Knowing the physicochemical properties of exhaled droplets and aerosol particles is a prerequisite for a detailed mechanistic understanding and effective prevention of the airborne transmission of infectious human diseases. This review provides a critical consideration and synthesis of scientific knowledge on the number concentrations, size distributions, composition, mixing state, and related properties of respiratory particles emitted upon breathing, speaking, singing, coughing, and sneezing. A parametrization of respiratory particle size distributions is derived and presented based on five log-normal modes related to different origins in the respiratory tract, which can be used to trace and localize the sources of infectious particles. This approach may support the medical treatment as well as the risk assessment for aerosol and droplet transmission of infectious diseases. It was applied to analyze which respiratory activities may drive the spread of specific pathogens, such as Mycobacterium tuberculosis, influenza viruses, and severe acute respiratory syndrome coronaviruses 2 (SARS-CoV-2). The results confirm the high relevance of vocalization for the transmission of SARS-CoV-2, as well as the usefulness of physical distancing, face masks, room ventilation, and air filtration as preventative measures against coronavirus disease 2019 and other airborne infectious diseases.

Accelerator mass spectrometry (AMS) was born in the late 1970s, when it was realized at nuclear physics laboratories that the accelerator systems can be used as a sensitive mass spectrometer to measure ultralow traces of long-lived radioisotopes. It soon became possible to measure radioisotope-to-stable-isotope ratios in the range from ${10}^{-12}$ to ${10}^{-16}$ by counting the radioisotope ions “atom by atom” and comparing the count rate with ion currents of stable isotopes ($1.6\mathrm{\mu }\mathrm{A}=1\times {10}^{13}$ singly charged ions/s). It turned out that electrostatic tandem accelerators are best suited for this, and there are now worldwide about 160 AMS facilities based on this principle. This review presents the history, technological developments, and research areas of AMS through the 45 yr since its discovery. Many different fields are touched by AMS measurements, including archaeology, astrophysics, atmospheric science, biology, climatology, cosmic-ray physics, environmental physics, forensic science, glaciology, geophormology, hydrology, ice core research, meteoritics, nuclear physics, oceanography, and particle physics. Since it is virtually impossible to discuss all fields in detail in this review, only specific fields with recent advances are highlighted in detail. For the others, an effort is made to provide relevant references for in-depth studies of the respective fields.

Diffusion occurs in numerous physical systems throughout nature, drawing its generality from the universality of the central limit theorem. Approximately a century ago it was realized that an extension to this type of dynamics can be obtained in the form of “anomalous” diffusion, where distributions are allowed to have heavy power-law tails. Owing to a unique feature of its momentum-dependent dissipative friction force, laser-cooled atomic ensembles can be used as a test bed for such dynamics. The interplay between laser cooling and anomalous dynamics bears deep predictive implications for fundamental concepts in both equilibrium and nonequilibrium statistical physics. The high degree of control available in cold-atom experiments allows for the parameters of the friction to be tuned, revealing transitions in the dynamical properties of the system. Rare events in both the momentum and spatial distributions are described by non-normalized states using tools adapted from infinite ergodic theory. This leads to new experimental and theoretical results that illuminate the various features of the system.

Fast radio bursts (FRBs), millisecond-duration bursts prevailing in the radio sky, are the latest large puzzle in the Universe and have been a subject of intense observational and theoretical investigations in recent years. The rapid accumulation of observational data has painted the following sketch about the physical origin of FRBs: They predominantly originate from cosmological distances, so their sources produce the most extreme coherent radio emission in the Universe; at least some, probably most, FRBs are repeating sources that do not invoke cataclysmic events; and at least some FRBs are produced by magnetars, neutron stars with the strongest magnetic fields in the Universe. Many open questions regarding the physical origin(s) and mechanism(s) of FRBs remain. This review addresses the phenomenology and possible underlying physics of FRBs. Topics include a summary of the observational data, basic plasma physics, general constraints on FRB models from the data, radiation mechanisms, source and environment models, and propagation effects, as well as FRBs as cosmological probes. Current pressing problems and future prospects are also discussed.

