Free-electron lasers are an exciting development for fields ranging from structural biology to nanotechnology. These lasers produce an intense and extremely short burst of X-rays, which could enable the structure of individual organic molecules to be collected without the need to first form them into a crystal (as is the case in conventional X-ray analysis). But the intensity of these pulses is such that they obliterate any sample they irradiate. In this issue, Henry Chapman, Janos Hajdu and colleagues report a proof-of-principle of a technique that reconstructs the image of a sample using scattered X-rays at the beginning of a pulse. Using a single 25-femtosecond soft X-ray pulse generated by the recently completed FLASH free-electron laser, they imaged two micrometre-sized stick figures patterned into a silicon nitride film ãƒâƒã‚âƒãƒâ‚ã‚âƒãƒâƒã‚â‚ãƒâ‚ã‚âƒãƒâƒã‚âƒãƒâ‚ã‚â‚ãƒâƒã‚â‚ãƒâ‚ã‚â‚ãƒâƒã‚âƒãƒâ‚ã‚âƒãƒâƒã‚â‚ãƒâ‚ã‚â‚ãƒâƒã‚âƒãƒâ‚ã‚â‚ãƒâƒã‚â‚ãƒâ‚ã‚â— just moments before it evaporated at a temperature of 60,000 K.

Letter p839 | News and Views p799

Following a flurry of activity in the 1990s, there is now a resurgence of interest in optical solitons: three papers in this issue present diverse insights into their manipulation, behaviour and potential practical use. First, Marco Peccianti and colleagues show how to control the path of spatial solitons travelling through anematic liquid crystal by electrically altering the refractive index of different regions of the material. Carmel Rotschild and colleagues report an extremely long-range interaction between spatial solitons — which usually only interact when they're less than a few beam widths apart — travelling within a heat-sensitive nonlinear medium. And finally, Joe Mok and colleagues demonstrate the ability to tune the speed of gap solitons, slowing them to a sixth of the speed of light in vacuum by launching them near the band edge of a fibre Bragg grating.

Letter by Peccianti et al | News and Views by Kivshar | Article by Rotschild et al | Article by Mok et al | News and Views by Povinelli

Quantum teleportation describes the transfer of a quantum state from one place to another, by a sender who knows neither the state to be teleported nor the location of the receiver. The phenomenon is one of the most intriguing examples of how quantum entanglement can assist in realizing practical tasks, and is involved in numerous quantum communication and quantum computation schemes. Quantum teleportation of single bits of quantum information has been demonstrated before, but large-scale applications require the transfer of composite systems. Now Qiang Zhang and colleagues report the teleportation of combined polarization states — including entanglement — of two photons, the key to their success being the development of an efficient six-photon interferometer.

Letter by Zhang et al | News and Views by Walther

The Klein Paradox enables a relativistic electron to pass straight through a potential barrier of at least twice its rest mass, as if the barrier were not even there. Such behaviour, which is just one of the many counterintuitive consequences of the Dirac equation, is usually expected to occur under only the most extreme of circumstances, such as in the vicinity of a black hole. But Mikhail Katsnelson and colleagues suggest that analogous conditions to those that support this paradox exist in a single sheet of graphene. They predict that when the massless Dirac fermions that carry charge in graphene encounter a square barrier at normal incidence, they too will pass through it with perfect efficiency. Moreover, analysis of related effects at oblique incidence and in bilayer graphene suggests this behaviour could be used for device applications.

Article by Katsnelson et al | News and Views by Calogeracos

In systems ranging from our unpredictable weather to flame acceleration of a supernova, the Rayleigh-Taylor instability is the underlying mechanism; liquids of different densities mix in such a way as to lower the energy of the system. For a fluid, the Reynolds number is a measure of whether the flow is laminar or turbulent, with turbulence – and eddies of varying scales – setting in above 2,300 or so. These eddies will affect flame propagation in type Ia supernovae, the 'standard candles' used to estimate the expansion rate of the Universe. As it is not possible to measure the dynamics of turbulent combustion in a supernova, William Cabot and Andrew Cook use the largest-to-date direct numerical simulation to study the self-similarity, scaling and growth rate of thermonuclear flames.

Article by Cabot et al | News and Views by Schmidt

The speed at which a semiconductor laser can be switched is related not only to the time it takes to populate its excited states to inversion, but also to how quickly these relax to their ground state. One way of shortening this process is to pump the laser harder, but in many instances doing so can be undesirable or simply impractical. Another way is to increase the rate at which excited electrons spontaneously decay to the ground state to give off photons and stimulate further emission. This rate is determined not by the material properties of the lasing medium but by its electromagnetic environment. In this issue, Hatice Altug and colleagues show that by exploiting the enhanced spontaneous emission rate of a laser cavity formed within a photonic crystal, they can achieve switching speeds in excess of 100 GHz. Article by Altug et al

Plasma instabilities known as edge–localized modes represent a significant challenge to the development of magnetically confined fusion as a clean, sustainable energy source. These instabilities are caused by the build–up of energy at the edge of toroidal plasmas produced in the so–called high–confinement mode, and result in the rapid discharge of energy to the walls of the chamber in which they are held. This in turn causes significant erosion of the walls, requiring their frequent replacement, which could severely limit the operation and viability of any future fusion reactor. But Todd Evans and colleagues may have found a solution. By weakly perturbing the edge of the magnetic field that confines a fusion plasma in a way that causes the field lines to become chaotic, these catastrophic modes can be all but eliminated.

Article by Evans et al | News and Views by Alberto Loarte

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 by Sola et al | Letter by Remetter et al | News and Views by Thomas Pfeifer

The biological cell is supported by a meshwork of polymers, notably the protein actin. But this cytoskeleton is no rigid scaffold – the dynamic response of the mesh is akin to a muscle, flexing and stretching, and enabling the cell to move. The physics of the polymer mesh is fascinating in itself, but biophysicists want to push further, piecing the details together to create basic components – or functional modules that mimic the behaviour of the cytoskeleton. In this issue, Andreas Bausch and Klaus Kroy review the progress made, using physics theory, modelling and experiment, to establish a bottom-up approach to understanding the complexity of biological cells.

Article by Bausch et al

Superconductivity arises when electrons form pairs that condense into a coherent ground–state. The symmetry of the pairing determines the overall superconducting behaviour, including the transition temperature, T c. In high–T c superconductors, the pairing has d–wave symmetry, which changes sign four times on rotation through 360°. Using their celebrated phase-sensitive technique, John Kirtley and co–authors revisit optimally doped YBa2Cu3O7–8 to map out the in–plane angular dependence of the pairing symmetry. A square pick–up loop scans over the arrangement of 72 rings to detect any spontaneous supercurrents (shown as peaks in the image). Owing to the presence of Cu–O chains along one crystalline axis, the superconducting energy gap is larger in that direction. The authors also find no evidence for time-reversal symmetry breaking and set an upper bound to any imaginary components of the main d–wave gap. [Article p190]

Article by Kirtley et al

Recently developed high-intensity, coherent X-ray sources — such as highharmonic-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 Xray 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 by Quiney et al. | News and views by Jacobsen

Cadmium ions in action

Artwork by Boris Blinov (Univ. Washington).

Letter by Stick et al