The spin state of two electrons in a double well is a promising qubit. Now, such qubits can be rotated around two different axes — enabling, for example, a spiral trajectory of the qubit along the surface of the Bloch sphere — by applying an appropriate sequence of voltage pulses to the two device contacts that control the charge configuration (measured as the conductance) of the double well. Article p903 Cover design by David Shand, based on original artwork by Matteo Giuricin (Thehouseofmouse)

High-harmonic generation could be the basis of frequency combs for vacuum–ultraviolet wavelengths. But first we need a better understanding of harmonic orders produced at photon energies lower than the gas-ionization threshold. It is now shown that harmonics as low as the seventh have contributions from different quantum paths and that their temporal coherence is long enough for a frequency comb. Letter p815 Cover design by David Shand

In semiconductor quantum dots, the interaction between electrons and the surrounding nuclear spins limits the attainable electron-spin-coherence time. But the nuclear-spin reservoir can take a constructive role, as two independent studies show, in which it provides a feedback mechanism to actively lock electronic resonances to changing external driving fields. Articles p758 and p764; News & Views p710 Cover design by David Shand

The dynamics at a black-hole event horizon are tricky to study experimentally. But by exploiting the strong nonlinear optical response of metamaterials it could be possible to engineer optical singularites that mimic the behaviour of black holes and other massive celestial objects – and bring experimental astrophysics into the lab. Article p687 Cover design by David Shand

The reversed-field pinch is a potential alternative to the tokamak for confining a hydrogen plasma in the development of fusion-based power generation. Until now, these devices had been plagued by instabilities that prevented them from sustaining the dense hot plasmas needed to cause hydrogen isotopes to fuse. A demonstration of the evolution of a plasma confined in a reversed-field-pinch device towards a stable self-organized state suggests a solution to such problems could soon be at hand. Letter p570 Cover design by David Shand

A crystal made of several hundred ions — its fluorescence signal is shown at the centre of the cover image — has been coupled strongly to the modes of an optical cavity. Cavity QED with ion crystals offers a fresh approach to creating efficient light–matter interfaces and quantum memories, and potentially to cavity optomechanics. Letter p494; News & Views p455 Cover design by David Shand

From a map, created using traction microscopy, of the physical forces at play in a sheet of canine kidney cells as the colony expands, it is clear that the cells many rows from the front do most of the work. This is contrary to the current belief that sheets of cells move through traction forces exerted by the cells at the leading edge. Letter p426; News & Views p377 Cover design by David Shand

The coupling between the electronic and vibration modes in suspended carbon nanotubes has now been shown to be so strong that it can suppress the flow of current — behaviour known as Franck–Condon blockade. This could enable the development of novel electromechanical devices and provide another means of studying mechanical motion at the quantum limit. Letter p327 Cover design by David Shand

Topological insulators are band insulators with large spin–orbit coupling, in which surface states give rise to gap-traversing modes that are robust against scattering. Theory now predicts that screw dislocations and other types of defects can give rise to one-dimensional topologically protected states in such systems. Article p298 Cover design by David Shand

Darwin and physics? The relevance of Darwin's ideas in physics, the wider context of Darwin's legacy and the controversy that rumbles on are explored in a special collection of articles marking the anniversary of the theory of evolution. (The background image is derived from spin polarization patterns in a spin–orbit-coupled 2D electron system, courtesy of Patrik Brusheim, iHPC, Singapore.) Editorial p161; Commentary p162 and p164; Thesis p167; Books & Arts p169 and p170; Progress Article p181 Cover design by David Shand

The interaction between single atoms and the electromagnetic field can be enhanced using a cavity, the signature of strong coupling being 'vacuum Rabi splitting'. An experiment in which a superconducting circuit acts as an 'artificial atom' explores the physics beyond this linear-response effect and shows how nonlinear phenomena lead to the rich structure of the cavity transmission peaks shown on the cover. Letter p105

Cover design by David Shand

Free electron lasers capable of generating intense bursts of coherent X-rays promise unprecedented spatial and temporal resolution in the study of the microstructure and dynamics of materials. Using a new algorithm, the structure of a molecule can be calculated from the faintest of diffraction patterns, produced by arbitrarily oriented individual molecules dropped one by one into a free electron laser's beam. Article p64; News & Views p17

Cover design by David Shand