Fundamental properties of light unavoidably impose features on images collected using fluorescence microscopes. Accounting for these features is often critical in quantitatively interpreting microscopy images, especially those gathering information at scales on par with or smaller than light’s emission wavelength. Here the optics responsible for generating fluorescent images, fluorophore properties, and microscopy modalities leveraging properties of both light and fluorophores, in addition to the necessarily probabilistic modeling tools imposed by the stochastic nature of light and measurement, are reviewed.

Spin-orbit coupling (SOC) relates to the interaction between an electron’s motion and its spin and is ubiquitous in solid-state systems. Although the effect of SOC in normal-state phenomena has been extensively studied, its role in superconducting hybrid structures and devices elicits many unexplored questions. In conjunction with broken symmetries and material inhomogeneities within superconducting hybrid structures, SOC may have contributions beyond its effects in homogeneous materials. Notably, even with well-established magnetic or nonmagnetic materials and conventional $s$-wave spin-singlet superconductors, SOC leads to emergent phenomena including equal-spin-triplet pairing and topological superconductivity (hosting Majorana states), a modified current-phase relationship in Josephson junctions, and nonreciprocal transport, including superconducting diode effects. SOC is also responsible for transforming quasiparticles in superconducting structures, which enhances the spin Hall effect and changes the spin dynamics. Taken together, SOC in superconducting hybrid structures and the potential for electric tuning of the SOC strength create interesting possibilities to advance superconducting spintronic devices for energy-efficient computing and enable topological fault-tolerant quantum computing. By providing a description of experimental techniques and theoretical methods to study SOC, this Colloquium describes the current understanding of resulting phenomena in superconducting structures and offers a framework to select and design a growing class of materials systems where SOC plays an important role.

New developments in superconductivity, particularly through unexpected and often surprising forms of superconducting materials, continue to excite the community and stimulate theory. It is now becoming clear that there are two distinct platforms for superconductivity: natural and synthetic materials. The study of these artificial materials has greatly expanded in the past decade or so, with the discoveries of new forms of superfluidity in artificial heterostructures and the exploitation of proximitization. Natural superconductors continue to surprise through Fe-based pnictides and chalcogenides, and nickelates as well as others. This review presents a two-pronged investigation into such superconductors, with an emphasis on those that have come to be understood to belong somewhere between the Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensation (BEC) regimes. The nature of this “crossover” superconductivity, which is to be distinguished from crossover superfluidity in atomic Fermi gases, is a focus here. Multiple ways of promoting a system out of the BCS and into the BCS-BEC crossover regime are addressed in the context of concrete experimental realizations. These involve natural materials, such as organic conductors, as well as artificial, mostly two-dimensional materials, such as magic-angle twisted bilayer and trilayer graphene, or gate-controlled devices, as well as one-layer and interfacial superconducting films. Such developments should be viewed as a celebration of BCS theory, as it is now clear that, even though this theory was initially implemented with the special case of weak correlations in mind, it can be extended in a natural way to treat the case of these more exotic strongly correlated superconductors.

Single-molecule technology stands as a powerful tool, enabling the characterization of intricate structural and dynamic information that would otherwise remain concealed within the averaged behaviors of numerous molecules. This technology finds extensive application across diverse fields including physics, chemistry, biology, and medicine. Quantum sensing, particularly leveraging nitrogen-vacancy (NV) centers within diamond structures, presents a promising avenue for single-molecule magnetic resonance, offering prospects for sensing and imaging technology at the single-molecule level. Notably, while significant strides have been made in single-molecule scale magnetic resonance using NV centers over the past two decades, current approaches still exhibit limitations in magnetic sensitivity, spectral resolution, and spatial resolution. In particular, the full reconstruction of three-dimensional positions of nuclear spins within single molecules remains an unattained goal. This review provides a comprehensive overview of the current state of the art in single-molecule scale magnetic resonance encompassing an analysis of various relevant techniques involving NV centers. Additionally, it explores the optimization of technical parameters associated with these methods. This detailed analysis serves as a foundation for the development of new technologies and the exploration of potential applications.

