Volume 7 Issue 11, November 2011

In his tragically short life, Alan Turing helped define what computing machines are capable of, and where they reach inherent limits. His legacy is still felt every day, in areas ranging from computational complexity theory to cryptography and quantum computing.

The 2011 Nobel Prize in Physics has been awarded to Saul Perlmutter, Brian Schmidt and Adam Riess, "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".

Generating intense bursts of high-energy radiation usually requires the construction of large and expensive facilities, such as free-electron lasers, which are based on conventional particle accelerators. Laser-driven accelerators offer a cheaper and smaller alternative, and they are now capable of generating intense bursts of gamma-rays.

An in-plane magnetic field usually destroys the isotropic fractional quantum Hall states of two-dimensional electron systems, and gives rise to anisotropic liquid-crystal-like states. An unexpected observation of the coexistence of both states at once suggests the emergence of a new quantum phase of matter.

Heavy electrons, formed through the quantum mixture of localized and itinerant electrons, can pair to create unconventional superconductivity in a two-dimensional lattice that is just one-unit-cell thick.

Time-reversal symmetry makes massless Dirac fermions in topological insulators ‘gapless’. When a gap opens, it breaks this symmetry and confers mass to the fermions. But now a quantum phase transition has been observed in a three-dimensional topological insulator that allows these particles to acquire mass without symmetry breaking.

In strong magnetic fields, clean two-dimensional electron systems support fractional Hall states that exhibit isotropically vanishing longitudinal resistance. At low field these states disappear and an anisotropic stripe phase emerges. And in between, contrary to expectation, these states can coexist.

Unconventional superconductivity is usually associated with a layered system. But how thin can a layered superconductor be and continue to be superconducting? Painstakingly grown superlattices of the heavy-fermion superconductor CeCoIn₅ suggest it could be as thin as a single layer.

Light can interact with the electrons in a crystalline solid, which in turn generates lattice vibrations or phonons. A related phenomenon was proposed 40 years ago in which it is the ions in the crystal rather than the electrons that mediate the interaction. This effect, known as ionic Raman scattering, is now observed experimentally.

Electron pumps usually deliver small numbers of electrons by using strong Coulomb blockade to limit their flow under an applied bias. By periodically modulating the wavefunction of the electrons in a hybrid superconducting device, they can be delivered without bias.

Laser-driven particle accelerators have the potential to be much cheaper than conventional accelerators. But so far, the reliability and energy spread of the beams they produce has been poor. A technique that decouples the particle-injection and acceleration stages of these devices could improve their performance.

The radiation produced when an intense laser interacts with a solid target could provide a cheaper source of X-rays to synchrotrons and free-electron lasers. But they can also produce short bursts of gamma rays, whereas synchrotrons do not.

Electron spin in quantum dots are extensively studied as a qubit for quantum information processing. However, the coherence of electron spin is deleteriously influenced by nuclear spin. Quantum-dot holes are a potential alternative. Full control over hole-spin qubits is now achieved using picosecond lasers.

A single quantum system comprising a nitrogen-vacancy in diamond is now coupled to a nanowire cantilever. Magnetic fields can then couple the nitrogen-vacancy spin and the oscillator enabling read-out of the nanometre-scale motion.

After decades of research, the microscopic details of the superconductor–insulator transition in two-dimensions, which is driven by the presence of disorder, are revealed by simulations. These include a phase transition from a gapped superconductor to a gapped insulator, for example.

It is widely believed that high-field superconductivity in heavy fermion metals is sustained only when the effective mass of its conduction electrons diverge. Measurements of magnetically driven changes in the electronic topology of URhGe suggest it is not divergence of the effective mass to infinity but a vanishing of the Fermi velocity to zero that supports this behaviour.

Superconductivity and magnetism have often been regarded as opposites. High magnetic fields usually destroy the superconducting state. But for superconductors constrained to two dimensions, a parallel magnetic field can actually enhance superconductivity.

At low temperatures and separated by sufficient distances, magnetic impurities embedded in non-magnetic metals lose their magnetic nature. But when two such atoms are brought close together, it reappears. By varying the distance between two cobalt atoms with a scanning tunnelling microscope, the quantum phase transition between these two states can be explored.

The robustness of edge states against external influence is a phenomenon that has been successfully applied to electron transport. A study now predicts that the same concept can also lead to improved optical devices. Topological protection might, for example, reduce the deleterious influence of disorder on coupled-resonator optical waveguides.

That the final energy of an isolated system in contact with a heat bath follows the Gibbs distribution is a classical result of statistical physics. But the situation is different when the system is non-adiabatically driven out of equilibrium. Theoretical work now shows that in these cases the energy distribution is non-Gibbsian and that two qualitatively different regimes with a transition between them emerge.