Research articles

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.

Quantum critical points in many-body systems are characterized by the appearance of long-range entanglement. These subtle quantum correlations are known to be extremely fragile with respect to thermal noise. But theoretical work now shows that, unexpectedly, another classical disturbance, the ubiquitous 1/f noise, does preserve the critical correlations.

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.

The speed with which symmetry breaking transitions occur in the solid state makes them difficult to study in the time domain. State-of-the-art pump–probe measurements of the dynamics of charge-density waves in terbium telluride enable the evolution of the symmetry breaking charge-order transition of this system to be studied with unprecedented temporal resolution.

Monitoring the photocurrent generated as a laser scans across a graphene field-effect device subjected to low temperature and high magnetic fields enables the spatial distribution of Landau levels across a graphene sheet to be mapped. This in turn allows the relative contribution of bulk and edge states to the macroscopic electrical characteristics of these devices to be determined.

Although solar flares are the most energetic events that occur in our Solar System, very little is known about their contribution to the total energy the Earth receives from the Sun. The identification of a measurable signal from a moderate-sized solar flare in total solar irradiance data suggests their impact on the variability of the Sun’s output could be larger than expected.

Amplifying a signal usually also amplifies the noise. A quantum-state amplifier is now demonstrated that can actually decrease uncertainty about the state’s phase. Counterintuitively, the concept involves the addition of thermal noise.

A pure spin current has no net charge current and is therefore difficult to detect. A new technique that takes advantage of nonlinear optical effects can measure pure spin currents non-invasively, non-destructively and in real-time.

Real-space mapping of the quantum Hall state at the Dirac point in epitaxial graphene reveals unexpected localized lifting of the degeneracy of this level. This could be the result of moiré interference caused by the twisting of the top layer with respect to underlying layers, suggesting possible new ways to understand and control the unusual properties of graphene.

Micrometre-scale superconducting circuits are at present explored as the building blocks for scalable quantum information processors. In a system where two such qubits are coupled to a resonant cavity, tripartite interactions and controlled coherent dynamics have now been demonstrated. This platform should allow for a fuller exploration of multipartite quantum states and their deterministic preparation.

A new technique for controlling the quantum state of a superconducting qubit is now presented. Microwave pulses are applied in such a way that they excite only one of a pair of degenerate states. The concept enables construction of a controlled-NOT gate, a device important for quantum logic.

The electronic properties of metals are usually well described by Fermi-liquid theory. However, whether it still applies to very thin metal films has been unclear. Now, measurement of the lifetime of hot electrons in lead films just a few monolayers thick suggests that it does, and in turn provides a reliable means for determining the lifetime of excited carriers in bulk metals.

Pinching is a process most commonly associated with the break-up of liquid streams in air. Time-resolved three-dimensional X-ray imaging of a eutectic Al–Cu alloy reveals that interfacial-energy-driven bulk diffusion can drive similar processes in liquid–solid systems

The Jaynes–Cummings model describes the interaction between a two-level system and a small number of photons. It is now shown that the model breaks down in the regime of ultrastrong coupling between light and matter. The spectroscopic response of a superconducting artificial atom in a waveguide resonator indicates higher-order processes.

As a measure of disorder, entropy is a central concept of statistical mechanics. In practice, however, it is typically determined thermodynamically, that is, by measuring heat. However, in arrays of interacting submicrometre-sized magnetic islands—known as artificial spin ice—entropy can be determined directly by ‘counting’ the microstate of the system.

The Heisenberg uncertainty principle bounds the uncertainties about the outcomes of two incompatible measurements on a quantum particle. This bound, however, changes if a memory device is involved that stores quantum information. New work now extends the uncertainty principle to include the case of quantum memories, and should provide a guide for quantum information applications.

Bosons in an optical lattice are used to simulate the Bose–Hubbard model. If the lattice is disordered, a Bose glass is predicted to exist. Transport measurements in such a lattice provide evidence for a disorder-driven superfluid–insulator transition into a Bose-glass state.

Single nitrogen–vacancy centres in diamond are a prime candidate for implementing scalable quantum information processing at room temperature. Work so far has been focused on using the ground state of these defects, but an experimental study now suggests that the excited state is a promising route to fast gate operation.