Letters in 2009

An approach that combines fluorescence and cavity-QED methods enables the fast and reliable detection of single atoms, and should be useful for a series of atomic-physics and quantum-information protocols.

Topological insulators are exotic states of matter that show quantum-Hall-like behaviour in the absence of a magnetic field. Surface states in such systems are protected against scattering and are thought to provide an avenue for the realization of fault-tolerant quantum computing. Experiments now reveal the observation of such a topological state of matter in Bi₂Se₃, a naturally occurring stoichiometric material with a simple surface-state structure and a bulk energy gap larger than k_BT at room temperature.

Interacting nuclear spins on a crystalline lattice are commonly believed to be well described within a thermodynamic framework that uses the concept of spin temperature. Demagnetization experiments now challenge this belief, showing that in general the spin-temperature concept fails to describe a nuclear-spin ensemble in a quantum dot when strong quadrupolar interactions are induced by strain.

A recent experimental study of the fractional quantum Hall state—a prototypical system exhibiting strong collective quantum behaviour—provided evidence for the existence of unexpected collective modes at a filling factor of 1/3. Fully microscopic calculations now explain these modes as arising from collective excitations within the composite fermion theory.

An experiment distributing entangled photons over 144 km significantly raises the bar on distance, channel loss and transmission time—encouraging news with regard to future long-distance quantum-communication networks.

It has been thought that sheets of cells move by traction forces exerted by the cells at the leading edge of the sheet. Using traction microscopy to create a map of physical forces, it is now shown that in fact it is cells many rows from the front that do most of the work.

A method for tomographic imaging of molecular orbitals—based on the alignment of molecules in the laboratory frame and linearly polarized laser fields—has now been extended to atoms, which cannot be naturally aligned.

A theoretical study predicts universal signatures of four-body physics in cold-gas experiments, and presents evidence that these have already been observed.

The separation between two electrons bound in a Cooper pair in a conventional superconductor can extend up to several hundred nanometres. A new study shows that these long-range interactions can reach beyond the confines of a superconductor itself to coherently couple electrons in two normal metals either side of the superconductor.

The Kadowaki–Woods ratio attempts to relate the temperature dependence of a metal to its heat capacity. However, as it takes different values for different classes of metals it is not universal. By including effects related to carrier density and spatial dimensionality, a much more universal ratio, which describes the properties of many different systems, has been achieved.

Conventional wisdom suggests that it should be impossible for information to pass across a singularity. A study of the behaviour of air bubbles as they disconnect from a submerged nozzle suggests that this isn’t always the case.

Hyperfine coupling to nuclei can be detrimental to the coherence of electron spins, but properly harnessed it can provide a mechanism for manipulation and storage of quantum information. Spin-blockade measurements in ¹³C carbon nanotubes now show surprisingly strong effects of electron–nuclear interaction, with a hyperfine coupling two orders of magnitude larger than previously anticipated.

The strength of interparticle interactions in cold gases can be tuned using magnetic fields. This widely used approach is now combined with laser manipulation, providing additional flexibility, such as the possibility of spatially modulating the interaction strength on short length scales.

The remarkably strong coupling between the electronic and vibrational modes of suspended carbon nanotube quantum dots provides a new way of studying quantized mechanical motion.

The creation and annihilation of magnetic vortex–antivortex pairs has been predicted to have a role in magnetic switching in permalloy nanostructures, but has never previously been observed. High-speed X-ray microscopy now enables the evolution and dynamics of this process to be studied in detail.

When a single strand of DNA is threaded through a nanopore, a direct test of the effect of pore size indicates that a hydrodynamic model for the process should include the coupled Poisson–Boltzmann and Stokes equations.

The discovery of an overlooked but apparently ubiquitous spike in the mid-infrared photoelectron spectra of molecular and atomic gases suggests that we don’t know as much as we thought we did about the ionization of matter in strong fields.

The scattering of laser light by acoustic phonons confined within a photonic crystal fibre reveals unexpected highly nonlinear acoustic modes that behave like the Raman-active modes of a molecule.

The broadening of a wave-packet can be suppressed as it propagates through a periodic potential. The first-order effect of this so-called dynamic localization has been seen in many different systems. Higher-order effects are now seen for the first time in an optical pulse guided along curved photonic lattices.

A proposed device—an optical analogue of the superconducting Josephson interferometer—might enable detailed studies of the role that dissipation has in strongly correlated quantum-optical systems.