Letters in 2011

The unavoidable coupling between a quantum state and its environment leads to decoherence. Weak measurements—indirectly observing a quantum state without disturbing it—are now shown to be a useful tool for reducing or even nullifying the effects of decoherence.

So-called topological defects appear in various forms, be it as monopoles, cosmic strings, vortex lines or domain walls. This work suggests that such localized entities can be put in non-local superpositions, and describes the decoherence behaviour of such quantum states.

An optically trapped colloidal particle serves as the first realization of a stochastic thermal engine, extending our understanding of the thermodynamics behind the Carnot cycle to microscopic scales where fluctuations dominate.

It has long been debated whether it is possible to approach a zero-temperature metallic state in a two-dimensional system. A study of the electrical characteristics of arrays of superconducting islands of varying thickness and spacing on a normal metal film suggests it is.

When an intense laser pulse hits a flat metal foil, it ejects a spray of high-energy protons. Laser irradiation of a curved foil covering the tip of a hollow cone focuses the protons to intensities that could be useful for generating extreme states of matter.

Electromagnons are excitations that exhibit both electric and magnetic dipole moments, and are expected to enhance the coupling of magnetization and polarization in multiferroic materials. The identification of electromagnons in a perovskite helimagent may be useful in the development of ways to manipulate light.

Manipulating the electrons trapped in quantum-dot pairs is one possible route to quantum computation. Translating this idea to three quantum dots would enable a whole host of extended functionality. Researchers now generate and manipulate coherent superpositions of quantum states using the spins across three electrical-gate-defined dots.

The discovery that potassium-doped iron selenide undergoes phase separation into a defect-free superconducting phase and an iron-vacancy-ordered insulating phase resolves many questions about the unusual behaviour of this iron-based superconductor.

Experimental progress has made it possible to load fermionic atoms into higher orbital bands. Such systems provide a platform for studying quantum states of matter that have no prior analogues in solid-state materials. This theoretical study predicts a semimetallic topological state in these systems, which can be turned into a topological insulating phase.

Photoelectron spectroscopy is an invaluable tool for better understanding the energy levels of molecules. However, many levels remain hidden because of transition selection rules or a high density of states. Using X-rays to excite core–shell electrons and monitoring their Auger decay enables the extraction of previously hidden molecular-potential curves.

The behaviour of molecules and solids is governed by the interplay of electronic orbitals. Superfluidity, in contrast, is typically considered a single-orbital effect. Now, a combined experimental and theoretical study provides evidence for a multi-orbital superfluid, with a complex order parameter, occurring in a binary spin mixture of atoms trapped in an hexagonal optical lattice.

A quantum particle can tunnel through an energy barrier that it would otherwise be unable to surmount. This phenomenon has an important role in atomic processes such as ionization. Researchers now use an attosecond ‘clock’ to take a precise look at the dynamics of this process and identify the trajectory taken by the escaping electron.

In strong magnetic fields, clean two-dimensional electron systems support fractional Hall states that exhibit isotropically vanishing longitudinal resistance. At low field these states disappear and an anisotropic stripe phase emerges. And in between, contrary to expectation, these states can coexist.

Orbital order is important to many correlated electron phenomena, including colossal magnetoresistance and high-temperature superconductivity. A study of a previously unreported structure transition in KCuF₃ suggests that direct interorbital exchange is important to understanding such order.

Helical Dirac fermion states in topological insulators could enable dissipation-free spintronics and robust quantum information processors. A study of the influence of disorder on these states shows that although they are resilient against backscattering by magnetic impurities, fluctuations caused by charge impurities could cause problems for such applications.

Unconventional superconductivity is usually associated with a layered system. But how thin can a layered superconductor be and continue to be superconducting? Painstakingly grown superlattices of the heavy-fermion superconductor CeCoIn₅ suggest it could be as thin as a single layer.

Disorder-induced Anderson localization usually causes conducting materials to become insulating at low temperature. Graphene is a notable exception. But by increasing the carrier density in one graphene layer, a metal–insulator transition can be induced in an isolated second layer stacked above it.

Constraint-satisfaction problems are among the computationally hardest tasks: solutions are efficiently checkable, but no efficient algorithms are known to compute those solutions. Fresh insight might come from physics. A study mapping optimization hardness onto the phenomena of turbulence and chaos suggests that constraint-satisfaction problems can be tackled using analog devices.

The electronic properties of graphene depends on how many layers are involved. Monolayer graphene is a zero-gapped semi-metal. Bilayer graphene is a small-gapped semiconductor. Magnetotransport measurements indicate trilayer graphene can be both, depending on its stacking.

Monolayer graphene has no electronic band gap. Bilayer graphene does, and can be controlled by an electric field. And for trilayer graphene, infrared transmission measurements indicate both situations are possible depending on the stacking of the layers.