Mechanical vibrations are being harnessed for a variety of purposes and at many length scales, from the macroscopic world down to the nanoscale. The considerable design freedom in mechanical structures allows one to engineer new functionalities. In recent years, this has been exploited to generate setups that offer topologically protected transport of vibrational waves (topological phonon transport), both in the solid state and in fluids. Borrowing concepts from electronic physics and being cross fertilized by concurrent studies for cold atoms and electromagnetic waves, this field of topological transport in engineered mechanical systems offers a rich variety of phenomena and platforms. In this Colloquium, a unifying overview of the various ideas employed in this area is provided, different approaches and experimental implementations are summarized, and the challenges as well as the prospects are commented upon.

Magnetotactic bacteria are swimming microorganisms able to follow magnetic field lines with the help of an organelle called the magnetosome that is made of biomineralized magnetic crystals assembled in a chain. In combination with this ability, these bacteria perform active oxygen sensing to reach the oxic-anoxic transition zone, which is often located in the upper part of the sediment. From a physicist’s perspective, magnetotactic bacteria can be seen at the interface between bacterial active matter and magnetic colloids, which gives them unique properties at both the individual and collective levels. In crowded media and/or when they are submitted to external flows, their motion can be efficiently driven by magnetic fields, which leads to surprising effects. In this Colloquium, the different features of magnetotactic bacteria at are reviewed at every scale, from single cell to collective motion, from simple to complex environments, and by emphasizing the differences from other bacterial species or passive magnetic colloids. The Colloquium concludes with a discussion of perspectives on using magnetotactic bacteria in active magnetorheology.

The striking success of the standard model in explaining precision data and, at the same time, its lack of explanations for various fundamental phenomena, such as dark matter and the baryon asymmetry of the Universe, suggest new physics at an energy scale that greatly exceeds the electroweak scale. In the absence of a short-range–long-range conspiracy, the standard model can be viewed as the leading term of an effective “remnant” theory (referred to as the SMEFT) of a more fundamental structure. In recent years, many aspects of the SMEFT have been investigated, and it has become a standard tool for analyzing experimental results in an integral way. In this review, after a presentation of the salient features of the standard model, the construction of the SMEFT is reviewed. The range of its applicability and bounds on its coefficients imposed by general theoretical considerations are discussed. Since new-physics models are likely to exhibit exact or approximate accidental global symmetries, especially in the flavor sector, their implications for the SMEFT are also discussed. The main focus of the review is the phenomenological analysis of experimental results. How to use various effective field theories to study the phenomenology of theories beyond the standard model is explicitly shown. Descriptions of the matching procedure and the use of the renormalization group equations are given, allowing one to connect multiple effective theories that are valid at different energy scales. Explicit examples from low-energy experiments and from high-$p_T$ physics illustrate the workflow. Also commented upon are the nonlinear realization of electroweak symmetry breaking and its phenomenological implications.

The remanent magnetization of ferromagnets has long been studied and used to store binary information. While early magnetic memory designs relied on magnetization switching by locally generated magnetic fields, key insights in condensed matter physics later suggested the possibility of doing it by electrical means instead. In the 1990s, Slonczewski and Berger formulated the concept of current-induced spin torques in magnetic multilayers through which a spin-polarized current generated by a first ferromagnet may be used to switch the magnetization of a second one. This discovery drove the development of spin-transfer-torque magnetic random-access memories (MRAMs). More recent fundamental research revealed other types of current-induced torques named spin-orbit torques (SOTs) and will lead to a new generation of devices including SOT MRAMs and skyrmion-based devices. Parallel to these advances, multiferroics and their magnetoelectric coupling, first investigated experimentally in the 1960s, experienced a renaissance. Dozens of multiferroic compounds with new magnetoelectric coupling mechanisms were discovered and high-quality multiferroic films were synthesized (notably of ${\mathrm{BiFeO}}_3$), also leading to novel device concepts for information and communication technology such as the magnetoelectric spin-orbit (MESO) transistor. The story of the electrical switching of magnetization, which is discussed in this review, is that of a dance between fundamental research (in spintronics, condensed matter physics, and materials science) and technology (MRAMs, MESO transistors, microwave emitters, spin diodes, skyrmion-based devices, components for neuromorphics, etc.). This pas de deux has led to major scientific and technological breakthroughs in recent decades (such as the conceptualization of pure spin currents, the observation of magnetic skyrmions, and the discovery of spin-charge interconversion effects). As a result, this field has not only propelled MRAMs into consumer electronics products but also fueled discoveries in adjacent research areas such as ferroelectrics or magnonics. In this review, recent advances in the control of magnetism by electric fields and by current-induced torques are covered. Fundamental concepts in these two directions are reviewed first, their combination is then discussed, and finally current various families of devices harnessing the electrical control of magnetic properties for various application fields are addressed. The review concludes by giving perspectives in terms of both emerging fundamental physics concepts and new directions in materials science.

