity of soft gravitons at an extremely high occupation number. The same is true about a de Sitter type Universe. Both systems carry a maximal amount of quantum information that is protected against the standard semi-classical evolution. This leads to some important consequences. In particular, the primordial quantum memory pattern carried by the de Sitter Hubble patch from which our Universe evolved as a result of cosmic inflation was not erased by the latter process and is of observational importance. This opens up a conceptually new opportunity of catching the glimpses of the Universes's primordial quantum hair. The universal nature of the enhanced memory phenomenon allows to study and simulate black hole and de Sitter type quantum information storage and processing in laboratory systems with cold bosons.

Colloquium: Programming Shape

Shape constrains and enables function across scales. But how can we design and control shape, i.e. solve the basic inverse problems in physical geometry. I will discuss our work in this area that focuses on 3 different paradigms: (i) using origami to fold flat sheets into curved surfaces, (ii) using kirigami to reshape planar domains and tile arbitrary surfaces, (iii) using biomimetic 3D and 4D printing combined with designer metrics that allow us to “grow” a flat surface into a complex 3D shape such as a flower or a face.

Colloquium: The formalism of open quantum systems and its application to quantum metrology

The description of open quantum systems is amenable to a broad range of theoretical tools. Those include the derivation of different types of reduced master equations from specific microscopic modes and under the validity of physically motivated assumptions. A conceptually different approach relies on the formulation of phenomenological master equations, which in the more abstract formalism yield to the description of open system dynamics in terms of quantum channels. Using quantum metrology as a thread, I will illustrate the interrelation and complementarity of different formalisms to describe open quantum systems in order to provide a comprehensive approach to determining fundamental limits to the ultimate achievable precision in metrology and sensing.

Colloquium: Biological photonics

The study of structural colour in brightly coloured animals is an exciting interdisciplinary area of research. Complex photonic bandgap (PBG) structures that occur naturally cross a broad range of animals and plants, suggest broad innovation both in nature’s use of materials and in its manipulation of light and colour. In certain butterflies for instance, ultra-long-range visibility of up to one half-mile is attributed to photonic structures that are formed by discrete multilayers of cuticle and air. This contrasts, in other butterfly species, to photonic structures designed more for crypsis and which not only produce strong polarisation effects but can also create additive colour mixing using highly adapted periodicity. Optical systems also exist that employ remarkable 2D and 3D photonic crystals of cuticle to produce partial PBGs, with the effect that bright colour is reflected, or fluorescence emission is inhibited, over specific angle ranges. From the perspective of modern optical technology, these structures arguably indicate a significant functional advance, since in principle, such 2D and 3D periodicities are potentially able to manipulate the flow of light more completely. This presentation will offer an overview, for a more general audience, of this emerging field of study, as well as describing several of the exciting recent discoveries that reflect nature’s optical design ingenuity, and some technological applications to which they are currently being applied.

Colloquium: Quantum Atom Optics: Ocean & Universality in Quantum Dynamics

The experimental platform of atoms manipulated by light offers answers to a broad spectrum of open questions. With two explicit and very different examples I will give you a glimpse how broad this spectrum is. I will start with a fundamental question in oceanography: At what time has the deep water in the ocean been in exchange with the atmosphere? Quantum atom optics offers the experimental possibility to detect the very rare Argon 39 atoms one by one and with that allows the dating of water samples as small as ten liters [1]. A very different question in physics is about the existence of universal behavior. Specifically in respect to time dynamics this has only recently been discussed theoretically in the context of the early phase of a heavy ion collision. 'Universal' means in this context, that the evolution does not depend on the initial condition and follows the scaling hypothesis in time and space. I will introduce the concept and present the first observation of this phenomenon in highly controlled ultracold Bose gases [2]. [1] Ar-39 dating with small samples provides new key constraints on ocean ventilation, Nature Comm. 9, 5046 (2018). [2] Observation of universal dynamics in a spinor Bose gas far from equilibrium, Nature 563, 217 (2018).

Colloquium: Exploiting the Malleability of Disorder to Design Biologically-Inspired Function

Physicists have been thinking for a century about ordered solids. But ordered systems can’t compete with what biological systems such as proteins, which aren’t ordered, can do. We’ve discovered that relaxing the constraint of order increases design flexibility enormously, so that systems can be tuned to perform complex biologically-inspired tasks. Allostery in a protein is a phenomenon in which a molecule binding locally to one site affects the ability of another molecule to bind at a second distant site. Inspired by the long-range coupled conformational changes that constitute allosteric function in proteins, we tune in “allostery” into disordered mechanical networks derived from granular packings by modifying the network architecture to control the local strain at one point in such a network in response to a strain applied elsewhere in the system. In another biological example, the vascular network in the brain contracts and dilates blood vessels in order to direct enhanced blood flow to multiple specific regions of the brain as it performs multiple tasks at once. We have designed flow networks to accomplish similar complex tasks. In both examples, we address a key question: what, precisely, are we tuning into the structure in order to accomplish the function? Surprisingly, we find that the structure/function relationship is not geometrical, but topological in nature.

Colloquium: Topological Magnetic Order far from Equilibrium

Topological spin textures in chiral magnets such as skyrmions attract great interest as a possible route towards spintronics devices. Central to the suitability for applications are the mechanisms controlling the long-term stability and decay processes. Starting from the spectrum of low-lying excitations the effects of kinetic arrest and supercooling will be compared with strongly driven non-equilibrium dynamics. These identify topological magnetic order far from equilibrium as an exciting approach for the exploration of the fundamentals of topological protection and in the search for dynamically stabilized electronic phases.

Colloquium: Light-matter-life interactions at the micron scale

In 1676, using candle light for illumination and a small glass sphere as the lens, Antonie van Leeuwenhoek revealed to the world the beauty and complexity of a microscopic universe densely populated by living microorganisms. Today, using lasers, spatial light modulators, digital cameras and computers, we investigate the statistical and fluid mechanics of microswimmers in ways that were unimaginable only 50 years ago. With light we can image bacteria as they swim rapidly through 3D environments, apply gentle and controllable force fields or sculpt the 3D structure of their environment. In addition to shaping the physical world outside cells we can also use light to control the internal biological state of microorganisms that are genetically engineered to respond to light signals. I will review our recent work on the interactions between light, matter and bacteria, from the fundamental problems of rectification and non-equilibrium steady states in active matter to the use of genetically modified bacteria as propellers for micro-machines or as a "living" paint that can be controlled by light.