The heavy-fermion metal ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ was the first discovered unconventional, non-phonon-mediated superconductor and, for a long time, was believed to exhibit single-band $d$-wave superconductivity, as inferred from various measurements hinting at a nodal gap structure. More recently, however, measurements using a range of techniques at low temperatures ($T⪅0.1\mathrm{K}$) provided evidence for a fully gapped superconducting order parameter. In this Colloquium, after a historical overview the apparently conflicting results of numerous experimental studies on this compound are surveyed. The different theoretical scenarios that have been applied to understanding the particular gap structure are then addressed, including both isotropic (sign-preserving) and anisotropic two-band $s$-wave superconductivity, as well as an effective two-band $d$-wave model, where the latter can explain the currently available experimental data on ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. The lessons from ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ are expected to help uncover the Cooper-pair states in other unconventional, fully gapped superconductors with strongly correlated carriers, and, in particular, highlight the rich variety of such states enabled by orbital degrees of freedom.

This review addresses the method of explicit calculations of interatomic exchange interactions of magnetic materials. This involves exchange mechanisms normally referred to as a Heisenberg exchange, a Dzyaloshinskii-Moriya interaction, and an anisotropic symmetric exchange. The connection between microscopic theories of the electronic structure, such as density functional theory and dynamical mean-field theory, and interatomic exchange is examined. The different aspects of extracting information for an effective spin Hamiltonian that involves thousands of atoms, from electronic structure calculations considering significantly fewer atoms (1–50), is highlighted. Examples of exchange interactions of a large group of materials is presented, which involves heavy elements of the $3d$ period, alloys between transition metals, Heusler compounds, multilayer systems as well as overlayers and adatoms on a substrate, transition metal oxides, $4f$ elements, magnetic materials in two dimensions, and molecular magnets. Where possible, a comparison to experimental data is made that becomes focused on the magnon dispersion. The influence of relativity is reviewed in a few cases, as is the importance of dynamical correlations. Development to theories that handle out-of-equilibrium conditions is also described here. The review ends with a description of extensions of the theories behind explicit calculations of interatomic exchange to nonmagnetic situations, such as those that describe chemical (charge) order and superconductivity.

The weak gravity conjecture holds that in a theory of quantum gravity any gauge force must mediate interactions stronger than gravity for some particles. This statement has surprisingly deep and extensive connections to many different areas of physics and mathematics. Several variations on the basic conjecture have been proposed, including statements that are much stronger but are nonetheless satisfied by all known consistent quantum gravity theories. These related conjectures and the evidence for their validity in the string theory landscape are reviewed. Also reviewed are a variety of arguments for these conjectures, which tend to fall into two categories: qualitative arguments that claim the conjecture is plausible based on general principles and quantitative arguments for various special cases or analogs of the conjecture. The implications of these conjectures for particle physics, cosmology, general relativity, and mathematics are also outlined. Finally, important directions for future research are highlighted.

In this review recent investigations are summarized of many-body quantum systems with long-range interactions, which are currently realized in Rydberg atom arrays, dipolar systems, trapped-ion setups, and cold atoms in cavities. In these experimental platforms parameters can be easily changed, and control of the range of the interaction has been achieved. The main aim of the review is to present and identify the common and mostly universal features induced by long-range interactions in the behavior of quantum many-body systems. Discussed are the case of strong nonlocal couplings, i.e., the nonadditive regime, and the one in which energy is extensive, but low-energy, long-wavelength properties are altered with respect to the short-range case. When possible, comparisons with the corresponding results for classical systems are presented. Finally, cases of competition with local effects are also reviewed.

This is an extended version of my Nobel Lecture, delivered on December 8, 2021. I will recall the genesis of the concept of multiple equilibria in natural sciences. I will then describe my contribution to the development of this concept in the framework of statistical mechanics. Finally, I will briefly mention the cornucopia of applications of these ideas both in physics and in other disciplines.

