Magnetic skyrmions are topologically stable nanoscale particlelike objects that were discovered in 2009. Since that time, intense research interest in the field has led to the identification of numerous compounds that support skyrmions over a range of conditions spanning from cryogenic to room temperatures. Skyrmions can be set into motion under various types of driving, and the combination of their size, stability, and dynamics makes them ideal candidates for numerous applications. At the same time, skyrmions represent a new class of system in which the energy scales of the skyrmion-skyrmion interactions, sample disorder, temperature, and drive can compete. A growing body of work indicates that the static and dynamic states of skyrmions can be influenced strongly by pinning or disorder in the sample; thus, an understanding of such effects is essential for the eventual use of skyrmions in applications. The current state of knowledge regarding individual skyrmions and skyrmion assemblies interacting with quenched disorder or pinning is reviewed. The microscopic mechanisms for skyrmion pinning, including the repulsive and attractive interactions that can arise from impurities, grain boundaries, or nanostructures, are outlined. This is followed by descriptions of depinning phenomena, sliding states over disorder, the effect of pinning on the skyrmion Hall angle, the competition between thermal and pinning effects, the control of skyrmion motion using ordered potential landscapes such as one- or two-dimensional periodic asymmetric substrates, the creation of skyrmion diodes, and skyrmion ratchet effects. Highlighted are the distinctions arising from internal modes and the strong gyrotropic or Magnus forces that cause the dynamical states of skyrmions to differ from those of other systems with pinning, such as vortices in type-II superconductors, charge density waves, or colloidal particles. Throughout this review future directions and open questions related to the pinning and dynamics in skyrmion systems are also discussed.

This is a review of the Sachdev-Ye-Kitaev (SYK) model of compressible quantum many-body systems without quasiparticle excitations, and its connections to various theoretical studies of non-Fermi liquids in condensed matter physics. The review is placed in the context of numerous experimental observations on correlated electron materials. Strong correlations in metals are often associated with their proximity to a Mott transition to an insulator created by the local Coulomb repulsion between the electrons. The phase diagrams of a number of models of such a local electronic correlation are explored, employing a dynamical mean-field theory in the presence of random spin exchange interactions. Numerical analyses and analytical solutions, using renormalization group methods and expansions in large spin degeneracy, lead to critical regions that display SYK physics. The models studied include the single-band Hubbard model, the $t\text{−}J$ model, and the two-band Kondo-Heisenberg model in the presence of random spin exchange interactions. Also examined are non-Fermi liquids obtained by considering each SYK model with random four-fermion interactions to be a multiorbital atom, with the SYK atoms arranged in an infinite lattice. Connections are made to theories of sharp Fermi surfaces without any low-energy quasiparticles in the absence of spatial disorder, obtained by coupling a Fermi liquid to a gapless boson; a systematic large-$N$ theory of such a critical Fermi surface, with SYK characteristics, is obtained by averaging over an ensemble of theories with random boson-fermion couplings. Finally, an overview of the links between the SYK model and quantum gravity is presented, and the review ends with an outlook on open questions.

Advances in machine learning methods provide tools that have broad applicability in scientific research. These techniques are being applied across the diversity of nuclear physics research topics, leading to advances that will facilitate scientific discoveries and societal applications. This Colloquium provides a snapshot of nuclear physics research, which has been transformed by machine learning techniques.

Interest in the multiple facets of optical vortices has flourished in the last three decades. This review examines the basic research and applications of the interplay between optical vortices and condensed-matter systems. This subfield of optical-vortex physics has rapidly developed in recent years thanks to a vigorous synergy between theory and experiment. After presenting self-contained and focused introductions to optical vortices and condensed-matter optics, theory and current progress in the research on the interaction of condensed-matter systems and optical vortices are examined. When one considers the interaction of optical vortices with condensed-matter systems, many aspects of the standard theory of the interaction of matter with plane-wave light need to be reformulated. In bulk, light-matter Hamiltonian matrix elements have to be recalculated and novel selection rules are obtained, reflecting the conservation of total angular momentum. Orbital angular momentum is transferred from the light beam to the photoexcited electrons, thereby generating macroscopic currents. Semiconductor nanostructures add the complexity of their own spatial inhomogeneity, which is handled adequately by the envelope-function approximation. Here again modified matrix elements for light-matter interactions dictate the allowed and forbidden optical transitions, which are distinct from those obtained in traditional optical excitation with smooth fields. Quantum rings play a central role due to their specially adapted geometry to the cylindrical nature of the optical-vortex beams. When the electron-electron interaction is taken into account, the rich physics of excitons and exciton polaritons comes into play and is modified by the finite orbital angular momentum of the structured light. Furthermore, the new features brought about by optical vortices in plasmonics and in the optical excitation of two-dimensional materials are reviewed. For all these systems theory and recent experiments are discussed. Finally, an overview of current and prospective applications of the interaction of optical vortices with condensed-matter systems in the fields of quantum technologies, communications, sensing, etc., is presented. Throughout this review an attempt has been made to present not only a survey of the relevant literature but also a perspective on the interesting and rapidly evolving field of optical-vortex–condensed-matter interactions.

