This Issue

    Cover story

    Recently developed high-intensity, coherent X-ray sources — such as high-harmonic-generation lasers and X-ray free-electron lasers — offer great potential for studying matter at the atomic scale. But to take full advantage of this opportunity, significant improvements will be needed in the optics used to manipulate the radiation from these sources. To this end, Keith Nugent and colleagues report a technique for imaging the optical field at the focus of an X-ray lens that does not require the use of secondary X-ray optics. This technique could not only reduce uncertainties in the characterization and development of X-ray optics, but, in certain contexts, it could also eliminate the need for lenses altogether. [Letter p101 ; News and Views p71 ]

    2D turbulence meets percolation

    Turbulence in a fluid creates vortices. The patterns produced by a strongly interacting system with an infinite number of degrees of freedom cannot be predicted analytically. However, they do display symmetries that help to tackle the problem. Using numerical simulations in two dimensions, Denis Bernard and co-workers show that the statistics of vorticity clusters exhibit conformal invariance — that is, if different sections of a pattern are scaled up by different amounts, the resulting pattern is indistinguishable from the original. This applies to critical percolation, a much simpler problem involving random inhomogeneous systems. Analytic solutions are available for this universality class, and so a connection between critical percolation and two-dimensional turbulence could lead to developments in the latter non-equilibrium process. [Article p124 ; News and Views p67 ]

    NMR in the earth's magnetic field

    NMR spectroscopy provides detailed information about the structure and dynamics of molecules, but strong magnets are needed to perform the experiments. This requirement makes the technique expensive and confines it to the laboratory. But Stephan Appelt and colleagues show that the Earth's magnetic field — which is five orders of magnitude weaker than the fields typically used in NMR experiments — might still be strong enough for chemical analysis applications. From spectra recorded 'out in the field', subject only to the magnetic field of the Earth, they can sensitively characterize millilitre amounts of liquid. Moreover, owing to the inherent homogeneity of the Earth's field, their simple setup produces sharper resonance lines than sophisticated high-field instruments. [Letter p105 ]

    Join the rich club

    Methods developed in the framework of statistical physics have been successfully applied to large-scale analysis of complex networks in areas as diverse as biology, technology and social sciences. Such studies have revealed a high degree of universality in the general self-organizing principles that shape the structure of real-world networks. Alessandro Vespignani and colleagues now take a close look at the tendency of the hubs in a network to be well interconnected —known as the 'rich-club' phenomenon — and provide a firm basis for detecting and quantifying such structures. When analysing a variety of real-world networks, they find that scientists tend to form oligarchies, but, perhaps more surprisingly, that hubs in the Internet do not. [Letter p110 ; News and Views p75 ]

    Neutron star two-step

    Short gamma-ray bursts could be the result of mergers of double-neutron-star binaries.

    The brightest cosmic light shows, known as gamma-ray bursts (GRBs), were first detected in the 1960s by a US military satellite on the lookout for nuclear explosions on Earth. They occur roughly once each day, from any direction in space. Long GRBs (lasting more than two seconds) are associated with distant supernovae, in active star-forming galaxies. But the more energetic short GRBs come from local elliptical galaxies containing quiescent globular clusters. Jonathan Grindlay and co-authors look to these dense clusters of ancient stars as natural environments for the dynamical evolution of compact binary stars. They calculate that a significant proportion (up to 30%) of short GRBs could originate from the merger of double-neutron-star binaries, which coalesce gravitationally on the timescale of a few hundred million years or more. [Article p116 ; News and Views p79 ]

    Vacuum splitting in semiconductors

    When two harmonic oscillators of the same resonant frequencies interact — as happens when an atom or quantum dot is placed inside a single-mode resonant cavity of the same energy — instead of reinforcing each other, the coupling causes the frequency of the system to become split in two. This effect, known as vacuum Rabi splitting (VRS), provides valuable insight into cavity quantum electrodynamics and the fundamental quantized nature of electromagnetic fields. Although the splitting is more pronounced when it involves many oscillators in a cavity, it only becomes truly quantum mechanical in nature for a single oscillator. In this issue, Galina Khitrova and colleagues review the progress that has been made towards realizing single-oscillator VRS in semiconductors — a goal that not only makes VRS easier to study, but improves the outlook for practical applications. [Review p81 ]

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    This Issue. Nature Phys 2, v (2006).

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