Volume 6 Issue 5, May 2010

The rich history of research involving photons is celebrated in a supplement to this month's Nature Physics.

Magnetic monopoles are sources or sinks of magnetic field that have been detected experimentally but remain abstract. Thanks to an artificial lattice of magnetic nanowires, it is now possible to observe monopoles and watch them move.

Clouds of uncharged particles such as sand or volcanic ash become charged by some undetermined mechanism. Experiments now show that nearby electric fields could be responsible.

Most computer processors work in series, performing one instruction at a time. This limits the speed with which they can carry out certain types of task. A parallel computational approach based on arrays of simultaneously interacting molecular switches could provide a more efficient solution.

An experiment with ultracold gases reveals how weakly interacting atoms cooperate to protect long-range coherence against disorder-induced localization, and should offer insight into long-standing questions on the complex interplay of interactions and disorder in quantum systems.

Ferromagnetism and superconductivity are eternal enemies, so a current of superconducting pairs of electrons travelling within a ferromagnet raises several questions.

Creating entangled photon states becomes technologically ever more difficult as the number of particles increases, and the current record stands at six entangled photons. However, using both their polarization and momentum degrees of freedom, up to ten-qubit states can be encoded in ‘only’ five photons, as has now been demonstrated.

The ability to produce spin-polarized currents in a quantum wire is crucial for spin-based electronics. Fortunately, the spin–orbit interaction can be exploited to deliver pure spin currents, without charge currents, that travel in one direction only.

It is well known that a spin-polarized current can be used to manipulate the orientation of nanometre-scale magnets. This ability has now been extended to control the spin orientation of magnetic atoms adsorbed on a surface.

Although the valence and conduction electrons of a solid exist in states that extend throughout the solid, those of the innermost shells of its constituent atoms are usually considered to be strongly localized. A study of the photoelectron emission spectrum from graphene suggests that this isn’t always the case.

‘Quantum’ lasers — with one atom interacting with a single optical mode — do not exhibit a conventional-laser-like threshold in the regime of strong light–matter coupling. A study now shows that a threshold is seen when the coupling strength is reduced, which represents the onset of classical lasing.

In bosonic many-body systems, disorder tends to localize particles, whereas weak repulsive interactions between the particles have a delocalizing effect. The crossover between these regimes has now been studied experimentally, using an optical lattice to control disorder and interactions independently.

Artificial spin ices contain magnetic defects that mimic magnetic monopoles, which move along strings as predicted by Paul Dirac. A study now shows that the formation and subsequent motion of monopole defects in an artificial spin-ice system can be visualized directly using magnetic force microscopy.

Granular flows, such as in silos or desert sandstorms, can form charged particle clouds in the presence of an electric field. Simulations and experiments on inert grains explain how significant electrical charges are able to accumulate.

The processors of most computers work in series, performing one instruction at a time. This limits their ability to perform certain types of tasks in a reasonable period. An approach based on arrays of simultaneously interacting molecular switches could enable previously intractable computational problems to be solved.

Mott insulators are driven by strong Coulomb repulsion and topological insulators by strong spin–orbit coupling. Although these effects are normally in competition, in some cases the Coulomb interaction can enhance the effects of spin–orbit coupling. Together these interactions could lead to gapless spin-only excitations on the surface of a strongly correlated insulator.

Building on recent experimental advances in controlling individual Rydberg atoms, theoretical work proposes a ‘Rydberg quantum simulator’. Such a system would be suitable for efficiently simulating other quantum systems with many-body interactions and strongly correlated ground states.

When a superconductor is in contact with a normal metal, Cooper pairs from the superconductor ‘leak’ into the metal, causing local superconductivity. When in contact with a ferromagnet, however, Cooper pairs do not stray very far. Therefore, the discovery that a ferromagnetic nanowire goes completely superconducting when placed between two superconducting electrodes is surprising indeed.