Articles in 2009

Back-action, the effect of a measurement on the subject system, limits precision when determining position. This is of particular importance in nanomechanical oscillators, which could soon enter the quantum regime. A technique that avoids back-action by coupling the oscillator to a microwave resonator has now been demonstrated.

A spectroscopic technique that enables momentum-resolved probing of excitations of atomic gases in optical lattices allows the full band structure of such systems to be measured for the first time. The method should facilitate the comparison of quantum-gas phases with their condensed-matter counterparts.

The composition of integral quantum number particles such as protons and neutrons from the strong confinement of fractional quantum number particles such as quarks is well known in high-energy physics. Now, similar behaviour has been found in condensed-matter physics, in the excitation spectra of a weakly coupled spin-ladder compound.

In a glassy system, a distribution of relaxation times indicates a system that continues to rearrange itself. Besides the main relaxations involved in the glass transition, there are faster dynamics associated with secondary relaxations, which are predicted to reconfigure structures that are stringy rather than tightly clustered.

Quantum oscillations in metals are a signature of electrons travelling in closed orbits in a magnetic field. Could such oscillations occur in the absence of closed orbits, as seems to be the case for the copper oxide superconductors that have arc-like segments instead of closed Fermi surfaces?

Intense optical beams can alter the way that a material interacts with X-ray radiation. This is now demonstrated by experiments that use femtosecond laser pulses to affect inner-shell processes in neon atoms, increasing the transmission of X-rays. This could allow imprinting of optical pulse trains onto much longer X-ray pulses.

Anisotropies in the response of ferromagnetic electrodes attached to a gold nanoparticle lead to Coulomb blockade and spin-valve-like magnetoresistance phenomena. Such behaviour could allow the development of magnetically gated single-electron transistors composed of just two terminals.

Optical tweezers use the forces exerted by light to manipulate objects at the micrometre scale. An approach in which the target particle itself plays an active part now achieves this using a lower light intensity. This reduction means that heat-sensitive targets such as viruses could be manipulated directly.

The spin state of two electrons in a double well is a promising qubit. Now, such qubits can be arbitrarily rotated around two different axes by applying a magnetic field of different magnitude to each electron. This can be done in nanoseconds, before the stored information is lost.

Coupling a nanometre-scale oscillator to a micrometre-scale optical resonator provides a way of measuring the small-amplitude motion. The scheme is applied to silicon nitride ’strings’, but it could be extended to many other types of tiny vibrating structures.

Ferromagnetism usually only occurs in materials containing elements that form covalent 3d and 4f bonds. Its occurrence in pure carbon is therefore surprising, even controversial. A systematic magnetic force microscope study indicates that ferromagnetism in graphite is the result of localized spins that arise at grain boundaries.

Optical lattices, generated by interfering laser beams, provide a platform for observing condensed-matter phenomena in ultracold-atom systems. By extending the lattice idea to a multimode cavity, it should be possible to observe even more complex effects, such as frustration, crystallization, glass phases and supersolidity.

Quantum many-body systems can show an elusive form of order known as topological order. Theoretical work now unifies several microscopic models whereby topological phases have been found, and predicts quantum phase transitions that are driven by quantum fluctuations of the topology.

Formulating a consistent framework for relativistic thermodynamics has been a subject of controversy over the past century. A new approach for defining thermodynamic quantities makes predictions that are, in principle, testable, and which might lead to a natural extension of thermodynamics to general relativity.

In a ‘striped’ superconductor, it may be possible to observe a superconducting state that, with increasing temperature, melts into a unique phase with charge-4e superconductivity, instead of the usual charge of 2e from paired electronic excitations.

The identification of the magnetic-fluctuation mode at a quantum phase transition of the archetypical heavy-fermion compound Ce_1−xLa_xRu₂Si₂ indicates that quantum criticality in this system is governed by collective antiferromagnetic behaviour, rather than by local magnetic moments as has been suggested.

Understanding the mechanical properties of DNA helps us to predict protein–DNA and DNA–DNA interactions. It is now shown that—with the aid of statistical physics—the melting temperature of DNA can be used to extract very detailed information about local flexibility.

Bound entanglement is a particular class that is not distillable—that is, it cannot be converted into a pure maximally entangled state by means of local operations and classical communication. A four-qubit bound entangled state, or Smolin state, has now been created experimentally.

When electrons are transported through a semiconductor quantum dot, they interact with nuclear spin in the host material. This interaction—often considered to be a nuisance—is now shown to provide a feedback mechanism that actively pulls the electron-spin Larmor frequency into resonance with that of an external microwave driving field.

Pumping an atomic system with light at a wavelength that is longer than its resonance can lead to cooling. Conversely, it is now shown that pumping with shorter-wavelength light can lead to the stimulated emission of phonons—in analogy to the amplification of photons in lasers.