Cover Story

Laser technology has advanced to a stage where we can generate pulses of light on timescales equivalent to those governing the dynamics of electrons within atoms and molecules. Two reports in this issue illustrate progress towards exploiting this capability to study and control the electronic properties and behaviour of such systems. The work by Inigo Sola and co-workers modulates the polarization of a train of ultrashort pulses to steer the return of an electron wavepacket to an atom from which it had been ionized. As a result, an isolated and much shorter secondary pulse emerges, towards the few-attosecond limit. Thomas Remetter and colleagues use a sequence of attosecond pulses to shear apart multiple electron wavepackets within an argon atom. By probing the subsequent interference between these wavepackets, valuable information about their phase is gained. [Letter p319 ; Letter p323 ; News and Views p297 ]

Smooth in 2D

For electrons, it is relatively easy to describe their behaviour when the –electron interactions are weak or strong. In the weak limit, the standard description of metals, Landau's Fermi liquid theory, treats interacting electrons of mass me as non-interacting quasiparticles of effective mass meff. For strong interactions, a low-density gas is expected to crystallize into a Wigner crystal. But what happens for moderately strong interactions? In this issue, Amit Ghosal and co-workers use state-of-the-art Monte Carlo simulations to explore the effect on the properties of electrons confined in a two-dimensional circular disk — a quantum dot. As they increase the interaction strength, ring structures form, but the inhomogeneous modulations develop smoothly, rather than through sharp transitions as they would in bulk systems. [Letter p326 ]

Broadband Reception

The range of physical stimuli our senses can process varies by several orders of magnitude in intensity — such as between the sounds of a pin drop and a bomb blast. Psychophysics is concerned with how our perceptions relate to these stimuli. One of the open questions is how our senses interpret real-life signals that cover a wide dynamic range. Puzzlingly, the dynamic range of the individual units of our sensory system is much smaller than that of the whole. In an attempt to resolve this mystery, Osame Kinouchi and Mauro Copelli model the dynamics that emerge from the interaction of coupled excitable elements, and provide a basis for understanding how a large dynamic range can be obtained in a network of lower-range elements. [Article p348 ; News and Views p301 ]

Topology Matters

Electron interference driven by attosecond lasers enables the collection of quantum phase information.

Systems with spins that can be arbitrarily manipulated into a desired state enable the construction of 'exotic' quantum states of matter that exhibit intriguing and unexpected behaviours. Spin models with so-called topological order are one such state, and beyond their potential for studying fundamental physics, are of interest for quantum-information applications, owing to their inherent robustness against perturbations. Systems of atoms or molecules stored in optical lattices — where they can be controlled with high precision — provide convenient testbeds for realizing such spin models experimentally. Andrea Micheli and colleagues present a theoretical framework for systematically engineering the hamiltonians that govern the behaviour of topologically ordered states in an optical lattice populated with polar molecules, providing a new route for the study, and perhaps practical use, of these exotic forms of matter. [Article p341 ; News and Views p309 ]

Light Gets Magnetic Bent

Light is usually immune to the influence of even the strongest magnetic fields. But, as Leon Karpa and Martin Weitz show, when photons interact with the matter to form composite quasiparticles known as 'dark polaritons', this isn't always so. When a beam of light is sent through a dense atomic medium, such as a Bose–Einstein condensate, that is subject to electromagnetically induced transparency, its path can be bent by an inhomogeneous magnetic field. This behaviour is analogous to the celebrated Stern–Gerlach experiment, which demonstrated the existence of quantum spin by using an inhomogeneous field to induce similar deviations in an atomic beam. The authors suggest that further study could provide insight into the quantum properties of dark polaritons, and perhaps find use in quantum-information processing. [Letter p332 ]

Insight With Isospin

Werner Heisenberg used the formalism in 1932, but it was Paul Wigner who, in 1937, coined the term 'isotopic spin'. Now more commonly known as 'isospin', the concept defines an exchange symmetry between protons and neutrons that has led to remarkable insights into the structure of the nucleus. In this issue, a review by David Warner, Michael Bentley and Piet Van Isacker examines the impact of isospin symmetry in nuclear physics — in particular for nuclei that have an equal number of neutrons and protons, and hence maximal symmetry. As such nuclei have become more accessible to experiment in recent years, it seems that this classic concept from the heyday of atomic theory is as relevant today as ever. [Review p311 ]