Letters in 2010

A potentially critical limiting factor in the use of free-electron lasers to determine the structure of organic molecules is the damage the procedure may cause. A model based on coherence theory and quantum electrodynamics suggests that it should be possible to reconstruct a molecule’s structure from the X-ray data obtained as it undergoes damage.

A study of autoresonant behaviour in a superconducting quantum pendulum reveals that fluctuations, both quantum and classical, only determine the initial oscillator motion, not its subsequent dynamics.

Light-emitting quantum dots are usually assumed to behave as perfect point-source emitters. It is now found that this assumption breaks down when quantum dots are placed near structures that support nanoscale optical modes — information that could be useful in building better nanophotonic devices.

Topological insulators are bulk insulators beneath conducting surface states with very special properties. By doping these surface states with iron, the surface band structure can be explored and controlled.

A neutron-scattering study provides quantitative evidence for magnetically mediated superconductivity close to a quantum critical point in the heavy fermion superconductor CeCu₂Si₂.

Electron spins in semiconductor structures are quantum bits with good prospects, but the information stored in the spin states tends to degrade quickly owing to interactions with nuclei in the host material. A study of GaAs quantum dots now provides a fuller understanding of this memory loss and how it can be suppressed. Quantum-memory times exceeding 200 μs are demonstrated, two orders of magnitude longer than previously reported for this system.

Ultrafast spectroscopy reveals the many-body effects behind the metallization of a one-dimensional Mott insulator. Unlike in ultracold gases, these femtosecond excitation studies of quantum dynamics occur at room temperature.

High-order harmonic generation is a nonlinear optical process that enables the creation of light pulses at frequencies much higher than that from a seed laser. The host medium for this interaction is typically a gas. Now, the process has been observed in a bulk crystalline solid with important implications for attosecond science.

The pseudogap state in the cuprate superconductors shows signs of electronic pair formation above the superconducting temperature. Is it just a ‘precursor’ state or a separate (and competing) state? In fact, both interpretations seem to be correct.

If vortex cores within a superconductor can trap electrostatic charge, the cores will experience a repulsive Coulomb interaction. Evidence from NMR measurements indeed suggests that above some threshold magnetic field, the Abrikosov vortex lattice becomes unstable.

Bound pairs consisting of a vortex and an antivortex are expected to dominate the low-temperature physics in a variety of two-dimensional systems. The observation of such bound pairs, however, remains elusive. A study now establishes non-equilibrium condensates of exciton-polaritons as a platform for exploring the physics of vortex–antivortex pairs.

Rydberg molecules—which involve atoms in highly excited electronic states and can be as large as 100 nanometres—have been created recently in cold gases of rubidium atoms. New work demonstrates that the inter-atomic interactions in these long-range molecules can be manipulated coherently, enabling controlled ‘making and breaking’ of the bond using laser light.

Graphene has a random edge structure. According to theory, this dirty and random edge affects the topological nature of bilayer graphene, which accounts for measurement discrepancies across different experimental probes.

For an ideal topological insulator, the metallic surface states should be easy to measure using transport techniques; however, the bulk is not completely insulating. Improving the ‘leaky’ bulk state proves crucial for measuring the surface Dirac fermions, including correlation effects.

Although carbon nanotubes are not superconductors they can carry supercurrents injected from superconducting contacts. Analysis of the tunnelling spectra of a nanotube connecting two superconductors reveals the detailed electronic structure of discrete entangled electron–hole states that carry the resulting supercurrent.

Feedback mechanisms such as the ‘demon’ in Maxwell’s well-known thought experiment can, in principle, enable the transformation of information into energy, without violating the second law of thermodynamics. Such information-to-energy conversion by feedback control has now been demonstrated experimentally.

Quantum information is often thought of in terms of manipulating discrete qubits. But continuous variables can also carry data. A method for storing continuous-variable states of light for up to a millisecond in room-temperature memories is now demonstrated.

A three-dimensional periodic structure focuses acoustic waves to a spot size that is one fiftieth of the wavelength—beating the classical diffraction limit by a long way. The device could lead to improved resolution for ultrasound imaging.

The rotation of polarized light in certain materials when subject to a magnetic field is known as the Faraday effect. Remarkably, just one atomic layer of graphene exhibits Faraday rotations that would only be measurable in other materials many hundreds of micrometres thick.

By varying the voltage on an isolated gate electrode beneath a graphene sheet, the ionization state of cobalt atoms on its surface can be controlled. This enables the electronic structure of individual ionized atoms, and the resulting cloud of screening electrons that form around them, to be obtained with a scanning tunnelling microscope.