Quantum random sampling is the leading proposal for demonstrating a computational advantage of quantum computers over classical computers. Recently the first large-scale implementations of quantum random sampling have arguably surpassed the boundary of what can be simulated on existing classical hardware. Here the theoretical underpinning of quantum random sampling is comprehensively reviewed in terms of computational complexity and verifiability, as are the practical aspects of its experimental implementation using superconducting and photonic devices and its classical simulation. Open questions in the field are discussed, and perspectives for the road ahead, including potential applications of quantum random sampling, are provided.

The spontaneous breaking of time-translation symmetry has led to the discovery of a new phase of matter: the discrete time crystal. Discrete time crystals exhibit rigid subharmonic oscillations that result from a combination of many-body interactions, collective synchronization, and ergodicity breaking. This Colloquium reviews recent theoretical and experimental advances in the study of quantum and classical discrete time crystals. The breaking of ergodicity is focused upon as the key to discrete time crystals and the delaying of ergodicity as the source of numerous phenomena that share many of the properties of discrete time crystals, including the ac Josephson effect, coupled map lattices, and Faraday waves. Theoretically, there is a diverse array of strategies to stabilize time-crystalline order in both closed and open systems, ranging from localization and prethermalization to dissipation and error correction. Experimentally, many-body quantum simulators provide a natural platform for investigating signatures of time-crystalline order; recent work utilizing trapped ions, solid-state spin systems, and superconducting qubits are reviewed. Finally, this Colloquium concludes by describing outstanding challenges in the field and a vision for new directions on both the experimental and theoretical fronts.

Inertial confinement fusion (ICF) has existed as a field of study since the 1970s, but the field was born out of the Cold War. In the decades since the 1960s, pioneering research developing the principles and technologies of ICF has culminated in the creation of three major Department of Energy facilities that still exist today: the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, the OMEGA laser at the Laboratory for Laser Energetics, and the $Z$ pulsed power facility at Sandia National Laboratories. While the technology of ICF facilities themselves is interesting, this review concentrates upon the physics principles of the targets fielded on U.S. ICF facilities and upon results from the last decade of research. While there have been periods of frustration on the road to ICF ignition, recent research has demonstrated great leaps in understanding what aspects of the implosions need more control. Tangible progress in ICF is evident as burning plasmas and ignited plasmas have recently been generated, repeatedly, on the NIF stemming from decades of science and engineering understanding generated from work at the three previously mentioned facilities and in the international community.

Innovations in fluid mechanics have been leading to better food since ancient history, while creativity in cooking has inspired fundamental breakthroughs in science. This review addresses how recent advances in hydrodynamics are changing food science and the culinary arts and, reciprocally, how the surprising phenomena that arise in the kitchen are leading to new discoveries across the disciplines, including molecular gastronomy, rheology, soft matter, biophysics, medicine, and nanotechnology. This review is structured like a menu, where each course highlights different aspects of culinary fluid mechanics. Our main themes include multiphase flows, complex fluids, thermal convection, hydrodynamic instabilities, viscous flows, granular matter, porous media, percolation, chaotic advection, interfacial phenomena, and turbulence. For every topic, an introduction and its connections to food are provided, followed by a discussion of how science could be made more accessible and inclusive. The state-of-the-art knowledge is then assessed, the open problems, along with the likely directions for future research and indeed future dishes. New ideas in science and gastronomy are growing rapidly side by side.

