Nitrogen-vacancy centers in diamond are sensitive to magnetic fields, and a single center permits detection of electron and nuclear spins and imaging of single molecules in its vicinity. This article reviews the achievements of advanced methods to obtain spectral and spatial resolution and it points to technical problems that remain to be solved for widespread and multidisciplinary adoption of single-molecule magnetic resonance spectroscopy.

Artificially engineered mechanical systems, sometimes called metamaterials, offer many promising applications on length scales ranging from macroscopic systems to the nanoscale. A topic of particular interest is the existence of topologically protected phononic edge states in such systems that are analogous to the electronic edge states that give rise to the quantum Hall effect. This Colloquium gives an introduction to topologically protected transport in metamaterials and its applications for controlling acoustic transport.

Magnetotactic bacteria have a built-in compass, in the form of a magnetosome chain made up of magnetic biominerals, that allows them to passively align along terrestrial magnetic field lines. They also sense oxygen gradients and swim using at least one flagellum. Hence, these bacteria are self-propelled active matter capable of displaying flocking behavior. This Colloquium explains the physics behind these various capabilities, as well as their interactions and biological significance.

The standard model is successful at describing most of the data at the electroweak scale, but there are indications that new physics should exist at a higher energy scale. To identify, quantify, and elucidate the new physics, one can use the framework of the standard model effective field theory. This article reviews the construction and theoretical tools provided by the effective field theory for analyzing the present and future experimental data, as well as theoretical ideas for new physics.

Electronic devices that incorporate magnetism, called spintronic devices, can increase the functionality of electronic circuits and lead to increases in efficiency. Such devices are useful if the magnetization can be manipulated electrically rather than by magnetic fields. This review covers the materials, underlying physics, and applications involved in such manipulation, focusing on two control mechanisms. The first is control by manipulating the magnetization through its coupling to ferroelectric order and the second is control by spin-polarized currents manipulating the magnetization through the angular momentum flowing into it.

Recent observations of compact astrophysical objects have opened the possibility to probe the nature of gravity in its strong-field regime. Such observations could reveal deviations from general relativity or the standard model. Spontaneous scalarization, which is controlled by scalar-field couplings to gravity, leads to a behavior that resembles a phase transition: the scalar induces measurable effects in the strong-field regime while remaining undetectable in weak-field gravitational experiments. This review presents the spontaneous scalarization mechanism, several scalarization models considered in the literature, and their astrophysical implications for neutron stars and black holes. It also discusses the generalization of such models to other types of fields and instabilities.

Time-resolved angle-resolved photoemission spectroscopy provides access to light-induced changes in the electronic band structure and interactions of solids, and to the out-of-equilibrium electron dynamics. This article reviews the history and future prospects for the development of the technique, and offers an overview of recent achievements in studying unoccupied and light-driven states, photoinduced phase transitions, electron-phonon scattering, and electron dynamics in quantum materials, including topological insulators, unconventional superconductors, traditional and novel semiconductors, excitonic insulators, and spin-textured systems.

Metamaterials are artificially patterned structures designed to behave as artificial materials with novel properties. A popular application is controlling electromagnetic waves with subwavelength patterning, leading to properties like negative indices of refraction. Metamaterials can also control diffusion processes, which are different from wave propagation. This review describes metamaterials in diffusive systems in terms of their underlying physics, the theory used to describe them, and their potential applications in areas such as heat management, drug transport, and particle separation.

Friction at highly lubric interfaces of two-dimensional materials is important yet incompletely characterized. This Colloquium discusses sliding and pinning between two-dimensional layers, using simulations of twisted graphene interfaces as a prototypical system. The resulting insights are of potential relevance for a larger category of bilayer and multilayer systems as well.

This article reviews recent literature and presents new calculations of the Lamb shift in light muonic atoms. Point-nucleus QED and nuclear structure effects are treated consistently among all muonic and electronic atoms to allow for improved determination of nuclear charge radii and fundamental constants.

Fractons are exotic excitations originally conceived as platforms for reliable quantum memories. They are characterized by highly restricted mobilities. In the continuum, they are described by tensor fields with higher gauge symmetries. In this Colloquium, the focus is on a class of duality mappings between fracton models and elasticity theory, building the reader’s intuition and understanding in a more familiar setting.