Scalarization is a mechanism that endows strongly self-gravitating bodies, such as neutron stars and black holes, with a scalar-field configuration. It resembles a phase transition in that the scalar configuration appears only when a certain quantity that characterizes the compact object, for example, its compactness or spin, is beyond a threshold. A critical and comprehensive review of scalarization, including the mechanism itself, theories that exhibit it, its manifestation in neutron stars, black holes and their binaries, potential extension to other fields, and a thorough discussion of future perspectives, is provided.

Angle-resolved photoemission spectroscopy (ARPES), with its exceptional sensitivity to both the binding energy and the momentum of valence electrons in solids, provides unparalleled insight into the electronic structure of quantum materials. Over the past two decades, the advent of femtosecond lasers, which can deliver ultrashort and coherent light pulses, has ushered the ARPES technique into the time domain. Currently time-resolved ARPES (TR-ARPES) can probe ultrafast electron dynamics and the out-of-equilibrium electronic structure, providing a wealth of information that is otherwise unattainable in conventional ARPES experiments. This review begins with an introduction to the theoretical underpinnings of TR-ARPES followed by a description of recent advances in state-of-the-art ultrafast sources and optical excitation schemes. It then reviews paradigmatic phenomena investigated by TR-ARPES thus far, such as out-of-equilibrium electronic states and their spin dynamics, Floquet-Volkov states, photoinduced phase transitions, electron-phonon coupling, and surface photovoltage effects. Each section highlights TR-ARPES data from diverse classes of quantum materials, including semiconductors, charge-ordered systems, topological materials, excitonic insulators, Van der Waals materials, and unconventional superconductors. These examples demonstrate how TR-ARPES has played a critical role in unraveling the complex dynamical properties of quantum materials. The conclusion outlines possible future directions and opportunities.

Diffusion driven by temperature or concentration gradients is a fundamental mechanism of energy and mass transport that inherently differs from wave propagation in both physical foundations and application prospects. Compared with conventional schemes, metamaterials provide an unprecedented potential for governing diffusion processes, based on emerging theories like the transformation and the scattering-cancellation theory that expanded the original concepts and suggested innovative metamaterial-based devices. The term diffusionics is used in the review to generalize these noteworthy achievements in various energy and mass diffusion systems. Examples include heat diffusion systems and particle and plasma diffusion systems. For clarity the numerous studies published over the past decade are categorized by diffusion field (i.e., heat, particles, and plasmas) and discussed from three different perspectives: the theoretical perspective, to detail how the transformation principle is applied to each diffusion field; the application perspective, to introduce various interesting metamaterial-based devices, such as cloaks and radiative coolers; and the physics perspective, to connect them with concepts of recent concern, such as non-Hermitian topology, nonreciprocal transport, and spatiotemporal modulation. The possibility of controlling diffusion processes beyond metamaterials is also discussed. Finally, several future directions for diffusion metamaterial research, including the integration of metamaterials with artificial intelligence and topology concepts, are examined.

A plethora of two-dimensional (2D) materials have been introduced in physics and engineering in the past two decades. Their robust, membranelike sheets permit (mostly require) deposition, giving rise to solid-solid dry interfaces whose mobility, pinning, and general tribological properties under shear stress are currently being understood and controlled, both experimentally and theoretically. In this Colloquium simulated case studies of twisted graphene systems are used as a prototype workhorse tool to demonstrate and discuss the general picture of 2D material interface sliding. First highlighted is the crucial mechanical difference, often overlooked, between small and large incommensurabilities, which corresponds to, for example, small and large twist angles in graphene interfaces. In both cases, focusing on flat, structurally lubric or “superlubric” geometries, the generally separate scalings with the area of static friction in pinned states and of kinetic friction during sliding are elucidated and reviewed, tangled as they are with the effects of velocity, temperature, load, and defects. The roles of island boundaries and elasticity are also discussed, and compared when possible to results in the literature for systems other than graphene. It is proposed that the resulting picture of pinning and sliding should be applicable to interfaces in generic 2D materials that are of importance for the physics and technology of existing and future bilayer and multilayer systems.

