Volume 6 Issue 12, December 2010

Precisely what are the electrons in a high-temperature superconductor doing before they superconduct? Strong electronic correlations may give rise to composite rather than fractionalized excitations, as is typical in other strongly coupled systems such as quark matter.

Recent advances in the formulation of the second law of thermodynamics have rekindled interest in the connections between statistical mechanics and information processing. Now a 'Brownian computer' has approached the theoretical limits set by the rejuvenated second law. Or has it?

Engineered decoherence enables tracking of multipartite entanglement as a quantum state decays.

Carbon nanotubes are not superconductors but they can carry a supercurrent injected from a superconducting contact. Analysis of the tunnelling spectra of a nanotube connecting two superconductors reveals details of the bound electron–hole states that carry such a supercurrent.

Rydberg molecules, which consist of one atom in its electronic ground state and one in a highly excited state, can extend to the size of a virus. But size is only one oddity of these molecules. As has now been demonstrated, the chemical bond that holds the atoms together in this fragile molecule can be coherently controlled using laser light.

A noisy environment is used to study the dynamics of a four-trapped-ion entangled state. The study shows that entanglement properties such as distillability and separability can be altered by controlling the degree of dephasing. The results provide an important insight into the nature of multiparticle entanglement.

The ability to generate entangled photon pairs from a quantum dot critically depends on the size of the fine-structure splitting of its exciton states. A demonstration of the ability to tune this splitting with an electric field represents a promising step in the use of quantum dots to generate entangled photon pairs on demand.

Loading only single atoms into an optical trap with an efficiency in excess of 80% has now been achieved by manipulating the collisions between pairs of atoms. Such a process has previously been limited to about 50% efficiency. The technique will aid the development of neutral-atom-based quantum logic gates.

A study of Mn-doped InAs quantum wells reveals unexpected metastable behaviour of magnetotransport phenomena at sub-kelvin temperatures, in structures that show at the same time the quantum Hall effect in high magnetic fields. These findings bridge the physics of two-dimensional carrier systems with phenomena specific to magnetically doped semiconductors.

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.

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.

A study of the electronic structure of molecular wires as a function of their length reveals strong coupling between electrons and molecular vibrations. The mechanism provides a means to coherently couple electronic levels by nuclear motion, and possibly to mechanically control electron transport in molecular electronics.

Betratron oscillations of electrons driven through a plasma by a high-intensity laser generate coherent X-rays. A new study demonstrates the intensity of these X-rays can be as bright as that generated by conventional third-generation synchrotrons, in a device a fraction of the size and cost.

Magnetic reconnection governs many astrophysical phenomena, but its details are poorly understood. The extreme magnetic fields generated by the interaction of a high-intensity laser with a plasma enables the study of magnetic reconnection processes similar to those that occur in solar flares.

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.

A dark exciton is an electron–hole pair with a very long radiative recombination time. Whereas their ’bright’ counterparts are studied in depth, dark states in quantum dots are often regarded as a nuisance. Now, a technique has been found for optically accessing dark excitons, which might make them more useful than first thought.

A major goal in the fields of ultracold quantum gases and quantum simulations is measuring the phase diagram of strongly interacting many-body systems. This has now been achieved in an optical-lattice-based quantum simulator. The simulation is validated through an ab initio comparison with large-scale numerical quantum Monte Carlo simulations.

Laser light can trap and manipulate small particles. Scientists now show that femtosecond near-infrared laser pulses can split a single trap into two. The effect is a result of third-order optical nonlinearities that arise once the laser power crosses a certain threshold, and the direction of the split is determined by the light’s polarization.

Laser beams travelling side-by-side through a medium usually only interact if they’re within a beam-diameter apart. An observation of the attraction and coalescence of high-power beams separated by several beam diameters in a plasma has implications for the development of laser-driven fusion.