Colloquium: Quantum Materials: Full of Frustration and Correlations

The pioneering work on geometrical frustration dates back to the 1920s, when Linus Pauling realized that the hydrogen bonds between H$_2$O molecules in ice can be allocated in multiple ways. A given oxygen atom in water ice is situated at the vertex of a diamond lattice and has four nearest-neighbor oxygen atoms, each connected via an intermediate proton according to the ice rule “two-in two-out”. Although these considerations used electric dipoles, Phil Anderson mapped them to a spin model possessing an extensive degeneracy of states. Quantum spin liquids attract great interest due to their exceptional magnetic properties characterized by the absence of long-range order down to low temperatures despite the strong magnetic interaction. Commonly these compounds are strongly correlated electrons systems, and their electrodynamic response is governed by the Mott gap in the excitation spectrum. Here we will summarize and discuss the optical properties of several two-dimensional quantum spin liquid candidates with different degrees of effective correlations. Placing organic molecules on a triangular lattice, a spin liquid ground state can be realized which allows us to investigate the genuine Mott state in the absence of magnetic order. Combining our optical data with pressure-dependent transport studies and theoretical calculations, we can construct a universal phase diagram of the correlation-controlled Mott insulator. But how important is the coupling of the fluctuating magnetic moments? How important is disorder for the electronic properties? If this resembles a quantum phase transition, is there a superconducting phase found in the vicinity and what is the superconducting glue? Can our findings be generalized, when going to a kagome or hexagonal lattice, realized in Herbert-smithites or $\alpha$-RuCl$_3$ for instance? Reference: A. Pustogow et al., Nature Materials 17, 773 (2018); Phys. Rev. Lett. 121, 056402 (2018). M. Dressel et al., J. Phys. Cond. Matter 30, 203001 (2018)

Colloquium: Critical and incoherent electron systems and superconductivity within the Sachdev-Ye-Kitaev approach

The established and highly successful description of metals is based on Landau’s Fermi liquid theory. The relevant phase space for electron-electron collisions is determined by the Pauli blocking of a degenerate Fermi gas due to its Fermi surface. In some strongly correlated systems narrow bands form and the energetics of the system at elevated energies or temperatures becomes dominated by strong interactions and is no longer restricted by Fermi-surface phase-space effects. To develop a well-controlled approach of this regime is an important question in the theory of strongly correlated electrons. In this talk I will give an overview over the description of incoherent and critical electronic systems using the Sachdev-Ye-Kitaev approach. We consider versions of the model where electrons interact with each other, with boson collective modes, and even with phonons. We also comment on the very direct relation to holographic approaches to strongly coupled quantum theories. Finally we address the emergence of superconductivity in such an incoherent metal and show that pairing and superconductivity of a fully incoherent electronic system is allowed and leads to a pairing state with high transition temperature but low condensation energy.

Colloquium: From single-cell variability and correlations across lineages to the population growth

Genetically identical microbial cells often display diverse phenotypes. Stochasticity at the single-cell level contributes significantly to this phenotypic variability, and cells utilize a variety of mechanisms to regulate noise. In turn, these control mechanisms lead to correlations in various cellular traits across the lineage tree. I will present recent models we developed for understanding cellular homeostasis, with special focus on protein levels and cell size. These models allow us to characterize single-cell variability, including the emerging correlations and distributions. I will discuss the implications of stochasticity on the population growth. In contrast to the dogma, we find that variability may be detrimental to the population growth, suggesting that evolution would tend to suppress it.

Special dynamics of water pearls

A few tricks (hydrophobic texture, Leidenfrost state, etc.) allow us to keep water drops with a spherical shape, which induces spectacular properties. We describe a few of them. We also challenge the water mobility in the cases where it is expected to be impeded - for instance by reducing the drop size or by increasing the water temperature.

Colloquium: New Phases of Quantum Magnets in and out of Equilibrium

In quantum magnets spins form well-defined lattices and serve as model systems to study many-body quantum phases such as interacting quantum-dimer qubits, spin Luttinger-liquids, Bose glasses, or magnon Bose-Einstein condensates. Neutron, muon and photon sources are unique tools for high-precision studies of such phases, and of their correlations and excitations in as well as out of equilibrium. An overview of current frontiers in the field will be presented with special focus on recent developments in computational physics and exciting new opportunities that free electron lasers like SwissFEL and European XFEL will offer to study the time-dependence and out-of-equilibrium quantum mechanics of such systems.

Colloquium: Topology and scale invariance in atomic Flatland

In a 2D world, most transitions towards ordered states of matter like crystals or magnets would not occur because of the increased role of fluctuations. However, non-conventional topological transitions can still occur, as understood by Kosterlitz and Thouless (2016 Nobel prize). In this talk I will present some important features of Flatland physics explored with cold atomic gases, and connect them with other prominent topological properties of matter, such as quantum-Hall type phenomena.

Colloquium: Using Machine Learning for Prediction of Large, Complex, Spatially Extended Systems

We first review the basic idea of using machine learning in conjunction with a limited duration of time series data to construct a closed-loop, autonomous, dynamical system that can predict the future evolution of the state of the unknown system that generated the data [1]. Using the reservoir computing type of machine learning, we then present examples of extensions and applications of this idea. These will include a parallel implementation enabling forecasting of the states of very large spatiotemporally chaotic systems with local interactions [2], a hybrid scheme where an imperfect knowledge-based model component is combined with a limited-size machine learning component to achieve prediction performance much better than that of either of the components acting alone [3], an architecture combining the parallel and hybrid schemes, and generalization of ensemble Kalman filtering to cyclic prediction using the parallel/hybrid machine learning schemes. As an example of the potential utility of these elements for large complex spatiotemporally chaotic systems, their use in our ongoing project on improving weather forecasting [4] will be outlined. [1] Jaeger, Haas, Science (2004). [2] Pathak, Hunt, Girvan, Lu, Ott, Phys Rev Lett (2018). [3] Pathak, Wikner, Fussell, Chandra, Hunt, Girvan. Ott, CHAOS (2018). [4] Collaborators on our current weather forecasting project: T. Acomano, M. Girvan, B. Hunt, G. Katz, J. Reggia, I. Szunyogh, C.-Y. Wang, A. Wikner.

