Volume 7 Issue 8, August 2011

Equilibrium free-energy landscapes supply important information about complex molecules such as nucleic acids and proteins. But can equilibrium landscapes be calculated from measurements on a non-equilibrium system?

In a Mott insulator, repulsive interactions suppress conductivity. Such behaviour has been demonstrated, individually, for both bosonic and fermionic atoms in optical lattices. Now, a Bose–Fermi mixture is found to be Mott insulating too, even when the individual components are not.

A single microwave photon in a superposition of two states of different frequency is now demonstrated using a superconducting quantum interference device to mediate the coupling between two harmonics of a resonator. Such quantum circuits bring closer the possibility of controlling photon–photon interactions at the single-photon level.

The linear and hyperbolic electronic bands of single- and bilayer graphene give rise to quantum Hall effects that are different from those seen in previously studied 2D systems. The electronic structure of trilayer graphene includes both types of band, giving rise to even richer behaviour.

A single microwave photon is prepared in a superposition of two states of different frequency. This is achieved by using a superconducting quantum interference device to mediate the coupling between two harmonics of a superconducting resonator.

The power of magnetic resonance imaging for investigating physical and biological systems is well established. Here, it is shown how the sensitivity of cavity atom optics, together with the control provided by atom chips, enables the implementation of a magnetic-resonance-imaging technique that provides a minimally destructive, state-sensitive detection modality for atoms in ultracold gases.

Twin photons — pairs of highly correlated photons — are one of the building blocks for quantum optics, and are used in both fundamental tests of quantum physics and technological applications. Now an efficient source for correlated atom pairs is demonstrated, promising to enable a wide range of experiments in the field of quantum matter-wave optics.

The study of many fundamental processes in chemistry relies on the understanding of the dynamics of the valence electrons, which make and break chemical bonds. A laser method now provides direct information on the dynamics of the valence electrons—separate from any vibrational motion—during a polyatomic chemical reaction, without the need for strong laser fields that unavoidably influence the motions of these electrons.

Edge effects matter in graphene, particularly in nanoribbons. A study using scanning tunnelling microscopy and spectroscopy reveals how chirality at the atomically well-defined edges of a graphene nanoribbon affects its electronic structure.

The charge carriers in single-layer graphene are effectively massless. In bilayer graphene, they are massive. In trilayer graphene, the two types coexist, which leads to an unusual quantum Hall response in which the Landau levels of massless and massive charge carriers repeatedly cross.

In the past few years, there have been a number of proposals for fabricating magnetic memories based on the current-induced motion of magnetic domain walls. A device that uses a novel geometry for injecting electrical currents into the sample is shown to work with current densities that are two orders of magnitude lower than in previous approaches.

The energy-landscape theory is an important tool for investigating how proteins fold. Hummer and Szabo conceived a simple method for constructing folding-energy landscapes from single-molecule pulling experiments. But are these non-equilibrium measurements a valid approach to equilibrium landscapes? The Hummer–Szabo formalism is now experimentally validated for the first time.

Exciton-polariton fluids—which are composed of composite light–matter bosons—provide an experimental means for studying quantum fluids that are intrinsically out of equilibrium. These authors demonstrate the nucleation and dynamics of vortex–anti-vortex pairs in the flow of exciton-polaritons passing around an obstacle, and establish these systems as a platform for studying quantum turbulence.

In a Mott insulator, strong repulsive interactions suppress conductivity. Such behaviour has been demonstrated, individually, for both bosonic and fermionic atoms confined to optical lattices. Now, a dual Mott insulator of bosons and fermions has been realized, and interspecies interaction are found to markedly modify each Mott insulator.

Its tunable energy bandgap makes bilayer graphene interesting both from a theoretical perspective and with a view to applications. But exactly how the bandgap is formed is still unclear. A scanning tunnelling spectroscopy study now finds that the microscopic picture of the gap is fundamentally different from what is expected from macroscopic measurements and currently developed theories.

A demonstration of attosecond control of the motion and directed emission of electrons from individual silica nanoparticles using few-cycle laser fields opens new possibilities to manipulate electronic processes in nanoscale systems.