Probing of electromagnetic fields in high-energy-density experiments is key to understanding questions in fusion processes such as how the fields are compressed, diffuse through the plasma, and can seed instabilities. Many kinetic processes studied, including collisionless shocks, filamentary instabilities, jets, magnetic reconnection, and turbulence, all depend on the field structure. In this review, an overview of experimental techniques and the underpinning theoretical principles and modeling of proton-based imaging is presented, followed by a review of experiments and an outlook for future frontiers in the technique.

The gravitational form factors encode fundamental particle properties including mass, spin, and $D$-term. Their physical interpretation promises, for composed particles, insights on spatial distributions of energy, angular momentum, and internal forces. This Colloquium reviews the theoretical and recent experimental advances in this field with focus on the quark-gluon structure of the proton in QCD.

Quantum technology is now at a point where practical work can begin on creating the quantum internet. However, numerous challenges must be overcome before this vision becomes a reality. A global-scale quantum internet requires the development of the quantum repeater, a device that stores and manipulates qubits while interacting with or emitting entangled photons. This review examines different approaches to quantum repeaters and networks, covering their conceptual frameworks, architectures, and current progress in experimental implementation.

In most of physics it is normal to obtain information by analysis of noisy data. The paradigm of quantum computing has been a simplified version of this – one measurement of a two-level system gives one bit of reliable information about the result of a computation. But real-world quantum computers do not work this way: the noisiness of quantum evolution also requires good strategies for extracting information. This review covers many error-mitigation strategies used in present-day quantum processors. These strategies make it much more feasible to obtain useful results before fault tolerance is achieved.

Kinematic variables are important tools for analyzing collider experiments. This article reviews a variety of such tools, which were designed primarily for the experiments at the Large Hadron Collider, but which have potential uses in other experiments. The article also discusses the interconnection and mutual complementarity of kinematic variables and modern machine-learning techniques.

The study of optics in correlated disordered media combines wave physics, complex media, and nanophotonics. Investigations have shown how subwavelength structural correlations control light scattering, transport, and localization. This article reviews the formalism behind light scattering in disordered media, experimental techniques, and achievements in studying light interaction with correlated disorder. It explores phenomena like optical transparency, superdiffusive transport, and photonic gaps, offering new perspectives for applications. The research covers systems from photonic liquids to hyperuniform disordered photonic materials, and addresses mesoscopic phenomena and disorder engineering for light-energy management.

Atmospheric nanoparticles can serve as nuclei for cloud droplets, thereby inducing significant but uncertain effects on the radiative forcing of the climate system. This article focuses on the physicochemical processes that govern the growth of these particles from formation of molecular clusters until the particles reach sizes where they can act as cloud condensation nuclei. The review describes the latest developments in measurement and modeling of these processes and connects these domains to the large-scale simulations such as Earth system models. The authors recommend closer coordination among laboratory studies, atmospheric measurements, and large-scale modeling to understand the importance of nanoparticles in the climate system.

The pandemic of coronavirus disease 2019 has led to a renewed focus on the physicochemical properties of the droplets and aerosol particles that are exhaled during breathing, speaking, singing, coughing, and sneezing. In this article, the properties of respiratory particles, including their number concentrations and size distributions, as well as their formation mechanisms at different sites in the respiratory system, are reviewed. The data in the literature are synthesized via a parametrization of the particle size distribution data using log-normal modes related to the different origin sites.

Accelerator mass spectrometry (AMS) is a mass-spectrometric method using entire accelerator systems to measure ultralow traces of long-lived radioisotopes. AMS spectrometers produce an ion beam from a sample of interest and separate ions according to their magnetic, electric, and atomic characteristics. It is thus possible to identify both the mass number and the atomic number of a very rare radioisotope, and count it atom by atom. The review describes the 45-year history since the discovery of AMS, detailed technical aspects, and a wide range of research fields.

The standard central limit theorem does not apply to sums of many random variables with heavy-tailed probability distributions. The anomalous statistics for such sums have exotic properties and they are applied phenomenologically across the natural sciences, economics, and the social sciences. This Colloquium reviews how anomalous statistics can be derived from first principles and how they govern the observed diffusive motion of ultracold atoms in laser fields.

Fast radio bursts, milliseconds-duration radio bursts predominantly originating from cosmological distances, figure among the unsolved puzzles of contemporary astrophysics. The rapid accumulation of observational data has generated an equally intense theoretical activity toward the understanding of the physical processes at the origin of these events. This review presents a thorough survey of the current knowledge about fast radio bursts, starting with the generic constraints that can be placed on theoretical models based on current observations and plasma physics considerations, then moving to a critical discussion of coherent radiation mechanisms and source models currently debated in the scientific community.