Colloquium: Fluid flows shaping morphology

Fluid flows can induce long-ranged interactions and propagate information on large scales. Especially during the development of an organism, coordination on large scales is essential. What are the principal mechanisms of how fluid flows induce, transmit and respond to biological signals and thus control morphology? Fluid flows are particularly prominent during the growth and adaptation of transport networks. Here, the network-forming slime mold Physarum polycephalum emerged as a model system. Investigating the pivotal role of fluid flows in this live transport network we find that flows are patterned in a peristaltic wave across the network thereby optimizing transport. In fact, flows are hijacked by signals to propagate throughout the network. This simple mechanism is sufficient to explain surprisingly complex dynamics of the organism like scaling of peristaltic wave with network size and finding the shortest path through a maze.

Colloquium: Making Quantum Matter from Light

In this talk I will discuss ongoing efforts at UChicago to explore matter made of light. I will begin with a broad introduction to the challenges associated with making matter from photons, focusing specifically on (1) how to trap photons and imbue them with mass and charge; (2) how to induce photons to collide with one another; and (3) how to drive photons to order, by cooling or otherwise. I will then provide as examples two state-of-the-art photonic quantum matter platforms: microwave photons coupled to superconducting resonators and transmon qubits, and optical photons trapped in multimode optical cavities and made to interact through Rydberg-dressing. In each case I will describe a synthetic material created in that platform: a Mott insulator of microwave photons, stabilized by coupling to an engineered, non-Markovian reservoir, and a Laughlin molecule of optical photons prepared by scattering photons through the optical cavity. Indeed, building materials photon-by-photon will provide us with a unique opportunity to learn what all of the above words mean, and why they are important for quantum-materials science. Finally, I will conclude with my view of the broad prospects of photonic matter in particular, and of synthetic matter more generally.

Colloquium: Topological Physics in HgTe based Quantum Devices

Topological insulators are a novel class of materials that exhibit a novel state of matter – while the inside (bulk) of the materials is electrically insulating, their surface is metallic. This effect occurs because the band structure of the materials is topologically different (in a mathematical sense) from the outside world. This talk describes our discovery of this type of behavior while studying the charge transport properties of thin, two-dimensional layers of the narrow-gap semiconductor HgTe. These layers exhibit the quantum spin Hall effect, a quantized conductance which occurs when the bulk of the material is insulating. Using various tricks one can show that the transport occurs along one-dimensional, spin-polarized channels at the edges of the sample. Also thicker HgTe samples can be turned into topological insulators, but now the surface states are two-dimensional metallic sheets. The metal in these sheets is rather exotic in that the band structure is similar to that encountered for elementary particles – the charge is carried by so-called Dirac fermions. This means that experiments on these layers can be used to test certain predictions from particle theory that are difficult to access otherwise. As an example, I will describe experiments where a supercurrent is induced in the surface states by contacting these structures with Nb electrodes. AC investigations indicate that the induced superconductivity is strongly influenced by the Dirac nature of the surface states. We present strong evidence for the presence of a gapless Andreev mode in our junctions. Finally, by playing with the strain in the layers, we can turn HgTe into a Dirac semimetal, which exhibits the ‘axial anomaly’ known from particle physics when the Fermi level is tuned to the Dirac points.

Colloquium: The long way to steady state fusion plasmas - the superconducting stellarator device Wendelstein 7-X

The stable generation of high temperature Hydrogen plasmas (ion and electron temperature in the range 10-20 keV) is the basis for the use of nuclear fusion to generate heat and thereby electric power. The most promising path is to use strong, toroidal, twisted magnetic fields to confine the electrically charged plasma particles in order to avoid heat losses to the cold, solid wall elements. Two magnetic confinement concepts have been proven to be most suitable: (a) the tokamak and (b) the stellarator. The stellarator creates the magnetic field by external coils only, the tokamak by combining the externally created field with the magnetic field generated by a strong current in the plasma. “Wendelstein 7-X” is the name of a large superconducting stellarator that goes into operation after 15 years of construction. With 30 $m^3$ plasma volume, 3 T magnetic field on axis, and 10 MW micro wave heating power, Hydrogen plasmas are generated that allow one to establish a scientific basis for the extrapolation to a future fusion power plant. It is a unique feature of Wendelstein 7-X to be able to operate high-power Hydrogen plasmas under steady-state conditions, more specifically for 1800 s (note that the world standard is now in the 10 s ballpark). This talk provides a review of the principles of nuclear fusion and discusses the key physics subjects of optimized stellarators. The sometimes adventurous undertaking to construct such a first-of-a-kind device is summarized as well as the most important findings during the first operation phase of Wendelstein 7-X. We finish with an outlook towards the fusion power station and address the most important remaining issues to be addressed in the framework of the world-wide fusion research endeavor.

Gutzwiller Colloquium: Two-dimensional melting: New algorithms, new insights

The hard-disk model has exerted outstanding influence on computational physics and statistical mechanics. Decades ago, hard disks were the first system to be studied by reversible Markov-chain Monte Carlo methods satisfying the detailed-balance condition and by molecular dynamics. It was in hard disks, through numerical simulations, that a two-dimensional melting transition was first seen to occur even though homogeneous short-range interacting particle systems cannot develop crystalline order. Analysis of the system was made difficult by the absence of powerful simulation methods. In recent years, we have developed a class of irreversible event-chain Monte Carlo algorithms that violate detailed balance. They realize thermodynamic equilibrium as a steady state with non-vanishing probability flows. A new factorized Metropolis filter turns them into a paradigm for general Monte Carlo calculations. I will in particular show how the event-chain Monte Carlo algorithm has allowed us to demonstrate that hard disks melt with a first-order transition from the liquid to the hexatic and a continuous transition from the hexatic to the solid. Event-chain computations have also lead to our new understanding of two-dimensional melting for soft disks, that has been intensely studied in experiment. Finally, I will discuss two-dimensional melting on a substrate (as it is realized in skyrmion systems), and for active particles, and will present a very recent application of the event-chain algorithm to Coulomb-type long-range-interacting systems.

Colloquium: Chern numbers counted in a synthetic-dimension quantum Hall strip

Ultra cold atoms are remarkable systems with a truly unprecedented level of experimental control and one application of this control is creating topological band structures. The most natural approach centers on creating suitable real-space lattice potentials that the atoms experience. Here we present our experimental work which uses the internal atomic states as an additional ``synthetic'' dimension. We engineered a two-dimensional magnetic lattice in an elongated strip geometry, with effective per-plaquette flux about 4/3 times the flux quanta. The long direction of this strip is formed from a 1D optical lattice while the short direction is built from the 5 mF states comprising the f=2 ground state hyperfine manifold of Rb-87. We imaged the localized edge and bulk states of atomic Bose-Einstein condensates in this strip, with single lattice-site resolution along the narrow direction. In this 5-site wide strip we are able to delineate between bulk behavior quantified by Chern numbers and edge behavior which is not.