A comprehensive theory of the Lamb shift in light muonic atoms such as $\mu \mathrm{H}$, $\mu \mathrm{D}$, ${\mu }^3{\mathrm{He}}^{+}$, and ${\mu }^4{\mathrm{He}}^{+}$ is presented, with all quantum electrodynamic corrections included at the precision level constrained by the uncertainty of nuclear structure effects. This analysis can be used in the global adjustment of fundamental constants and in the determination of nuclear charge radii. Further improvements in the understanding of electromagnetic interactions of light nuclei will allow for a promising test of fundamental interactions by comparison with “normal” atomic spectroscopy, in particular, with H-D and ${}^3\mathrm{He}\text{−}{}^4\mathrm{He}$ isotope shifts.

The burgeoning field of “fractons,” a class of models where quasiparticles are strictly immobile or display restricted mobility that can be understood through generalized multipolar symmetries and associated conservation laws, is reviewed. With a focus on merely a corner of this fast-growing subject, it is demonstrated how one class of such theories, symmetric tensor and coupled-vector gauge theories, surprisingly emerge from familiar elasticity of a two-dimensional quantum crystal. The disclination and dislocation crystal defects, respectively, map onto charges and dipoles of the fracton gauge theory. This fracton-elasticity duality leads to predictions of fractonic phases and quantum phase transitions to their descendants that are duals of the commensurate crystal, supersolid, smectic, and hexatic liquid crystals, as well as amorphous solids, quasicrystals, and elastic membranes. It is shown how these dual gauge theories provide a field-theoretic description of quantum melting transitions through a generalized Higgs mechanism. It is demonstrated how they can be equivalently constructed as gauged models with global multipole symmetries. Extensions of such gauge-elasticity dualities to generalized elasticity theories are expected to provide a route to the discovery of new fractonic models and their potential experimental realizations.

Proton imaging has become a key diagnostic for measuring electromagnetic fields in high-energy-density (HED) laboratory plasmas. Compared to other techniques for diagnosing fields, proton imaging is a measurement that can simultaneously offer high spatial and temporal resolution and the ability to distinguish between electric and magnetic fields without the protons perturbing the plasma of interest. Consequently, proton imaging has been used in a wide range of HED experiments, from inertial-confinement fusion to laboratory astrophysics. An overview is provided on the state of the art of proton imaging, including a discussion of experimental considerations like proton sources and detectors, the theory of proton-imaging analysis, and a survey of experimental results demonstrating the breadth of applications. Topics at the frontiers of proton-imaging development are also described, along with an outlook on the future of the field.

The physics of the gravitational form factors of the proton, as well as their understanding within quantum chromodynamics, has advanced significantly in the past two decades through both theory and experiment. This Colloquium provides an overview of this progress, highlights the physical insights unveiled by studies of gravitational form factors, and reviews their interpretation in terms of the mechanical properties of the proton.

A quantum internet is the holy grail of quantum information processing, enabling the deployment of a broad range of quantum technologies and protocols on a global scale. However, numerous challenges must be addressed before the quantum internet can become a reality. Perhaps the most crucial of these is the realization of a quantum repeater, an essential component in the long-distance transmission of quantum information. As the analog of a classical repeater, extender, or booster, the quantum repeater works to overcome loss and noise in the quantum channels constituting a quantum network. Here the conceptual frameworks and architectures for quantum repeaters, as well as the experimental progress toward their realization, are reviewed. Various near-term proposals to overcome the limits to the communication rates set by point-to-point quantum communication are also discussed. Finally, the manner in which quantum repeaters fit within the broader challenge of designing and implementing a quantum internet is overviewed.