The spin degree of freedom of an electron or a nucleus is one of the most basic properties of nature and functions as an excellent qubit, as it provides a natural two-level system that is insensitive to electric fields, leading to long quantum coherence times. This coherence survives when the spin is isolated and controlled within nanometer-scale, lithographically fabricated semiconductor devices, enabling the existing microelectronics industry to help advance spin qubits into a scalable technology. Driven by the burgeoning field of quantum information science, worldwide efforts have developed semiconductor spin qubits to the point where quantum state preparation, multiqubit coherent control, and single-shot quantum measurement are now routine. The small size, high density, long coherence times, and available industrial infrastructure of these qubits provide a highly competitive candidate for scalable solid-state quantum information processing. Here the physics of semiconductor spin qubits is reviewed, with a focus not only on the early achievements of spin initialization, control, and readout in GaAs quantum dots but also on recent advances in Si and Ge spin qubits, including improved charge control and readout, coupling to other quantum degrees of freedom, and scaling to larger system sizes. First introduced are the four major types of spin qubits: single spin qubits, donor spin qubits, singlet triplet spin qubits, and exchange-only spin qubits. The mesoscopic physics of quantum dots, including single-electron charging, valleys, and spin-orbit coupling, are then reviewed. Next a comprehensive overview of the physics of exchange interactions is given, a crucial resource for single- and two-qubit control in spin qubits. The bulk of the review is centered on the presentation of results from each major spin-qubit type, the present limits of fidelity, and an overview of alternative spin-qubit platforms. A physical description of the impact of noise on semiconductor spin qubits, aided in large part by an introduction to the filter-function formalism, is then given. Last, recent efforts to hybridize spin qubits with superconducting systems, including charge-photon coupling, spin-photon coupling, and long-range cavity-mediated spin-spin interactions, are reviewed. Cavity-based readout approaches are also discussed. The review is intended to give an appreciation for the future prospects of semiconductor spin qubits while highlighting the key advances in mesoscopic physics over the past two decades that underlie the operation of modern quantum-dot and donor spin qubits.

The discovery of neutrinoless $\beta \beta$ decay could soon be within reach. This hypothetical ultrarare nuclear decay offers a privileged portal to physics beyond the standard model of particle physics. Its observation would constitute the discovery of a matter-creating process, corroborating leading theories of why the Universe contains more matter than antimatter, and how forces unify at high energy scales. It would also prove that neutrinos and antineutrinos are not two distinct particles but can transform into each other, with their mass described by a unique mechanism conceived by Majorana. The recognition that neutrinos are not massless necessitates an explanation and has boosted interest in neutrinoless $\beta \beta$ decay. The field stands now at a turning point. A new round of experiments is currently being prepared for the next decade to cover an important region of parameter space. In parallel, advances in nuclear theory are laying the groundwork to connect the nuclear decay with the underlying new physics. Meanwhile, the particle theory landscape continues to find new motivations for neutrinos to be their own antiparticle. This review brings together the experimental, nuclear theory, and particle theory aspects connected to neutrinoless $\beta \beta$ decay to explore the path toward, and beyond, its discovery.

Positronium is the simplest bound state, built of an electron and a positron. Studies of positronium in vacuum and its decays in medium tell us about quantum electrodynamics (QED) and about the structure of matter and biological processes of living organisms at the nanoscale, respectively. Spectroscopic measurements constrain our understanding of QED bound state theory. Searches for rare decays and measurements of the effect of gravitation on positronium are used to look for new physics phenomena. In biological materials positronium decays are sensitive to the intermolecular and intramolecular structure and to the metabolism of living organisms ranging from single cells to human beings. This leads to new ideas of positronium imaging in medicine using the fact that during positron emission tomography (PET) as much as 40% of positron annihilation occurs through the production of positronium atoms inside the patient’s body. A new generation of the high sensitivity and multiphoton total-body PET systems opens perspectives for clinical applications of positronium as a biomarker of tissue pathology and the degree of tissue oxidation.

The past decade has witnessed dramatic progress related to various aspects of emergent topological polar textures in oxide nanostructures displaying vortices, skyrmions, merons, hopfions, dipolar waves, or labyrinthine domains, among others. For a long time, these nontrivial structures (the electric counterparts of the exotic spin textures) were not expected due to the high energy cost associated with the dipolar anisotropy: the smooth and continuous evolution of the local polarization to produce topologically protected structures would result in a large elastic energy penalty. However, it was discovered that the delicate balance and intricate interplay between the electric, elastic, and gradient energies can be altered in low-dimensional forms of ferroelectric oxide nanostructures. These can be tuned to manipulate order parameters in ways once considered impossible. This review addresses the historical context that provided the fertile background for the dawning of the polar topological era. This has been possible thanks to a fruitful, positive feedback between theory and experiment: advances in materials synthesis and preparation (with a control at the atomic scale) and characterization have come together with great progress in theoretical modeling of ferroelectrics at larger length and timescales. An in-depth scientific description to formalize and generalize the prediction, observation, and probing of exotic, novel, and emergent states of matter is provided. Extensive discussions of the fundamental physics of such polar textures, a primer explaining the basic topological concepts, an explanation of the modern theoretical and computational methodologies that enable the design and study of such structures, what it takes to achieve deterministic, on-demand control of order-parameter topologies through atomically precise synthesis, the range of characterization methods that are key to probing these structures, and their thermodynamic field-driven (temperature-driven, stress-driven, etc.) susceptibilities are included. The new emergent states of matter join together with exotic functional properties (such as chirality, negative capacitance, and coexistence of phases) that, along with their small size and ultrafast dynamical response, make them potential candidates in multifunctional devices. Finally, some open questions and challenges for the future are presented, underlining the interesting future that is anticipated in this field.