Colloquium: The Quantum Way of Doing Computations

Since the mid-nineties of the 20th century, it became apparent that one of the centuries’ most important technological inventions, computers in general and many of their applications could possibly be further enhanced by using operations based on quantum physics. This is timely since the classical roadmap for the development of computational devices, commonly known as Moore’s law, will cease to be applicable within the next decade. This is due to the ever-smaller sizes of the electronic components that will enter the realm of quantum physics. Computations, whether they happen in our heads or with any computational device, always rely on real physical devices and processes. Data input, data representation in a memory, data manipulation using algorithms and finally, data output require physical realizations with devices and practical procedures. Building a quantum computer then requires the implementation of quantum bits (qubits) as storage sites for quantum information, quantum registers and quantum gates for data handling and processing as well as the development of quantum algorithms. In this talk, the basic functional principle of a quantum computer will be reviewed. It will be shown how strings of trapped ions can be used to build a quantum information processor and how basic computations can be performed using quantum techniques. In particular, the quantum way of doing computations will be illustrated with analog and digital quantum simulations, which range from the simulation of quantum many-body spin systems over open quantum systems to the quantum simulation of a lattice gauge theory.

Colloquium: Statistical Mechanics, Localization and Periodically Driven Quantum Systems

The statistical mechanics of equilibrium systems is characterized by two fundamental ideas: that closed systems approach a late time thermal state and that of phase structure wherein such late time states exhibit singular changes as various parameters characterizing the system are changed. Recent progress has established generalizations of these ideas which apply to periodically driven, or Floquet, closed quantum systems. I will describe this progress, which centrally uses other recent advances in our understanding of many body localization. I will describe how it has resulted in the discovery of entirely new phases such as the Pi-spin glass/Floquet time crystal which exist only in driven quantum systems.

Colloquium: Lasing to Photon Bose-Einstein-Condensation Crossover: Open versus Closed System Dynamics

Bose-Einstein condensation has been observed with cold atomic gases, exciton-polaritons, and more recently with photons in a dye-solution filled optical microcavity. I will here describe measurements of my Bonn group observing the transition between usual lasing dynamics and photon Bose-Einstein condensation. The photon Bose-Einstein condensate is generated in a wavelength-sized optical cavity, where the small mirror spacing imprints a low-frequency cutoff and photons confined in the resonator thermalize to room temperature by absorption re-emission processes on the dye molecules. This allows for a particle-number conserving thermalization, with photons showing a thermodynamic phase transition to a macroscopically occupied ground state, the Bose-Einstein condensate. When the thermalization by absorption and re-emission is faster than the photon loss rate in the cavity, the photons accumulate at lower energy states above the cavity cutoff, and the system finally thermalizes to a Bose-Einstein condensate of photons. On the other hand, for a small reabsorption with respect to the photon loss, the state remains laser-like. I will also report recent measurements of the heat capacity of the photon gas, which were performed under the conditions of the thermalization being much faster than both photon loss and pumping. At the Bose-Einstein phase transition, the observed specific heat shows a cusp-like singularity, as in the $\lambda$-transition of liquid helium, illustrating critical behavior of the photon gas. In my talk, I will begin with a general introduction and give an account of current work and future plans of the Bonn photon gas experiment.

Colloquium: Engineering and Visualizing Majoranas in Chains and Hinges

A variety of one-dimensional electronic systems can be engineered to host topological superconductivity and Majorana zero modes. In this talk, I’ll describe experiments on two different one-dimensional platforms. One is based on chains of magnetic atoms the work on which grew out of early efforts on the study of localized Bogoliubov quasi-particles near individual magnetic atoms on a superconductors. We have now performed a series of experiments to establish the presence of Majorana’s in this system, including a recent study of their spin polarization. A second system is based on introducing superconductivity on the hinge states of a high order topological insulator. Using combination of magnetism and superconductivity, we are exploring how Majorana zero mode can emerge in this system.

Colloquium: Molecular motors interacting with actin filaments

Actin filaments are the essential components of the cytoskeleton that provide the elasticity of a cell. In a cell they interact with many proteins and in particular molecular motors. This talk will present 3 biological situations where actin filaments interact with molecular motors in relation with important cellular functions : the control of actin polymerization, intracellular transport and cell migration. Actin filaments are treadmilling : they grow at one end and depolymerize at the other end. The first example of interaction between actin and molecular motors deals with the induced depolymerization of actin filaments by myosin1b molecular motors studied in motility assays. The depolymerization requires a catch bond motor with a detachment rate decreasing strongly when the motor is under force Molecular motors navigate the cytoskeleton to position vesicles and organelles at specific locations in the cell. In order to understand this transport process, the group of Pascal Martin at Institut Curie has used an antiparallel network of overlapping filaments. Beads coated with myosin motors accumulate at the midline of the pattern. The accumulation is well described by a three-state model of bead transport, in which active beads locally sense the net polarity of the filament network by frequently detaching from and reattaching to the filaments. The migration of immune cells is guided by several chemical signals, but also by physical cues such as the hydraulic resistance of the vessels in which they travel. This barotaxis effect has been studied in vitro by the groups of M. Piel and A.M. Lenon using microfluidic channels. We show that barotaxis results from a force imbalance at the scale of the cell, amplified at the scale of a network of vessels.

Colloquium: Transitions and transients in complex spatio-temporal dynamics of the heart

Electro-mechanical excitation waves in the heart may exhibit different spatio-temporal dynamics ranging from repeated plane waves to scroll waves or spatio-temporal chaos, resulting in life threatening arrhythmias. This kind of chaotic dynamics in excitable cardiac media is often characterised by significant complexity fluctuations (including laminar phases) and can be non-persistent exhibiting supertransients, with lifetimes of the chaotic phases increasing exponentially with the system size. Terminating or at least shortening chaotic transients can be life saving in the medical context of cardiac arrhythmias. Therefore, we study the impact of perturbations on the duration of transients and features of the terminal phase of chaotic transients. Practically, such perturbations can be applied via so-called virtual electrodes where electrical heterogeneities of the cardiac muscle act as local excitation sites when subjected to a global electric field. In the talk we shall present novel results on the nonlinear dynamics of the heart including features of the terminal phase of transient chaos, parameter and state estimation, as well as experimental studies and modalities.