Laser-produced plasmas (LPPs) engulf exotic and complex conditions ranging in temperature, density, pressure, magnetic and electric fields, charge states, charged particle kinetics, and gas-phase reactions based on the irradiation conditions, target geometries, and background cover gas. The application potential of the LPP is so diverse that it generates considerable interest for both basic and applied research areas. The fundamental research on LPPs can be traced back to the early 1960s, immediately after the invention of the laser. In the 1970s, the laser was identified as a tool to pursue inertial confinement fusion, and since then several other technologies have emerged out of LPPs. These applications prompted the development and adaptation of innovative diagnostic tools for understanding the fundamental nature and spatiotemporal properties of these complex systems. Although most of the traditional characterization techniques developed for other plasma sources can be used to characterize the LPPs, care must be taken to interpret the results because of their small size, transient nature, and inhomogeneities. The existence of the large spatiotemporal density and temperature gradients often necessitates nonuniform weighted averaging over distance and time. Among the various plasma characterization tools, optical-based diagnostic tools play a key role in the accurate measurements of LPP parameters. The optical toolbox contains optical spectroscopy (emission, absorption, and fluorescence), as well as passive and active imaging and optical probing methods (shadowgraphy, Schlieren imaging, interferometry, Thomson scattering, deflectometry, and velocimetry). Each technique is useful for measuring a specific property, and its use is limited to a certain time span during the LPP evolution because of the sensitivity issues related to the selected measuring tool. Therefore, multiple diagnostic tools are essential for a comprehensive insight into the entire plasma behavior. Recent improvements in performance in laser and detector systems have expanded the capability of the aforementioned passive and active diagnostic tools. This review provides an overview of optical diagnostic tools frequently employed for the characterization of the LPPs and emphasizes techniques, associated assumptions, and challenges. Considering that most of the industrial and other applications of the LPP belong to low to moderate laser intensities (${10}^8–{10}^{15}{\mathrm{W}\mathrm{cm}}^{-2}$), this review focuses on diagnostic tools pertaining to this regime.

A complete understanding of the brain requires an integrated description of the numerous scales and levels of neural organization. This means studying the interplay of genes and synapses, but also the relation between the structure and dynamics of the whole brain, which ultimately leads to different types of behavior, from perception to action, while asleep or awake. Yet multiscale brain modeling is challenging, in part because of the difficulty to simultaneously access information from multiple scales and levels. While some insight has been gained on the role of specific microcircuits on the generation of macroscale brain activity, a comprehensive characterization of how changes occurring at one scale or level can have an impact on other ones remains poorly understood. Recent efforts to address this gap include the development of new frameworks originating mostly from network science and complex systems theory. These theoretical contributions provide a powerful framework to analyze and model interconnected systems exhibiting interactions within and between different layers of information. Recent advances for the characterization of the multiscale brain organization in terms of structure-function, oscillation frequencies, and temporal evolution are presented. Efforts are reviewed on the multilayer network properties underlying the physics of higher-order organization of neuronal assemblies, as well as on the identification of multimodal network-based biomarkers of brain pathologies such as Alzheimer’s disease. This Colloquium concludes with a perspective discussion of how recent results from multilayer network theory, involving generative modeling, controllability, and machine learning, could be adopted to address new questions in modern physics and neuroscience.

Geometric phases are ubiquitous in physics; they act as memories of the transformation of a physical system. In optics, the most prominent examples are the Pancharatnam-Berry phase and the spin-redirection phase. Recent technological advances in phase and polarization structuring have led to the discovery of additional geometric phases of light. The underlying mechanism for all of these is provided by fiber bundle theory. This Colloquium reviews how fiber bundle theory not only sheds light on the origin of geometric phases of light but also lays the foundations for the exploration of high-dimensional state spaces, with implications for topological photonics and quantum communications.

Quantum theory has been successfully validated in numerous laboratory experiments. But would such a theory, which effectively describes the behavior of microscopic physical systems and its predicted phenomena such as quantum entanglement, still be applicable on large length scales? From a practical perspective, how can quantum key distribution (where the security of establishing secret keys between distant parties is ensured by the laws of quantum mechanics) be made technologically useful on a global scale? Owing to photon loss in optical fibers and terrestrial free space, the achievable distance using direct transmission of single photons has been limited to a few hundred kilometers. A promising route to testing quantum physics over long distances and in the relativistic regimes, and thus realizing flexible global-scale quantum networks, is via the use of satellites and space-based technologies, where a significant advantage is that the photon loss and turbulence predominantly occurs in the lower $\sim 10\mathrm{km}$ of the atmosphere, and most of the photons’ transmission path in space is virtually in vacuum, with almost zero absorption and decoherence. Progress in free-space quantum experiments, with a focus on the fast-developing Micius satellite–based quantum communications, is reviewed. The perspective of space-ground integrated quantum networks and fundamental quantum optics experiments in space conceivable with satellites are discussed.

