Live neuronal cultures patterned into quasi-one-dimensional arrays can function, depending on the configuration, as diodes (shown on the cover) or as logic gates. The work also illustrates the role of redundancy in such complex networks; the devices work with lower error rates than the constituent neurons, suggesting that redundancy-based error correction is at work. Article p967; News & Views p905

Cover design by David Shand

The precision of interferometric measurements can be enhanced by using entangled states; these, however, are typically difficult to generate, fragile and low in intensity. A classical analogue to the exemplary Hong–Ou–Mandel quantum interferometer offers all the metrological advantages of the quantum version — the characteristic interference dip is seen at the centre of the cover image — but at higher signal intensity. Letter p864

Cover design by David Shand

Fifty years ago, Philip Anderson published a groundbreaking paper suggesting that the scattering of electrons in a disordered lattice could effectively bring them to a standstill. The signature of this behaviour, known as Anderson localization, has since been observed in many different systems, yet achieving the same for light in three dimensions is remarkably difficult. Massively parallel simulations of defect-induced scattering in a photonic crystal suggest that Anderson localization of light may only occur when the amount of disorder in such a system is within a certain range. Article p794; News & Views p755

Cover design by David Shand

All electric and magnetic fields must satisfy Maxwell's equations. This constrains the shapes that field-lines can take. There are exotic solutions to Maxwell's equations that produce complex interlinked and knotted field-line topologies, but until now, it has not been clear how they might be generated. William Irvine and Dirk Bouwmeester study these solutions in the context of optical fields and propose experiments to create novel beams of light, which they expect to have applications ranging from particle trapping to plasma confinement. Letter p716

Cover design by David Shand

Emission coherence is crucial to the potential of future X-ray sources based on high-order harmonic generation from laser-driven plasmas. A demonstration of the interference between three highorder harmonic beams produced from a plasma mirror proves that such coherence is possible, but only if the laser pulses that drive them are temporally sharp. Letter p631

Cover design by David Shand

Progress in the emerging field of attosecond science continues apace. The emission of attosecond pulses by the interaction of a high-intensity laser with an atomic gas is well known. But for greater versatility and control it could be preferable to use molecules. To this end, Willem Boutu and colleagues exploit quantum interference effects between intramolecular electronic states of aligned carbon dioxide molecules to coherently control the emission of attosecond pulses of extreme-ultraviolet light. Letter p545

Cover design by David Shand

Solitons are encountered in a wide range of nonlinear systems, in water channels and in optical fibres, but also in Bose–Einstein condensates (BECs). Christoph Becker and colleagues now study the dynamics of 'BEC solitons' in detail. They create dark solitons — basically a notch in the condensate — as well as dark–bright solitons, where the notch is filled with atoms in another quantum state (the cover pictures the two components of such a dark–bright soliton). The experiments reveal the solitons' particle-like motion, including collisions between the two types of solitons. Article p496; News & Views p437

Cover design by David Shand

Spin-polarized neutrons are sensitive to magnetic fields — a property that Nikolay Kardjilov and colleagues use to visualize the three-dimensional distribution of magnetic fields, by analysing the precession of the neutron's magnetic moment. Pictured is the field surrounding a dipole magnet, as detected by the neutrons. But as neutrons can penetrate through matter relatively easily, the approach can also be used to probe magnetic fields inside massive objects. Letter p399

Cover design by David Shand

Geological landscapes usually take millions of years to form. It is therefore difficult to model their growth or predict how the formations will evolve. However, the spectacular limestone terraces and cascades that form at geothermal hotsprings can grow at a rate of 5 mm per day. This expedited growth rate has enabled John Veysey and Nigel Goldenfeld to test their simulations of pond growth (pictured) in real-time. Compared to the actual landscape at Mammoth Hot Springs, Yellowstone National Park, USA – which has been captured on film for a period of two years – their simulation agrees well and displays self-similarity on all length-scales. Letter p310; News & Views p265 Image courtesy of John Veysey and Nigel Goldenfeld, rendered with the assistance of Nicholas Guttenberg.

Phase transitions are familiar occurrences — for example, the freezing of water to ice. When the transition occurs at absolute zero, it is known as a 'quantum phase transition'. As distinct states of matter coexist at a transition, there are quantum fluctuations between them. This Focus explores the resulting — and often surprising — collective behaviour. [Focus p167-204] Cover design by David Shand

A superconducting current running in a closed loop is an ideal demonstration of quantum physics. As all the electrons have the same wavefunction, the amount of magnetic field that can pass through the ring – the magnetic flux – must be quantized in units of the Planck constant, h, divided by the total charge. But in a superconductor, the electrons are bound into pairs, so the total charge is 2e, yielding a fluxoid of h/2e – a hallmark of superconductivity. Now, however, Florian Loder and co-workers show that for certain superconductors, a fluxoid of h/e is possible.

String theory involves more dimensions than we can see, but our Universe may be restricted to a three-dimensional 'brane' embedded in a higher-dimensional space – an explanation that is convenient yet impossible to test. Branes break a symmetry in a way similar to superfluid 3He, in which the atoms pair up and form a ground-state condensate. Could the interaction of two interfaces within 3He mimic a collision of two branes – say a brane-antibrane pair that some believe led to the Big Bang? Such a collision would leave certain topological defects behind, which is indeed what D. Ian Bradley and co-workers have observed in 3He. [Letter p46]; [News & Views p11]