Colloquium: Stochastic thermodynamics: From principles to the cost of precision

For the macroscopic world, classical thermodynamics formulates the laws governing the transformation of various forms of energy into each other. Stochastic thermodynamics extends these concepts to micro- and nano-systems embedded or coupled to a heat bath where fluctuations play a dominant role. Examples are colloidal particles in time-dependent laser traps, single biomolecules manipulated by optical tweezers or AFM tips, and transport through quantum dots. For these systems, exact non-equilibrium relations like the Jarzynski relation, fluctuation theorems and, most recently, a thermodynamic uncertainty relation have been discovered. First, I will introduce the main principles and show a few representative experimental applications. In the second part, I will discuss the universal trade-off between the thermodynamic cost and the precision of any biomolecular, or, more generally, of any stationary non-equilibrium process. By applying this thermodynamic uncertainty relation to molecular motors, I will introduce the emerging field of "thermodynamic inference" where relations from stochastic thermodynamics are used to infer otherwise yet inaccessible properties of (bio)physical and (bio)chemical systems.

Colloquium: Hybrid active matter: when particles and living cells play together

We introduce the broad field of active matter, a novel class of non-equilibrium materials composed of many interacting units that individually consume energy and collectively generate motion or mechanical stresses. Unlike swarms of fish and flocks of birds, cells or ants can support static loads because cells are bound by transient links. This leads to the concept of “entangled active matter” , which emerged recently to provide a unified understanding of the behavior of swarms of adhesive particles ranging from vibrating staples, to ants, and to cellular aggregates [1]. Recently we have focused on cellular aggregate – nanoparticles hybrid systems. This system is of great interest as it has been previously shown that particles can modify the mechanical properties of cells in terms of adhesion area, proliferation and motility. We study both small particles (up to few microns), which are digested by cells by endocytosis and phagocytosis, and larger particles, which do not enter in the cells, leading to a completely different physics. Nanostickers (size 20 nm ) [2] We show that nanoparticles, within a limited size range, can be used as a glue “nanostickers”to enable the formation of self-assembled aggregates by promoting cell–cell interactions. We model the cell-cell adhesion induced by the nanostickers using a three states dynamical model where the NPs are free, adsorbed on the membrane or internalized by endocytosis. We find that carboxylated polystyrene NPs are more efficient than the silica NPs of the same size, which were reported to induce fast wound healing and to glue soft tissue by Leibler et al. Nanostickers by increasing the cohesion of tissues and tumors may have important applications for cellular therapy and cancer treatment. Microparticles ( size 1micron): gluttonous cells[3]. We study the spreading of cell aggregates deposited on adhesive substrates decorated with microparticles. A cell monolayer expands around the aggregate. The cells at the periphery uptake the microparticles by phagocytosis, clearing the substrate and forming an aureole of cells full of particles. We study the dynamics of spreading, the width of the aureole, and the level of cell internalization as a function of the size, nature and density of the beads. The radius and width of the aureole allow quantification of the MP volume fraction incorporated by the cell, leading to an easy, fast, and inexpensive cell – particle internalization measurement. Macroparticles ( size 2-20 microns) ”Activated Brownian motion” Performing the same experiment with particles which are too big to be eaten, we observed that they are put into motion by the cells.We also describe also the mechanical properties of hybrid particles-cells aggregates (surface tension, elastic modulus, viscosity) using pipette aspiration technique [4], the phase separation between dead and living matter and how they spread . [1] F Brochard-Wyart et al, Entangled Active Matter: from ants to living cells EPJE 2015 [2] B.Brunel et al Nanostickers for cells: a model study using cell–nanoparticle hybrid aggregates Soft Matter 12 (38), 7902-7907. 2015 [3] Grégory Beaune^1 et al *5,6How gluttonous cell aggregates clear substrates coated with microparticles, Scientific Reports 7, Article number: 15729 (Nov 2017).[4] David Gonzalez-Rodriguez et al. Soft Matter Models of developing Tissues and Tumors. Science 338, 910; 1226418 (2012)

Colloquium: Weather and climate: Deterministic or stochastic?

The atmosphere, ocean, and other components of the climate system behave in a fairly irregular way. A major surprise of the late 20th century was the realization that such behavior could be produced by natural systems with a small number of degrees of freedom, governed by fully deterministic, but nonlinear laws. Still, the climate system has a large number of degrees of freedom, and ample room for random factors to intervene. Can we reconcile a low-order, deterministically nonlinear description of weather or climate with a high-order, possibly linear but random one? This talk will present some steps on the road to such a "grand unification," and implications for climate variability, sensitivity and predictability will be discussed. References: 1. Ghil, M., 2002: Natural climate variability, in Encyclopedia of Global Environmental Change, T. Munn (Ed.), Vol. 1, J. Wiley & Sons, Chichester/New York, pp. 544–549. 2. Ghil, M., 2014: Climate variability: Nonlinear and random aspects, in Encyclopedia of Atmospheric Sciences, 2nd edn., G. R. North, J. Pyle and F. Zhang (Eds.), Elsevier, vol. 2, pp. 38–46. 3. Ghil, M., 2017: The wind-driven ocean circulation: Applying dynamical systems theory to a climate problem, Discr. Cont. Dyn. Syst. – A, 37(1), 189–228, doi:10.3934/dcds.2017008.

Colloquium: Tensor Network Machine Learning Models

Tensor networks are an efficient representation of interesting many-body wavefunctions and underpin powerful algorithms for strongly correlated systems. But tensor networks could be applied much more broadly than just for representing wavefunctions. Large tensors similar to wavefunctions appear naturally in certain families of models studied extensively in machine learning. Decomposing the model parameters as a tensor network leads to interesting algorithms for training models on real-world data which scale better than existing approaches. In addition to training models directly for recognizing labeled data, tensor network real-space renormalization approaches can be used to extract statistically significant "features" for subsequent learning tasks. I will also highlight other benefits of the tensor network approach such as the flexibility to blend different approaches and to interpret trained models.

Colloquium: Neural-Network Quantum States: from Condensed Matter to Quantum Computing

Once Upon a Time in Kamchatka - The Extraordinary Search for Natural Quasicrystals

Abstract: Quasicrystals are exotic materials with symmetries that were once thought to be impossible for matter. The first known examples were synthesized in the laboratory over 35 years ago, but could Nature have beaten us to the punch? This talk will describe the search that took over a dozen years to answer this question, resulting in one of the strangest scientific stories you are ever likely to hear.