Quantum cryptography exploits principles of quantum physics for the secure processing of information. A prominent example is secure communication, i.e., the task of transmitting confidential messages from one location to another. The cryptographic requirement here is that the transmitted messages remain inaccessible to anyone other than the designated recipients, even if the communication channel is untrustworthy. In classical cryptography, this can usually be guaranteed only under computational hardness assumptions, such as when factoring large integers is infeasible. In contrast, the security of quantum cryptography relies entirely on the laws of quantum mechanics. Here this physical notion of security is reviewed, with a focus on quantum key distribution and secure communication.

Two processes of photon-pair creation can be arranged such that the paths of the emitted photons are identical. The path information is thereby not erased but rather never born in the first place due to this path identity. In addition to its implications for fundamental physics, this concept has recently led to a series of impactful discoveries in the fields of imaging, spectroscopy, and quantum information science. Here the idea of path identity is presented and a comprehensive review of recent developments is provided. Specifically, the concept of path identity is introduced based on three defining experimental ideas from the early 1990s. The three experiments have in common that they contain two photon-pair sources. The paths of one or both photons from the different sources overlap such that no measurement can recognize from which source they originate. A wide range of noteworthy quantum interference effects (at the single- or two-photon level), such as induced coherence, destructive interference of photon pairs, and entanglement generation, are subsequently described. Progress in the exploration of these ideas has stagnated and has gained momentum again only in the last few years. The focus of the review is the new development in the last few years that modified and generalized the ideas from the early 1990s. These developments are overviewed and explained under the same conceptual umbrella, which will help the community develop new applications and realize the foundational implications of this sleeping beauty.

Modern electron linear accelerators are often designed to produce smooth bunch distributions characterized by their macroscopic ensemble-average moments. However, an increasing number of accelerator applications call for finer control over the beam distribution, such as by requiring specific shapes for its projection along one coordinate. Ultimately, the control of the beam distribution at the single-particle level could enable new opportunities in accelerator science. This review discusses the recent progress toward controlling electron beam distributions on the “mesoscopic” scale with an emphasis on shaping the beam or introducing complex correlations required for some applications. This review emphasizes experimental and theoretical developments of electron-bunch shaping methods based on bounded external electromagnetic fields or via interactions with the self-generated velocity and radiation fields.

The successes and limitations of statistical sampling for a sequence of models studied in the context of lattice QCD are discussed and the need for new methods to deal with finite-density and real-time evolution is emphasized. It is shown that these lattice models can be reformulated using tensorial methods where the field integrations in the path-integral formalism are replaced by discrete sums. These formulations involve various types of duality and provide exact coarse-graining formulas that can be combined with truncations to obtain practical implementations of the Wilson renormalization group program. Tensor reformulations are naturally discrete and provide manageable transfer matrices. Truncations with the time continuum limit are combined, and Hamiltonians suitable for performing quantum simulation experiments, for instance, using cold atoms, or to be programmed on existing quantum computers, are derived. Recent progress concerning the tensor field theory treatment of noncompact scalar models, supersymmetric models, economical four-dimensional algorithms, noise-robust enforcement of Gauss’s law, symmetry preserving truncations, and topological considerations are reviewed. Connections with other tensor network approaches are also discussed.

In undergraduate courses on classical electromagnetism, it is taught that a perfect conductor expels the electromagnetic (EM) field, and hence its surface is not able to support the propagation of bound EM waves. However, when the surface of a perfect conductor is structured at a length scale much smaller than the operating wavelength, geometrically induced surface EM modes can be supported. Owing to their similarities with the surface plasmon polaritons (SPPs) in the optical regime, these surface EM modes were named spoof surface plasmons. The concept of spoof surface plasmons has opened up a new line of research within plasmonics with the aim of transferring all the potentialities of SPPs in the optical regime to lower frequencies (microwave, terahertz, and midinfrared regimes) in which a metal behaves as a quasiperfect conductor. In recent years, several research groups have extended this concept from planar surfaces to waveguides, and eventually to resonators, covering the entire range of structures studied in standard plasmonics. This review provides a detailed perspective on the recent developments in spoof surface plasmon photonics from both the fundamental and applied sides.