For half a century after the discovery of superconductivity, materials exploration for better superconductors proceeded without knowledge of the underlying mechanism. The 1957 BCS theory cleared that up. The superconducting state occurs due to strong correlation in the electronic system: pairing of electrons over the Fermi surface. Over the following half century a higher critical temperature $T_c$ was achieved only serendipitously as new materials were synthesized. Meanwhile, the formal theory of phonon-coupled superconductivity at the material-dependent level became progressively more highly developed: by 2000, given a known compound, its value of $T_c$, the corresponding superconducting gap function, and several other properties of the superconducting state became available independent of further experimental input. In this century, density-functional-theory-based computational materials design has progressed to a predictive level; new materials can be predicted from free energy functionals on the basis of various numerical algorithms. Taken together these capabilities enable theoretical predictions for new superconductors, justified by applications to superconductors ranging from weak to strong coupling. Limitations of the current procedures are discussed; most of them can be handled with additional procedures. Here the process that has resulted in the three new highest temperature superconductors is recounted, with compressed structures predicted computationally and values of $T_c$ obtained numerically that have subsequently been confirmed experimentally: the designed superconductors ${\mathrm{SH}}_3$, ${\mathrm{LaH}}_{10}$, and ${\mathrm{YH}}_9$. These hydrides have $T_c$ in the 200–260 K range at megabar pressures; the experimental results and confirmations are discussed. While the small mass of hydrogen provides the anticipated strong coupling at high frequency, it is shown that it also enables identification of the atom-specific contributions to coupling, in a manner that was previously possible only for elemental superconductors. The following challenge is posed: that progress in understanding of higher $T_c$ is limited by the lack of understanding of screening of H displacements. Ongoing activities are mentioned and current challenges are suggested, together with regularities that are observed in compressed hydrides that may be useful to guide further exploration.

The Helfrich-Hurault (HH) elastic instability is a well-known mechanism behind patterns that form as a result of strain upon liquid crystal systems with periodic ground states. In the HH model, layered structures undulate and buckle in response to local, geometric incompatibilities in order to maintain the preferred layer spacing. Classic HH systems include cholesteric liquid crystals under electromagnetic field distortions and smectic liquid crystals under mechanical strains, where both materials are confined between rigid substrates. However, richer phenomena are observed when undulation instabilities occur in the presence of deformable interfaces and variable boundary conditions. Understanding how the HH instability is affected by deformable surfaces is imperative for applying the instability to a broader range of materials. In this review, the HH mechanism is reexamined and special focus is given to how the boundary conditions influence the response of lamellar systems to geometrical frustration. Lamellar liquid crystals confined within a spherical shell geometry are used as the model system. Made possible by the relatively recent advances in microfluidics within the past 15 years, liquid crystal shells are composed entirely of fluid interfaces and have boundary conditions that can be dynamically controlled at will. Past and recent work that exemplifies how topological constraints, molecular anchoring conditions, and boundary curvature can trigger the HH mechanism in liquid crystals with periodic ground states is examined. The review ends by identifying similar phenomena across a wide variety of materials, both biological and synthetic. The fact that the HH mechanism is a generic and often overlooked response of periodic materials to geometrical frustration is highlighted.