Colloquium: Dark matter challenges of the solid state

At the turn of the 20th century, physicists faced an uncanny range of unsolved problems: simple questions, such as why hot objects change color, why matter is hard and why the sun keeps on shining, went unanswered. These problems heralded a new era of quantum physics. What was truly remarkable about discovery in this heroic era, was the intertwined nature of research in the lab and in the cosmos: solving superconductivity really did help answer why the sun keeps on shining, while looking at the stars provided clues as to why matter is hard. The challenges facing us today, epitomized by our failure to quantize gravity and the mysteries of dark matter and energy, are not just problems facing particle physics and astronomy, but problems that challenge physics to its core. What is perhaps less well known, is that physics in the lab and cosmos are just as intertwined today, as they were a hundred years ago. I will talk today about the less well-known dark matter challenges of the solid state, epitomized by the strange metals with linear resistivity that accompany high temperature superconductivity, the recent discovery of insulators with Fermi surfaces and quantum criticality - the solid-state version of a black hole in the phase diagram. The solution of these laboratory-scale problems fundamentally challenge our understanding of emergent quantum matter, and they are no less intertwined with their cosmological counterparts than they were a hundred years ago. I will highlight three Dark-Matter challenges that have arisen in heavy fermion physics[1-4], emphasizing their connections with other strongly correlated quantum materials and discussing some of our recent theoretical efforts to make progress on them: quantum criticality, hidden order and the possibility of new classes of broken symmetry outside the Hartree-Fock/BCS paradigm and topology [5]. [1] Piers Coleman, “Heavy Fermions and the Kondo Lattice, a 21st Century Perspective”, arXiv:1509.05769 (2015). [2] Joe Thompson and Zachary Fisk, “Progress in Heavy Fermion Superconductivity: Ce115 and other materials”, J. Phys. Soc. Jpn. 81, 011002 (2012). [3] Philipp Gegenwart, Qimiao Si and Frank Steglich, ”Quantum criticality in heavy-fermion metals”,Nature Physics 4, 186 - 197 (2008). [4] Maxim Dzero et al, “Topological Kondo Insulators”, arXiv 1506.05635, Ann. Rev. Cond. Matt. Phys., Volume 7:249-280 (2016). [5] B. S. Tan et. al, “Unconventional Fermi surface in an insulating state”, Science 349, 287-290 (2015).

Colloquium: Optical thermodynamics of highly multimode systems

The prospect of judiciously utilizing both optical gain and loss has been recently suggested as a means to control the flow of light. This proposition makes use of some newly developed concepts based on non-Hermiticity and parity-time (PT) symmetry-ideas first conceived within quantum field theories. By harnessing such notions, recent works indicate that novel synthetic structures and devices with counter-intuitive properties can be realized, potentially enabling new possibilities in the field of optics and integrated photonics. Non-Hermitian degeneracies, also known as exceptional points (EPs), have also emerged as a new paradigm for engineering the response of optical systems. In this talk, we provide an overview of recent developments in this newly emerging field. The use of other type symmetries in photonics will be also discussed.

Colloquium: The Brief But Fascinating History of Silicon Spin Chains

Thirty years ago, scientists first observed that when a small amount of gold is deposited on the surface of silicon, both the gold and silicon atoms automatically organize themselves into parallel linear rows, so-called "atom chains", with nearly perfect structural order. This observation marked the beginning of a new research direction in which theoretical predictions about "physics in one dimension" could now be investigated experimentally using the standard tools of surface science such as scanning tunneling microscopy, x-ray diffraction, and photoemission. A particularly striking discovery, first reported ten years ago, was that at low temperature the silicon chains can develop local magnetic moments, which form regular highly ordered, periodic patterns. These "silicon spin chains" have now become the main focus of this research field. This talk will describe three recent theoretical and experimental aspects of silicon spin chains: the possibility of magnetic ordering in these one-dimensional systems; the prospects for using surface chemistry to tailor spin chains by creating or destroying individual spins; and the properties and dynamics of solitons in the spin chains.

Colloquium: Thermodynamics of superconducting quantum circuits

Thermodynamics of superconducting quantum circuits Superconducting circuits provide a platform for stochastic thermodynamics experiments in both classical and quantum regimes (“circuit Quantum Thermodynamics”). I first review the ideas, principles and examples of classical experiments utilizing single electron charge as the stochastic variable. I present experiments over the past several years on classical fluctuation relations and Maxwell Demons (MD), the latter in form of both non-autonomous Szilard Engines and autonomous MDs. Recent highlights on entropy reduction and rare events will be reviewed. In the second part of the talk I focus on open quantum systems formed of superconducting qubits and resonators, coupled to heat baths. In this context microwave photons carry the heat between the system and bath. I present experiments on quantum heat transport mediated by a transmon qubit, progress on superconducting quantum heat engines and refrigerators, and on detecting single microwave photon quanta. Success in the last item would allow us to perform true stochastic thermodynamics experiments in the quantum regime, and to realize quantum MDs.

Colloquium: Probing many-body localization using ultracold atoms

A fundamental assumption in statistical physics is that generic closed quantum many-body systems thermalise under their own dynamics. Recently, the emergence of many-body localised (MBL) systems has questioned this concept, challenging our understanding of the connection between statistical physics and quantum mechanics. In my talk, I will report on several recent experiments carried out in our group on the observation of Many-Body Localisation in different scenarios, ranging from 1D fermionic quantum gas mixtures in driven and undriven Aubry-André type disorder potentials and 2D systems of interacting bosons in 2D random potentials. It is shown that the memory of the system on its initial non-equilibrium state can serve as a useful indicator for a non-ergodic, MBL phase. Furthermore, I will present new results on the slow relaxation dynamics in the ergodic phase below the MBL transition and experiments that explore the resilience of a 2D MBL phase when coupled to a finite thermal bath. Our experiments represent a demonstration and in-depth characterisation of many-body localisation, often in regimes not accessible with state-of-the-art simulations on classical computers.

Colloquium: Exploration in Reinforcement Learning

In Reinforcement Learning an agent learns how to act in an environment in order to achieve desired goals. In general, the agent only has access to partial information from the world, and has to learn how to act appropriately in face of this uncertainty. One of the earliest results in the theory of Optimal Control (Feldbaum 1965) demonstrates that in order to act appropriately, the agent must also probe the environment in order to gain information and reduce uncertainty. In other words, exploring the environment is an essential component of learning to act. However, very few provably effective procedures were proposed in the control literature to solve this problem. In recent years, several elegant solutions to the problem of exploration have been proposed in the Machine Learning literature for the case of finite state and action spaces (which was of little concern to the control theorists). However, the problem of effective exploration in continuous spaces is still very much open, and of increasing importance in many applications. We survey some recent approaches to exploration in continuous domains, in both model based and model free settings, and suggest open problems for future research.

