A number of mechanisms for generation of x rays from the interaction of short-pulse lasers with underdense plasmas have recently been identified and explored, through both simulation and experiment. The x rays result from electrons accelerated in the plasma. The radiation is well collimated and incoherent within the femtosecond regime. This article reviews these mechanisms including the parameters needed for effective generation as well as the characteristics of the generated radiation, and discusses the perspectives for using a laser-plasma accelerator to realize a compact free-electron laser in the x-ray spectrum.

Pions, the lightest mesons, are considered to be the Goldstone bosons associated with the spontaneously broken chiral symmetry. The neutral pion plays an important role in our understanding of QCD, the theory of the strong force. This short-lived particle primarily decays into two gamma ray photons. Its mean lifetime carries information about the anomalous nonconservation of a chiral current, the so called chiral anomaly. The experimental and theoretical history of the neutral pion is reviewed, from discovery to the present, including suggestions for future work.

Individual electrons and holes confined to a semiconductor quantum dot can be manipulated in optics and transport schemes that aim for control on a single spin and single photon level. The coherence properties of these single carrier states are governed by the interaction with the mesoscopic nuclear spin ensemble of the lattice nuclei that form the dot. This article reviews the highly nonlinear electron spin–nuclear spin interaction in quantum dot optics experiments that explore ways of controlling this coupled spin system.

In a living cell, the transport of the proper molecules to the proper locations within the cell is critical. Since Brownian motion dominates at the scale of a cell, models for understanding cellular transport must be stochastic. In this review analytical techniques for stochastic models of intracellular transport via the two basic mechanisms of passive diffusion and motor-driven transport are presented. These models have a wide application to biological problems, ranging from axonal transport to protein-DNA interactions to the self-organization of subcellular structures.

When particles strongly interact, new phenomena can emerge. In this Colloquium, the significance and implications of interactions between three and more particles in atoms and nuclei are discussed. Particular emphasis is on the challenges this creates for understanding ultracold atoms, nuclear structure and astrophysics, as well as for fundamental symmetries.

Magnetism is the prototypical many-body phenomenon. It is only through spin-spin interactions that symmetries can be broken and magnetic order or a spin-liquid-like state can be achieved. Collective magnetic waves, or magnons, are the low-energy excitations in magnetic materials. These “particles,” created by the spin-spin interactions, can also interact with each other and their environment and, just as electrons and phonons in solids, have a finite lifetime. In this Colloquium, the theoretical underpinnings and experimental results of how these particles behave in complex magnets are discussed.

Even stars die, and when those most massive do, they do it in the most spectacular ways. However, our theoretical understanding of the mechanisms of one of nature’s most extraordinary events is still in its infancy. In this Colloquium, some theoretical underpinnings of core-collapse supernovae and the intriguing questions that remain to be answered are discussed.

A comprehensive review of radiative corrections to electroweak processes is presented, in both the Fermi theory of weak interactions and the standard model. The historical development of the study of radiative corrections, their crucial role in verifying the standard model, and advances in theoretical particle physics closely connected with them are reviewed in detail. The role of radiative corrections in the analysis of important signals of new physics is also discussed.

In vacuum an assembly of photons is a textbook example of a noninteracting Bose gas, each photon crossing the container along a straight line independently from all others. In a medium, the situation can be much richer due to the effective photon-photon interaction that appears in the presence of optical nonlinearity. The many interesting collective features that these fluids of light can then exhibit are reviewed, such as superfluid flow, solitons, vortices, and even the strongly correlated regime in which new quantum phases are expected.

Magnetic clusters, that is, isolated assemblies of spins ranging in number from a few to several hundred, are both ideal model systems for the study of magnetic interactions and important for the understanding of magnetic molecules. This review focuses on theoretical and experimental techniques for studying excitations of such clusters, which give information about their physical properties.

Semiconductor lasers were invented nearly two years after the demonstration of the first laser. Nowadays, they account for more than 50% of current lasers. They are used in optical storage and communication systems, as pump sources, for material processing, just to name a few applications. This review discusses work devoted to harnessing the complex dynamics of semiconductor lasers with delayed coupling, with applications in encrypted communications, classical key exchange, remote sensing, and fast random bit sequence generation.