Volume 8 Issue 1, January 2012

Manipulating the electrons trapped in quantum-dot pairs is seen as one possible route to quantum computation. This idea is now extended to three quantum dots, enabling a whole host of extended functionality.

Anisotropies in conductivity measurements of bismuth point to the spontaneous breaking of intrinsic degeneracies in its electronic structure — and suggest there may be still more to learn from this well-studied material.

Is it possible for a metal to exist in a strictly two-dimensional system? This may seem trivial, but it is actually a longstanding problem. The electrical characteristics of an array of superconducting islands on a normal metal suggests that the answer could be 'yes'.

Topological defects are encountered in fields ranging from condensed-matter physics to cosmology. These broken-symmetry objects are intrinsically local, but theoretical work now suggests that non-local quantum superpositions of such local defects might arise in a quantum phase transition.

Reductionism, as a paradigm, is expired, and complexity, as a field, is tired. Data-based mathematical models of complex systems are offering a fresh perspective, rapidly developing into a new discipline: network science.

A completely ordered universe is as unexciting as an entirely disordered one. Interesting ‘complex’ phenomena arise in a middle ground. This article reviews the tools that have been developed to quantify structural complexity and to automatically discover patterns hidden between order and chaos.

Networks have proved to be useful representations of complex systems. Within these networks, there are typically a number of subsystems defined by only a subset of nodes and edges. Detecting these structures often provides important information about the organization and functioning of the overall network. Here, progress towards quantifying medium- and large-scale structures within complex networks is reviewed.

Vast amounts of data are available about complex technological systems and how we use them. These data provide the basis not only for mapping out connectivity patterns, but also for the study of dynamical phenomena, including epidemic outbreaks and routing of information through computer networks. This article reviews the fundamental tools for modelling such dynamical processes and discusses a number of applications.

Aspects concerning the structure and behaviours of individual networks have been studied intensely in the past decade, but the exploration of interdependent systems in the context of complex networks has started only recently. This article reviews a general framework for modelling the percolation properties of interacting networks and the first results drawn from its study.

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.

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.

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.

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.

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.

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.

The controlled creation of one-dimensional conductive channels at the cores of topological defects in the multiferroic material BiFeO₃ demonstrates that such defects can drive metal–insulator phase transitions, and might provide a route towards high-density information storage.

Multiple valleys in the electronic structure of certain crystal lattices could enable the development of so-called valleytronic devices. But to do so, the degeneracy of these valleys must be lifted. Measurements of the anisotropic magnetoelectric response of bismuth suggest that its three-fold valley degeneracy breaks spontaneously at low temperatures and high fields.

Laser-driven proton accelerators could enable more effective cancer treatment, but to fulfil this function proton beams with a higher energy and narrower energy spread will need to be produced. Discovery of a laser–plasma acceleration mechanism that generates 20 MeV proton beams with a 1% spread is a promising step.

In many large ensembles, the property of the system as a whole cannot be understood from studying the individual entities alone — these ensembles can be made up by neurons in the brain, transport users in traffic networks or data packages in the Internet. The past decade has seen important progress in our fundamental understanding of what such seemingly disparate 'complex systems' have in common; some of these advances are surveyed here.