Colloquium: On Growth and Form of Microorganisms on Liquid Substrates

The interplay between fluid flows and living organisms plays a major role in the competition and organization of microbial populations in liquid environments. Hydrodynamic transport leads to the dispersion, segregation or clustering of biological organisms in a wide variety of settings. To explore such questions, we have created microbial range expansions in a laboratory setting by inoculating two identical strains of S. cerevisiae (Baker’s yeast) with different fluorescent labels on a nutrient-rich fluid 10^4 to 10^5 times more viscous than water. The yeast metabolism generates intense flow in the underlying fluid substrate several times larger than the unperturbed colony expansion speed. These flows dramatically impact colony morphology and genetic demixing, triggering in some circumstances a fingering instability that allows these organism to spread across an entire Petri dish in roughly 24 hours. We argue that yeast colonies create fluid flow by consuming nutrients from the surrounding fluid, decreasing the fluid’s density, and ultimately triggering a baroclinic instability when the fluid’s pressure and density contours are no longer parallel. Our results suggest that microbial range expansions on viscous fluids will provide rich opportunities to study the interplay between advection and spatial population genetics.

Colloquium: Active field theories: Statistical physics and the arrow of life

Active matter comprises particles whose microscopic dynamics breaks time-reversal symmetry (TRS) via the continuous conversion of fuel into motion, resulting in entropy production at the microscopic scale. Examples include bacteria and synthetic self-propelled colloids. Equilibrium statistical physics concepts, for instance the Boltzmann distribution, are not applicable to active matter because these concepts assume the TRS of the underlying microscopic laws. When viewed at a coarse grained level, the microscopic absence of TRS may either remain obvious at large scales or become almost undetectable. In the latter case, we retain the hope that equilibrium concepts, at least in some modified form, may be applicable. This talk will discuss the conceptual toolbox needed to characterize the presence or absence of TRS at coarse-grained level, focusing on global and local measures of entropy production. Put differently: when, and how, can you tell whether life's movie is running backwards?

Colloquium: An effective molecular orbital approach to electron phonon and pairing interactions in skipped valence and negative charge transfer gap Oxides

In high oxidation state oxides like the trivalent Nickel oxides, tetravalent Co and Fe oxides as well as the parent superconductors BaBiO3 and SrBiO3 and High Tc hole doped cuprates, the cation electron affinity in the formal valence could end up larger than the O 2- ionization potential leading to a so called negative charge transfer gap. If the charge transfer energy is strongly negative, then we should really adopt starting electronic configurations such as Ni2+ rather than 3 + or Bi 3+ rather than 4+ with compensation holes in the O 2p valence band for charge neutrality. If in addition the lowest energy cation ionization states are strongly hybridized with the valence O 2p states the low energy scale electronic structure and be well described by a molecular orbital type of approach (1,2). This is a new approach to the Wannier function description (3) but with explicit inclusion of the O states which provides a natural path to inclusion of the electron phonon coupling, charge density wave formation, potential bipolaron formation and paring interactions in superconductors. We discuss recent developments in this approach and show that the effective electron phonon coupling involving these molecular like orbitals is much stronger than that estimated from density function approaches. We also show that this leads to Peierls like charge density wave like ground states and we describe how the electron phonon coupling involving the hopping integrals rather than the on-site energies evolves into a large effective attractive interaction between low energy scale electrons. I will also briefly describe how these effects lead to our coelution that the ion battery material LiNiO2 should be viewed as an “entropy-stabilized charge- and bond-disproportionated glass”.

Colloquium: Quantum Optomechanics: exploring mechanical motion in the quantum regime

The quantum optical control of solid-state mechanical devices, quantum optomechanics, has emerged as a new frontier of light-matter interactions. Devices currently under investigation cover a mass range of more than 17 orders of magnitude - from nanomechanical waveguides of some picograms to macroscopic, kilogram-weight mirrors of gravitational wave detectors. This development has been enabled by the insight that quantum optics provides a powerful toolbox to generate, manipulate and detect quantum states of mechanical motion, in particular by coupling the mechanics to an optical or microwave cavity field. Originally, such cavity optomechanical systems have been studied from the early 1970s on in the context of gravitational wave antennas. Advancements in micro-fabrication and micro-cavities, however, have resulted in the development of a completely new generation of nano- and micro-optomechanical devices. Today, 10 years after the first demonstrations of laser cooling of micromechanical resonators, the quantum regime of nano- and micromechanical motion is firmly established. Recent experimental achievements include the generation of genuinely non-classical states of micromechanical motion such as quantum squeezing and entanglement. This level of control over solid-state mechanical degrees of freedom is now also being utilized in diverse application domains ranging from classical sensing, to low-noise optical coatings for precision interferometry, and also to photon-phonon quantum interfaces. From the fundamental physics point of view, one of the fascinating prospects of quantum optomechanics is to coherently control the motional degree of freedom of a massive object in an unprecedented parameter regime of large mass and long coherence time, hence opening up a new avenue for macroscopic quantum experiments. The availability of quantum superposition states involving increasingly massive objects could enable a completely new class of experiments, in which the source mass character of the quantum system starts to play a role. This addresses directly one of the outstanding questions at the interface between quantum physics and gravity, namely “how does a quantum system gravitate?”.