The cell, as the most fundamental unit of life, is a microcosm of biology in which the confluence of nearly all aspects of classical physics (mechanics, statistical physics, condensed matter, and electromagnetism) plays out. This leads to a rich and complex emergent behavior that determines the entire gamut of biological functions. Specifically, at the cellular scale, mechanical forces and deformations are inextricably linked to electrical fields (and, to a lesser degree, magnetic fields). This in turn is responsible for phenomenology such as cell-cell communication, morphological evolution, cell fusion, self-assembly, cell fission, magnetoreception, endocytosis, and adhesion, among others. From the viewpoint of biomedicine, cellular response to the combined influence of electrical, magnetic, and mechanical fields has applications in cancer treatment, targeted transfer of medicine, gene therapy, and wound amelioration. As an example of the profound influence of the combined electrical-mechanical coupling, one needs to take cognizance only of the operation of ion channels that form the basis for our sensing system (such as hearing, sight, and tactile sense). The coupled mechanical and electromagnetic behavior of a cell is a highly interdisciplinary endeavor and this review provides a distillation of both the theoretical underpinnings of the subject and the pertinent biological interpretation. The key developments pertaining to this topic are reviewed, a unified mathematical framework that couples nonlinear deformation and electromagnetic behavior as germane for soft biological entities is summarized, gaps in current knowledge are pointed out, and the central issues that are pertinent to future research are commented upon.

Interfacial thermal resistance (ITR) is the main obstacle for heat flows from one material to another. Understanding ITR becomes essential for the removal of redundant heat from fast and powerful electronic and photonic devices, batteries, etc. In this review, a comprehensive examination of ITR is conducted. Particular focus is placed on the theoretical, computational, and experimental developments in the 30 years after the last review given by Swartz and Pohl in 1989. To be self-consistent, the fundamental theories, such as the acoustic mismatch model, the diffuse mismatch model, and the two-temperature model, are reviewed. The most popular computational methods, including lattice dynamics, molecular dynamics, the Green’s function method, and the Boltzmann transport equation method, are discussed in detail. Various experimental tools in probing ITR, such as the time-domain thermoreflectance, the thermal bridge method, the $3\omega$ method, and the electron-beam self-heating method, are illustrated. This review covers ITR (also known as the thermal boundary resistance or Kapitza resistance) of solid-solid, solid-liquid, and solid-gas interfaces. Such fundamental challenges as how to define the interface, temperature, etc. when the materials scale down to the nanoscale or atomic scale and the opportunities for future studies are also pointed out.

The new era of gravitational wave astronomy truly began on September 14, 2015, with the detection of GW150914, the sensational first direct observation of gravitational waves from the inspiral and merger of two black holes by the two Advanced LIGO detectors. In the subsequent first three observing runs of the LIGO-Virgo network, gravitational waves from $\sim 50$ compact binary mergers have been announced, with more results to come. The events have mostly been produced by binary black holes, but two binary neutron star mergers have been observed thus far, as well as the mergers of two neutron star–black hole systems. Furthermore, gravitational waves emitted by core-collapse supernovae, pulsars, and the stochastic gravitational wave background are within the LIGO-Virgo-KAGRA sensitivity band and are likely to be observed in future observation runs. Beyond signal detection, a major challenge has been the development of statistical and computational methodology for estimating the physical waveform parameters and quantifying their uncertainties in order to accurately characterize the emitting system. These methods depend on the sources of the gravitational waves and the gravitational waveform model that is used. This review examines the main waveform models and parameter estimation methods used to extract physical parameters from gravitational wave signals detected to date by LIGO and Virgo and from those expected to be observed in the future, which will include KAGRA, and how these methods interface with various aspects of LIGO-Virgo-KAGRA science. Also presented are the statistical methods used by LIGO and Virgo to estimate detector noise, test general relativity, and draw conclusions about the rates of compact binary mergers in the Universe. Furthermore, a summary of major publicly available gravitational wave parameter estimation software packages is given.

The rich and diverse dynamics of particle-based systems ultimately originates from the coupling of their degrees of freedom via internal interactions. To arrive at a tractable approximation of such many-body problems, coarse graining is often an essential step. Power functional theory provides a unique and microscopically sharp formulation of this concept. The approach is based on an exact one-body variational principle to describe the dynamics of both overdamped and inertial classical and quantum many-body systems. In equilibrium, density-functional theory is recovered, and hence spatially inhomogeneous systems are described correctly. The dynamical theory operates on the level of time-dependent one-body correlation functions. Two- and higher-body correlation functions are accessible via the dynamical test-particle limit and the nonequilibrium Ornstein-Zernike route. The structure of this functional approach to many-body dynamics is described, including much background as well as applications to a broad range of dynamical situations, such as the van Hove function in liquids, flow in nonequilibrium steady states, motility-induced phase separation of active Brownian particles, lane formation in binary colloidal mixtures, and both steady and transient shear phenomena.