Letters in 2011

Similar to atoms in cold gases, exciton–polaritons in semiconductor microcavities can undergo Bose–Einstein condensation. Now, formation of a condensate in an excited orbital state has been observed in such a system, underlining the potential of exciton–polariton condensates to emulate condensed-matter physics.

The energy-landscape theory is an important tool for investigating how proteins fold. Hummer and Szabo conceived a simple method for constructing folding-energy landscapes from single-molecule pulling experiments. But are these non-equilibrium measurements a valid approach to equilibrium landscapes? The Hummer–Szabo formalism is now experimentally validated for the first time.

The charge carriers in single-layer graphene are effectively massless. In bilayer graphene, they are massive. In trilayer graphene, the two types coexist, which leads to an unusual quantum Hall response in which the Landau levels of massless and massive charge carriers repeatedly cross.

Quantization of the current flowing across a nanometre-scale constriction in graphene is usually destroyed through charge-scattering from rough edges and impurities. But by using high-quality suspended samples and a constriction whose length is shorter than its width, conductance quantization in graphene has now been demonstrated.

The chaotic nature of turbulence makes it difficult to develop simple rules of thumb to predict the behaviour of a turbulent flow. But analysis of the motion of three tracer particles with respect to a fourth suggests at least one: that the local alignment of turbulent rotation conserves angular momentum, similar to an ice-skater performing a pirouette.

Transferring graphene onto hexagonal boron nitride enables high-mobility multiterminal quantum Hall devices to be built. This makes it possible to study graphene's unique fractional quantum Hall behaviour more easily and more directly than previously.

The full phase diagram of supercooled silicon has not been accessible experimentally, so the critical behaviour is highly debated. Numerical simulations now reveal a liquid–liquid critical end-point at negative pressure. This study further supports the similarity between silicon and water.

Single electron spins have been detected before, but the methods used proved difficult to extend to multi-spin systems. A magnetic resonance imaging technique is now demonstrated that resolves proximal spins in three dimensions with nanometre-scale resolution. In addition to spatial mapping, the approach allows for coherent control of the individual spins.

Edge effects matter in graphene, particularly in nanoribbons. A study using scanning tunnelling microscopy and spectroscopy reveals how chirality at the atomically well-defined edges of a graphene nanoribbon affects its electronic structure.

Bell’s theorem experiments, which test the completeness of quantum mechanics, have a number of loopholes. However, one type—detection loopholes—becomes smaller when the measurement has more possible outcomes. Bell’s inequality is now violated in tests with as many as 11 different results.

Twin photons — pairs of highly correlated photons — are one of the building blocks for quantum optics, and are used in both fundamental tests of quantum physics and technological applications. Now an efficient source for correlated atom pairs is demonstrated, promising to enable a wide range of experiments in the field of quantum matter-wave optics.

Point defects in diamond known as nitrogen-vacancy centres have been shown to be sensitive to minute magnetic fields, even at room temperature. A demonstration that the spin associated with these defect centres is also sensitive to electric fields holds out the prospect of a sensor that can resolve, under ambient conditions, single spins and single elementary charges at the nanoscale.

Spectroscopic techniques are mostly used to study the interaction between matter and electromagnetic fields. Here, an experiment that probes the transitions between quantum states of neutrons in the Earth’s gravitational field demonstrates an exotic variant of spectroscopy, and one that might lead to sensitive fundamental tests of gravity laws.

The study of many fundamental processes in chemistry relies on the understanding of the dynamics of the valence electrons, which make and break chemical bonds. A laser method now provides direct information on the dynamics of the valence electrons—separate from any vibrational motion—during a polyatomic chemical reaction, without the need for strong laser fields that unavoidably influence the motions of these electrons.

The power of magnetic resonance imaging for investigating physical and biological systems is well established. Here, it is shown how the sensitivity of cavity atom optics, together with the control provided by atom chips, enables the implementation of a magnetic-resonance-imaging technique that provides a minimally destructive, state-sensitive detection modality for atoms in ultracold gases.

Intense femtosecond pulses of infrared light are frequently used to manipulate molecules. It is now shown that such control even extends to making different molecular eigenstates interfere with each other — an effect that could potentially pave the way to using molecules for quantum information processing.

In the past few years, there have been a number of proposals for fabricating magnetic memories based on the current-induced motion of magnetic domain walls. A device that uses a novel geometry for injecting electrical currents into the sample is shown to work with current densities that are two orders of magnitude lower than in previous approaches.

Magnetic reconnection is important to the dynamics of many astrophysical and fusion plasmas but our understanding of it is incomplete. Petaflop-scale simulations of the evolution of turbulent magnetic reconnection in a three-dimensional plasma indicate that it proceeds in a way that is dramatically different from classical theory.

Although evidence indicates that defects induce magnetism in graphite, it’s unclear whether this extends to graphene. An observation of the gate-tunable Kondo effect in ion-beam-damaged graphene suggests it does.

An atom recoils as it emits a photon. Researchers now show that the two possible recoil trajectories become coherently superimposed when a mirror is placed near the atom. This is because the mirror prevents the photon from giving away any information about the recoil direction.