Letters in 2012

A thermodynamic probe of the recently discovered charge-density-wave order in YBa₂Cu₃O_y reveals a biaxial modulation in magnetic fields up to 40 T.

Quantum dots are a promising host for spin-based qubits. Whereas nuclear-field fluctuations adversely affect electron-spin coherence, the smaller hyperfine interaction between holes and nuclei makes holes a promising alternative. A sensitive measurement of the hyperfine constant of the holes in different quantum-dot material systems now demonstrates how this interaction can be tuned and perhaps further reduced.

The elusive effects of quantum gravity could be betrayed by subtle deviations from standard quantum mechanics. An experiment using the gravitational wave bar detector AURIGA explores the limits of quantum gravity-induced modifications in the ground state of a mechanical oscillator cooled to the sub-millikelvin regime.

Many-particle entangled states and entanglement between continuous properties are valuable resources for quantum information, but are notoriously difficult to generate. An experiment now entangles the energy and emission times of three photons, creating generalized Einstein–Podolsky–Rosen correlations.

Liquid water inclusions in quartz can withstand negative pressures in excess of −100 MPa. Other techniques report much lower thresholds—suggesting that water in inclusions is stabilized by impurity effects. Experiments on a single inclusion in quartz now provide evidence consistent with a homogeneous mechanism for cavitation.

In the highly degenerate spin-ice ground state, flipped spins give rise to magnetic charges, or monopoles, which form a measurable current in a magnetic field. The low-temperature relaxation dynamics of spin-ice materials now reveal that defects can impede monopole flow—creating a magnetic analogue of electrical resistance.

Long-distance quantum communication is limited by optical absorption and scattering. A noiseless amplifier for photonic qubits coherently encoded across two optical modes is now demonstrated, which could combat this negative effect. The method enabled a fivefold increase in the transmission fidelity of the polarization state of a single photon.

Non-local quantum correlations between distant particles cannot be explained by signals propagating slower than the speed of light. It is now shown that they cannot be explained by hidden influences propagating faster than the speed of light either, because that would permit faster-than-light communication.

X-ray diffraction experiments reveal that spatial charge ordering occurs in the pseudogap state of YBa₂Cu₃O_6.67. Moreover, this charge ordered state competes with high-temperature superconductivity, and their relative strengths can be tuned using a magnetic field.

Current shot-noise for a relativistic electron beam—proportional to the average current and frequency bandwidth of the beam—can be suppressed below the shot-noise limit at optical frequencies, through the exploitation of collective Coulomb interactions.

Entanglement is an important resource in quantum-enhanced technologies, but it is difficult to generate, especially in solid-state systems. An experiment now demonstrates the entanglement of two nuclear spins via a parity measurement of the electron spin in a nitrogen-vacancy centre in diamond.

Extreme ultraviolet and X-ray radiation can be generated when the high harmonics of incident laser light are reflected by a dense plasma, the so-called relativistically oscillating mirror mechanism. Theoretical studies have, however, predicted an alternative regime in which short-wavelength light is generated by dense electron nanobunches that form at the plasma–vacuum boundary. Signatures of this coherent synchrotron emission are now experimentally observed.

The so-called braking index calculated for the spin-down of rotating neutron stars, or pulsars, doesn’t tally well with observations. But a model accounting for a changing moment of inertia, as an increasing fraction of the stellar core becomes superfluid, can explain the rotational evolution of young pulsars.

Stable structures can self-assemble in plasmas flowing at supersonic speeds, as evident in many astronomical objects. But now it is also seen in the laboratory using two plasmas travelling in opposite directions, each created by ablating a plastic disc with high-power lasers.

A topological insulator has surface metallic states that are topologically protected by time-reversal symmetry. Tin telluride is now shown to be a ‘topological crystalline insulator’, in which the surface metallic state is instead protected by the mirror symmetry of the crystal.

The fractional alternating-current Josephson effect produces a series of steps in the current–voltage characteristics of a superconducting junction driven at radiofrequencies. This unusual phenomenon is now observed in a semiconductor–superconductor nanowire. What is more, a doubling in step size when a strong magnetic field is applied could be a possible signature of Majorana fermions, particles that are their own antiparticle.

A two-level quantum system driven by an electromagnetic field can oscillate between its two states. The effects of these so-called Rabi oscillations are usually obscured in many-body systems by the variation in properties of the particles involved. Now, however, coherent many-body Rabi oscillations are observed in a vapour made up of several hundred cold rubidium atoms.

Chirality is usually manifested by differences in a material’s response to left- and right-circularly polarized light. This difference is the result of the specific distribution of charge within chiral materials. A similar response has now been found to result from the chiral spin structure of an antiferromagnet.

Optical vortices exhibit a corkscrew-like shape as they travel. The study of this phenomenon, known as singular optics, is now extended to the high-power regime where high-harmonic processes become evident. This type of radiation could help illuminate novel attosecond phenomena in atoms and molecules.

An analogue of a magnetic monopole is now observed in a condensed state of light–matter hybrid particles known as cavity polaritons. Spin-phase excitations of the polariton fluid are accelerated along the cavity under the influence of a magnetic field—just as if they were single magnetic charges.