Letters in 2010

The complex wrinkling patterns produced when a sheet or membrane is compressed are often difficult to predict. Observations of unexpected spatial period-doubling bifurcation instability in the wrinkling of a rigid membrane attached to a soft substrate can be described within a framework similar to that used for the parametric resonance of nonlinear oscillators.

In magnetic nanostructures, the core of a vortex points either up or down, and the polarity can be reversed by alternating-field pulses. An experiment now demonstrates deterministic and coherent control of vortex-core polarity using sequences of resonant microwave pulses and highlights routes to optimizing the technique, which might find application in magnetic-storage devices.

The absorption of one photon of an entangled pair by a lone trapped atom is identified by a correlation between the atomic absorption process and the detection of the second photon.

Betratron oscillations of electrons driven through a plasma by a high-intensity laser generate coherent X-rays. A new study demonstrates the intensity of these X-rays can be as bright as that generated by conventional third-generation synchrotrons, in a device a fraction of the size and cost.

One-way quantum computing requires an entangled multiqubit system. So-called cluster states have been proposed to provide this resource, but they are difficult to generate. An alternative that uses the ground state of a one-dimensional chain of spins is now experimentally realized and used to construct a quantum logic gate.

A study of the electronic structure of molecular wires as a function of their length reveals strong coupling between electrons and molecular vibrations. The mechanism provides a means to coherently couple electronic levels by nuclear motion, and possibly to mechanically control electron transport in molecular electronics.

Magnetic reconnection governs many astrophysical phenomena, but its details are poorly understood. The extreme magnetic fields generated by the interaction of a high-intensity laser with a plasma enables the study of magnetic reconnection processes similar to those that occur in solar flares.

The ability to generate entangled photon pairs from a quantum dot critically depends on the size of the fine-structure splitting of its exciton states. A demonstration of the ability to tune this splitting with an electric field represents a promising step in the use of quantum dots to generate entangled photon pairs on demand.

A study of Mn-doped InAs quantum wells reveals unexpected metastable behaviour of magnetotransport phenomena at sub-kelvin temperatures, in structures that show at the same time the quantum Hall effect in high magnetic fields. These findings bridge the physics of two-dimensional carrier systems with phenomena specific to magnetically doped semiconductors.

A noisy environment is used to study the dynamics of a four-trapped-ion entangled state. The study shows that entanglement properties such as distillability and separability can be altered by controlling the degree of dephasing. The results provide an important insight into the nature of multiparticle entanglement.

Loading only single atoms into an optical trap with an efficiency in excess of 80% has now been achieved by manipulating the collisions between pairs of atoms. Such a process has previously been limited to about 50% efficiency. The technique will aid the development of neutral-atom-based quantum logic gates.

The experimental demonstration of heat currents driving the injection of spins from a ferromagnetic into a non-magnetic metal establishes a new source of pure spin currents. The approach might provide an alternative mechanism for switching processes in memory devices and for other ‘spintronics’ applications.

When doped with copper, the topological insulator Bi₂Se₃ becomes superconducting. But for new physics and applications the search is not for just any superconductor; the material must retain its topological character. And indeed that is the case with doped Bi₂Se₃.

Erwin Schrödinger introduced in 1935 the concept of ‘steering’, which generalizes the famed Einstein–Podolsky–Rosen paradox. Steering sits in between quantum entanglement and non-locality — that is, entanglement is necessary for steering, but steering can be achieved, as has now been demonstrated experimentally, with states that cannot violate a Bell inequality (and therefore non-locality).

Single crystals of a two-dimensional quantum spin system with geometric frustration lead to the observation of a ‘pinwheel’ valence-bond ground state. In this case, the distortion of the ideal kagome lattice structure helps to stabilize the quantum spin state.

Introducing a phase shift between diffracted and undiffracted light from a sample is one of the oldest techniques for generating phase contrast in optical microscopy. A similar approach should help improve the contrast and clarity of images collected by scanning X-ray microscopy.

Self-organized criticality has been observed in a number of complex systems, including neuronal networks. Another property of cortical networks is that a high proportion of neurons collectively alternate between high activity (so-called up states), and quiescence (down states). Theoretical work now shows these two phenomena are intimately related.

Spreading of information, ideas or diseases can be conveniently modelled in the context of complex networks. An analysis now reveals that the most efficient spreaders are not always necessarily the most connected agents in a network. Instead, the position of an agent relative to the hierarchical topological organization of the network might be as important as its connectivity.

Long-lived polariton condensates can propagate well beyond the area of their initial excitation while still maintaining spatial coherence. This enables direct and controllable manipulation of the condensate wavefunction.

The Peregrine soliton — a wave localized in both space and time — is now observed experimentally for the first time by using femtosecond pulses in an optical fibre. The results give some insight into freak waves that can appear out of nowhere before simply disappearing.