Volume 8 Issue 9, September 2012

The unexpected deceleration of the Pioneer space probes has, at last, an explanation.

Technologies aimed at single-molecule resolution of non-equilibrium systems increasingly require sophisticated new ways of thinking about thermodynamics. An elegant extension to standard fluctuation theory grants access to the kinetic intermediate states of these systems — as DNA-pulling experiments now demonstrate.

In most electrical conductors, we expect charge and heat to be transported in the same direction. However, in certain two-dimensional electron systems, fractional quantum Hall states can cause charge and heat to flow in opposite directions.

Hybrid traps for laser-cooled ions and neutral atoms make excellent cold-chemistry laboratories. Experiments now show that engineering quantum states can provide additional control for accessing and manipulating chemical reaction rates.

In two-dimensional systems, superfluidity occurs in the absence of the long-range order associated with Bose–Einstein condensates. This phenomenon is illustrated in the direct observation of superfluidity in a 2D atomic Bose gas.

Two-dimensional Bose fluids—such as liquid-helium films, or confined ultracold atoms—cannot form a condensate, but become superfluid instead. Frictionless flow, proving superfluid behaviour, has now been observed in an ultracold two-dimensional Bose gas that is stirred with a laser beam.

Chemical reactions between a single trapped ion and a condensate of ultracold neutral atoms are investigated by controlling the quantum states of both ion and atoms—revealing the effect of the hyperfine interaction on the reaction dynamics.

In metals, the Coulomb potential of charged impurities is strongly screened, but in graphene, the potential charge of a few-atom cluster of cobalt can extend up to 10 nm. By measuring differences in the way electron-like and hole-like Dirac fermions are scattered from this potential, the intrinsic dielectric constant of graphene can be determined.

In systems of oscillators, phase-locking behaviour can, in theory, coexist with incoherent dynamics—invoking the fabled chimera state. Now, experimental realization of a coupled-map lattice reveals dynamical states displaying coexisting spatial domains of coherence and incoherence.

Chimera states describing the stable coexistence of synchronous and incoherent dynamics have so far only been realized numerically. An experimental demonstration of these states in a network of discrete chemical oscillators reveals behaviour that differs from that predicted by existing phase-oscillator models.

Quantum discord is the total non-classical correlation between two systems. This includes, but is not limited to, entanglement. Photonic experiments now demonstrate that separable states with non-zero quantum discord are a useful resource for quantum information processing and can even outperform entangled states.

Entanglement is not the only type of quantum correlation. Quantum discord is a broader measure of such non-classical interactions. An experimental investigation now shows how quantum discord can be consumed to encode information, even in the absence of entanglement.

In most electrical conductors, heat is transported by charge carriers and so both usually flow in the same direction; but in two-dimensional electron systems subject to strong magnetic fields, certain fractional quantum Hall states can cause charge and heat to flow in opposite directions.

Magnetic vortices confined to thin films gyrate with a dynamics determined by the vortex–core polarity, which switches when the gyration is fast enough. Fine-tuning these core-reversal oscillations reveals rich nonlinear behaviour, including commensurate and chaotic states.

Short-lived kinetic states between equilibria are difficult to access experimentally, despite being crucial in many dynamical processes. Single-molecule experiments demonstrate that an extended fluctuation relation allows extraction of the free energies of these metastable states under non-equilibrium conditions.