THIS ISSUE Cover story Vol.3 No.5 May 2007 All physicists know that light carries both linear and angular momentum. What is perhaps less well known, however, is that its angular momentum can be broken down into spin and orbital components. Spin angular momentum is associated with polarization, whereas orbital angular momentum arises from a more complex combination of the phase and amplitude profiles of an optical field. Although the spin momentum is the predominant property used in optical- based quantum information applications, orbital momentum is potentially more powerful for encoding and processing such information in high-dimensional quantum spaces. In this issue, Gabriel Molina-Terriza and colleagues review progress in the generation, understanding and use of the orbital angular momentum of light. [Progress Article p305] FLASH-FROZEN FLUID small size makes them diffi cult to study. crosslinking proteins, they identify three Despite the importance of water we still By improving the energy resolution of regimes of nematic phase behaviour — do not understand all of its phases and a transmission electron microscope’s ‘loose’, ‘semi-loose’ and ‘tight’, the latter phase transitions, particularly at elevated energy loss spectrometer, Odile Stéphan resembling the cytoskeletal structure of temperatures and pressures. And although and colleagues demonstrate a technique hair cells in the ear, and having properties it may seem counter-intuitive that heating for mapping surface plasmons directly, appropriate for directed sound propagation. water to high temperatures will cause it to at a resolution well below that of optical [Article p354] freeze, Daniel Dolan and co-workers have imaging techniques. [Article p348] found that extreme compression does create BREAK IT UP frozen water. Th ey used the Z machine Paired electrons (or holes) are responsible at Sandia to compress a sample of water for superconductivity. Break up those magnetically. At 7 gigapascals, the water pairs — by changing the magnetic underwent a fi rst-order transition to ice on fi eld, temperature or carrier density, for a time scale of nanoseconds, without the instance — and long-range phase coherence need for any nucleation centres. Th e most disappears. In reduced dimensions, pairs likely phase is ice VII. [Letter p339] can exist well beyond the confi nes of the superconducting state. Lu Li and colleagues QUANTUM CASCADE LASER HOTS UP investigate how such pairs are destroyed in Quantum cascade lasers (QCLs) are one the low-density region of the phase diagram of the most promising compact sources of of a high-temperature superconductor, terahertz radiation. But their electrically where superconductivity, magnetism unstable nature and the need to drive and glassiness compete for ground-state large currents through them to achieve dominance. Th eir sensitive magnetization population inversion for lasing, means measurements reveal that quantum phase terahertz QCLs have only been able to fl uctuations break up the pairs, but that operate at temperatures of around 100 K. short-range superconductivity survives in To overcome this, Romain Terazzi and the vortex-liquid state in which spontaneous colleagues have developed a QCL structure vortex creation occurs. [Letter p311] that induces electrons to undergo ‘Bloch Surface plasmon modes of a silver nanoprism mapped in oscillations’, which relaxes the population- an electron microscope. GENTLE PROBE inversion condition. Th e device can p348 Th e quantum properties of Bose–Einstein therefore operate at temperatures well above condensates are typically probed by releasing those of conventional terahertz QCLs. the constituent atoms from their trap, and [Letter p329; News & Views p298] ACTIN NETWORKS letting them interfere. Igor Mekhov and Biological cells are supported by a colleagues propose that combining ideas PLASMONS RESOLVED ‘cytoskeleton’, a crosslinked network from the physics of ultracold quantum gases Surface plasmon polaritons (SPPs) are of fi laments made of the protein actin. and cavity quantum electrodynamics could composite quasiparticles arising from Paul Dalhaimer and colleagues note provide a much less destructive approach. the interaction of light with electronic that these networks and liquid-crystal Th ey show that when a quantum gas is placed excitations that exist at the surface of a elastomers have similar behaviour. Using inside a high-Q cavity, diff erent quantum conductor. SPPs could hold the key to the principles of polymer and liquid-crystal phases — such as the superfl uid state or the photonic devices that operate at nanometre physics in simulations of actin networks, Mott-insulator phase — have characteristic scales, as they are much smaller than they arrive at a fuller understanding of signatures in the transmission spectra of optical wavelengths, and their properties their mechanics and phase behaviour. the cavity. Th is might provide more detailed can be controlled by patterns etched into For networks with long fi laments, and for information about quantum phase transitions the surfaces they travel across. Yet their varying length and connectivity of the than has been possible so far. [Letter p319] nature physics | VOL 3 | MAY 2007 | www.nature.com/naturephysics.