Articles in 2010

A genetic-algorithm approach to analysing quantum oscillations in a high-temperature superconductor reveals that quasiparticles behave as nearly free spins—split into spin-up and spin-down populations, known as the Zeeman effect.

Birefringent particles manipulated with an optical torque wrench exhibit strongly nonlinear, ‘excitable’ behaviour similar to that governing the firing of neurons. This technique could be used to detect small perturbations in the local environment of such a particle.

Optical control over electron spins embedded in semiconductor structures is an efficient way of manipulating quantum information. But a fully fledged quantum information processor will require control over two-spin states. This has now been demonstrated, including the implementation of ‘ultrafast’ two-qubit gate operations that take less than a nanosecond.

Single-molecule techniques and femtosecond pulse shaping are now combined to investigate quantum coherence in biomolecules. The creation and manipulation of such coherence enables a basic single-qubit operation with terrylene diimide at room temperature.

Atoms trapped in optical lattices have been used successfully to study many-body phenomena. But the shape that bosonic ground-state wavefunctions can take is limited, compromising the usefulness of this approach. Such limitations, however, do not apply to excited states of bosons. An atomic superfluid that has now been realized in such a higher-energy band promises to provide insight into a wider range of many-body effects.

Quantum cascade lasers can operate at terahertz frequencies because they use intraband, rather than interband, transitions in semiconductor nanostructures. However, they seemed to have reached a ceiling in terms of maximum operating temperature. This trend has now been broken with the introduction of a new scheme for charge injection.

Detecting and counting individual microwave photons is important for processing quantum information, but it is made challenging by an absence of detectors that are sensitive enough to radiation at this wavelength. Correlations between microwave photons have now been measured using a series of amplifiers and digital analysis.

A study of a non-liquid glass former reveals a correlation length that decreases as the transition temperature is approached from above, which is the opposite of what is expected. It suggests that ‘strong’ and ‘fragile’ liquids exist on opposite sides of an order–disorder phase transition.

Simply cooling down an artificial spin-ice does not necessarily lead to ground-state magnetic order. But as-grown artificial square ice reaches a thermodynamic ground state, with monopole dynamics possibly involved in the thermalization.

Networks of atom–cavity systems necessarily require that single atoms sit near dielectric interfaces. Real-time monitoring of caesium atoms just 100 nm from the surface of a micro-toroid resonator now demonstrates that the Casimir effect plays an important role in these systems.

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.

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.

Tunnelling measurements reveal the emergence of a thickness-dependent in-built potential across LaAlO₃ thin films grown on SrTiO₃ substrates. As well as being useful for developing novel LaAlO₃/SrTiO₃ devices, these observations help explain the origin of the two-dimensional electron gas that is known to arise at the interface between these two insulators.

An array of nanomagnets in a kagome lattice structure should support the creation and separation of oppositely charged monopoles, which are connected by Dirac strings of flipped dipoles. And indeed, such monopoles and their Dirac strings can be observed at room temperature.

The Landau–Zener model of a two-state system is a standard method for studying quantum dynamics. This textbook example of single-particle dynamics has now been generalized to a many-body system represented by two coupled ultracold Bose liquids.

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.

Quantitative measurements that establish the existence and evolution of quasiparticles across the whole phase diagram of a cuprate superconductor help to distinguish the many theoretical models for high-temperature superconductivity.

Raman amplification has been proposed as a means to generate high-power laser pulses without the bulky and expensive components of conventional lasers, but with limited success. Large-scale three-dimensional simulations enable researchers to identify conditions under which these limitations might be overcome.

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.

The energy potentials created by laser light can trap atoms. An analogous effect that traps electrons in solid-state systems is now proposed. The electron traps are created in quantum wells and wires in the presence of quasiparticles composed of two electrons and a hole. The idea could lead to advances in ultrafast optical and new optoelectronic devices.