Articles in 2009

In semiconductor quantum dots, interactions between the confined electrons and the surrounding reservoir of nuclear spins limit the attainable electron-spin coherence. But the nuclear-spin reservoir can also take a constructive role, as it facilitates the locking of the optical quantum-dot resonance to the changing frequency of an external driving laser, as an experiment now demonstrates.

A technique that allows the electrical detection of spin-polarized transport in semiconductors without disturbing the spin-polarized current or using magnetic elements has now been demonstrated. The approach could lead to the integration of spintronics elements into semiconductor microelectronic circuits.

The first experimental demonstration of saturable absorption in core-electron transitions in aluminium paves the way for investigating warm dense matter, which potentially has an important role in planetary science and the realization of inertial confinement fusion.

The effect of disorder in conventional two-dimensional electron systems is usually described in terms of individual electrons interacting with an underlying disorder potential. Scanning single-electron transistor measurements of graphene in a strong magnetic field indicate that in this system, coulombic interactions between electrons must also be taken into account.

The size-distribution profile of raindrops when they reach the ground was previously thought to be governed by complex interactions between neighbouring droplets as they fall. High-speed videos of falling water droplets suggest this is not the case, and that their size distribution can be explained by the fragmentation of individual droplets alone.

Black holes are difficult to study experimentally, owing to their distance from us and indeed their very nature. A theoretical study suggests that optical metamaterials that exhibit behaviour that is reminiscent of that of black holes, could enable us to learn more about these and other astrophysical objects.

The first evidence of supersolidity—the potential ability of solids to move without friction—in solid ⁴He was obtained in torsional-oscillator experiments. But later observations raised the possibility that the characteristic frequency changes were simply due to stiffening of the solid. Now, the results from a series of experiments comparing ³He and ⁴He rule out that explanation.

In 1970, Vitaly Efimov predicted that three interacting particles can form an infinite series of bound trimer states, even when none of the two-particle subsystems is stable. Experimental evidence for such an exotic state was obtained in 2006, but now an Efimov spectrum, containing two such states with the predicted scaling between them, has been observed.

Simultaneous coherent control of internal and motional states of a Bose–Einstein condensate has been demonstrated on an ‘atom chip’. The method should provide a route to generating many-particle entangled states, which are needed for entanglement-based technologies such as quantum-information processing or quantum-enhanced metrology.

A quantum computer requires quantum systems that are well-isolated from external perturbations, but which can still be easily manipulated with external fields. A scheme that uses spatially inhomogeneous fields to selectively address neutral-atom qubits while they are in field-insensitive superposition states satisfies these competing needs.

Bound macrodimers have now been directly observed for the first time. Macrodimers comprise two Rydberg atoms with a separation as large as 9 μm. The unique properties of macrodimers mean that they enable new experiments for investigating ultracold gases.

It is not surprising that a microfluidic channel whose walls have a ratchet-like structure can preferentially direct the flow of large particles in one direction. But a study of the movement of living cells through such channels provides the remarkable observation that the direction of preferred motion can be different for different species of cell.

Optomechanical systems in which a high-quality optical resonator is coupled to a mechanical oscillator hold great promise for examining quantum effects in relatively large structures. As a step towards this, a silica microtoroid has now been cooled to the point that it has just 63 thermal quanta.

Lévy flights, a form of random walk, are quite common in nature. However only macroscopic signatures, obtained by averaging over many steps, have been measured so far. Now, the individual steps are observed directly as light scatters in a hot atomic vapour.

Femtosecond laser pulses can demagnetize ferromagnetic metallic thin films on an ultrafast timescale. Studying how a single optical pulse interacts with a magnetic film now provides a better understanding of this so-called femtomagnetism.

Magnetic reconnection—the process by which magnetic field-lines break and reform in a plasma—is believed to be an important part of many astrophysical phenomena, but is poorly understood. The recreation of 3D reconnection events in a laboratory plasma provides a powerful means of studying the parameters that govern the onset, evolution and decay of this process.

A potentially general mechanism for symmetry breaking in mesoscopic quantum systems is revealed in a theoretical study, which shows how, in a rotating Bose–Einstein condensate, the symmetry properties of the true many-body state are related to those of its mean-field approximation.

Measurements of the distribution of stochastic switching currents in homogeneous, ultra-narrow superconducting nanowires provide strong evidence that the low-temperature current-switching in such systems occurs through quantum phase slips—topological quantum fluctuations of the superconducting order parameter via which tunnelling occurs between current-carrying states.

First-principles calculations predict that Bi₂Se₃, Bi₂Te₃ and Sb₂Te₃ are topological insulators—three-dimensional semiconductors with unusual surface states generated by spin–orbit coupling—whose surface states are described by a single gapless Dirac cone. The calculations further predict that Bi₂Se₃ has a non-trivial energy gap larger than the energy scale k_BT at room temperature.

The distributions of the sizes of cities or earthquakes, for example, follow a power law, but in physical systems different distributions of critical properties are usually seen. A scaling argument provides a practical rule to relate the type of distribution to an experimental quantity.