Colloquium: Quantum optics using atomic arrays

Ensembles of atoms or other quantum emitters are envisioned to be an important component of quantum applications, ranging from quantum memories for light to photon-photon gates to metrology. It has historically been an outstanding challenge to exactly solve for the quantum dynamics of an optical field as it propagates through and interacts with an ensemble. The standard axiomatic approach is to use the one-dimensional Maxwell-Bloch equations, which treats the interaction between the ensemble and a quasi-1D optical mode of interest, while the interaction with the remaining 3D continuum of modes is assumed to result in independent spontaneous emission of excited atoms. Strictly speaking, this assumption cannot be correct, as the emission of light is a wave phenomenon, and thus the emitted intensity must depend on interference and correlations between the atoms. Here, we discuss an alternative theoretical approach, which accounts for interference and the precise atomic positions. In this formalism, an interacting quantum spin model describes the dynamics of the atomic internal degrees of freedom under multiple photon scattering, while the field properties can subsequently be re-constructed from the spin correlations. Using this model, we then show how interference can be exploited as an extremely powerful resource to suppress the unwanted emission of light and the subsequent loss of information into undesirable directions. The effects of interference are particularly prominent in ordered arrays of emitters. As two specific examples, we construct a new protocol for a quantum memory for light based upon an ordered array, whose error rate as a function of system resources scales exponentially better than previously known bounds. We also show the interrogation time in an optical lattice clock can be significantly extended, through the excitation of collective subradiant atomic states whose spontaneous emission rates are strongly suppressed. These results raise the intriguing question of whether interference can be used to broadly re-define the performance limits of all applications involving atomic light-matter interfaces.

Colloquium: The Amplitude Mode in Condensed Matter : Higgs Hunting on a Budget

Transport and other related electronic properties in model lattice structures

Ultrafast and nano-optics: Watching the motion of electrons in nanostructures

Soft X-ray shines on the new attosecond horizon through the water window

Magnons, spinons, Majorana fermions and quantum spin liquids

In recent years many physicists researching quantum materials have become very excited about an unusual state of matter: the quantum spin liquid (QSL). In this talk I will explore the difference between “conventional” ordered magnetic systems and QSLs. To understand these systems the way physicists do one needs to appreciate the importance of collective elementary excitations. In ordinary magnets these are called spin-waves, or, in the quantum version, magnons. These will be explained and discussed along with other excitations that are important in quantum magnetism sometime called “triplons” and spinons. I will discuss a major technique for characterizing these excitations: inelastic neutron scattering. The last part of the talk will examine one material of particular recent interest: $\alpha-RuCl_3$. This material orders magnetically, but it is close to a special kind of quantum spin liquid called the “Kitaev quantum spin liquid”, and exhibits unusual excitations that might be related to particles known as Majorana Fermions. Finally, I will discuss how the magnetic order can be suppressed by either site-dilution of the magnetic $Ru^{3+}$ ions, or the application of a magnetic field. The resultant consequences on the magnetic excitation spectrum are seen to be of particular interest.

Topological sound and odd viscosity in chiral active matter

Abstract: Active materials are composed of interacting particles individually powered by motors. In this talk, we focus on chiral active materials that violate parity and time reversal symmetry. First, we show how to generate topological sound in fluids of self-propelled particles exhibiting a spontaneous chiral active flow under confinement. These topological sound modes propagate unidirectionally, without backscattering, along either sample edges or domain walls and despite overdamped particle dynamics. Next, we discuss an exotic transport coefficient characteristic of quantum Hall fluids, called odd viscosity, which controls the hydrodynamics of classical fluids composed of active rotors. This odd viscosity couples pressure to vorticity leading to transverse flow in piston compression experiments. We envision that such transverse response may be exploited to design self-assembled hydraulic cranks that convert between linear and rotational motion in microscopic machines powered by active rotors fluids.

Topological Photonics and Topological Insulator Lasers

Martin Gutzwiller-Fellowship Award Ceremony: Unusual few-body quantum states and their many-body implications

Simple systems with only 3 or 4 interacting particles can behave in a bizarre, counter-intuitive manner. Examples to be discussed in this Colloquium include ultra-long-range Rydberg molecules with enormous electric dipole moments, as well as states of a few neutral particles that resonantly form a cluster. Moreover, insights from a few-body viewpoint can help to understand some of the rich many-body systems being actively explored, from the unitary Bose gas to the fermionic or bosonic flavors of the fractional quantum Hall effect.

Cotranscriptional Folding of RNA Nanostructures and Devices

RNA nanotechnology has the great potential to allow us to produce well-defined nanostructures and devices inside cells and thus open up a wide range of design opportunities in synthetic biology. To achieve this goal we need to understand the design principles of geometry, folding kinetics and topology that will allow us to genetically encode well-defined RNA nanostructures that self-assemble during the transcription process. We have recently introduced the single-stranded RNA origami method and validated the architecture by transcribing RNA tiles that assemble into lattices of different geometries. I will introduce new software tools that allow interactive design of RNA origami structures using a library of functional modules and new sequence design approaches that allow large structures to be designed. Also I will show our latest progress in developing larger three-dimensional RNA origami structures and functional RNA nanodevices with applications in biosensing and diagnostics.

The multi-scale structure and dynamics of macaque visual cortex at cellular and synaptic resolution

El Nino as a Topological Insulator: A Surprising Connection Between Climate, and Quantum, Physics

Symmetries and topology play central roles in our understanding of physics. Topology explains the precise quantization of the Hall effect and the protection of surface states in topological insulators against scattering from disorder or bumps. However discrete symmetries and topology have so far played little role in thinking about the fluid dynamics of oceans and atmospheres. I show that, as a consequence of the rotation of the Earth that breaks time reversal symmetry, equatorially trapped Kelvin and Yanai waves emerge as topologically protected edge modes. Thus the oceans and atmosphere of Earth naturally share basic physics with topological insulators. As equatorially trapped Kelvin waves in the Pacific ocean are an important component of El Nino Southern Oscillation, these new results demonstrate that topology plays a surprising role in Earth’s climate system.

Localization: Moving beyond Statistical Mechanics

The central assumption of statistical mechanics is that interactions between particles establish local equilibrium. Isolated quantum systems, however, need not equilibrate; this happens, for example, when sufficient quenched disorder causes localization. The many-body localized (MBL) phase transports neither heat nor charge; may possess orders disallowed in equilibrium; and, may exhibit quantum coherence even when highly excited. In this talk, I will review the emerging understanding of how quantum localization can lead to new quantum phenomena even in highly excited states. I will give some theoretical intuition about how this might be used to build a better quantum computer and also review some of the latest experiments investigating localization.

Quantum Metrology, the art of parameter estimation

Gutzwiller Colloquium: Making Living Matter from the bottom up

There are many ways to study life, and one that is particularly appealing to physicists is regarding it as self-organized active soft matter that is away from equilibrium ``just the right way’’. In this Colloquium, I will discuss this notion, and provide a number of examples of how we can begin to put together simple systems - from basic ingredients that we fully understand - that would exhibit the kind of active behaviour we find in living systems. I will address the question of stability of a living system made of active components and propose a fundamentally new mechanism in which a competition between chemical signalling and cell division can determine the homeostatic conditions at the systemic level.