Helen Dell

Biomaterials: silk spin-off.

821 particles of half-integer spin, such as neutrons, getting too close to one another. Similar quantum-mechanical effects can be reproduced, at very low temperatures only billionths of a degree from absolute zero, on the entirely different scale of atomic physics in the matter phase known as a Bose–Einstein condensate. What is surprising is that, in the quantum world, attraction isn’t even necessary to form a stable bound system: repulsion can be enough. This prediction, like so much of quantum mechanics, is counterintuitive, as one would expect two repelling particles simply to fall apart to minimize their interaction energy. But in the presence of a periodic spatial perturbation this should no longer be true. Here, the energy of a particle cannot vary continuously, but is restricted to particular ranges of values. A pair of two repelling particles can therefore be stable simply because, if it fell apart, conservation of energy would require the two isolated atoms to have energies that would fall in a forbidden range. Although periodic structures are the natural forms in which the atoms arrange themselves into crystals — and as such are very commonin physics — interactions with the environment in the solid state dissipate energy, preventing the observation of the bound, repulsive pairs predicted by the theoretical model. Winkler and colleagues circumvent this obstacle by using a periodic structure made of light. In such an ‘optical lattice’, ultracold atoms are ordered in a regular array of microscopic traps caused by the interference of two or more laser beams. The resulting ordered system of atoms resembles a solid-state configuration, and neutral atoms in an optical lattice do indeed share many properties with electrons in a metal. In contrast to a solid-state crystal, however, light crystals are free from defects and dissipative vibrations, and have thus proved ideal systems for the investigation of otherwise elusive quantum-physical phenomena. In Winkler and colleagues’ experiments, the capabilities of the optical lattice are combined with a powerful tool that atomic physicists can now use to change the nature of the interactions between two atoms, transforming attractions into repulsions, and vice versa. This trick is achieved by placing cold atoms in a uniform magnetic field, and tuning the intensity of the field across a resonance — known as a Feshbach resonance — that occurs when the energy of a bound molecular state is exactly equal to the energy of two colliding atoms. Following on from the observation of Bose–Einstein condensation (which involves ‘bosonic’ atoms of integer spin; atoms of half-integer spin are termed ‘fermionic’), Feshbach resonances have made possible the observation of striking phenomena — including the formation of cold molecules and culminating in the observation of superfluidity among ultracold fermionic atoms. Winkler et al. take a cold gas of bosonic rubidium (Rb) atoms, which naturally have repulsive interactions, and first fill a threedimensional, cubic optical lattice such that each lattice site confines either no atoms, or two identical atoms (Fig. 1). The authors then use a Feshbach resonance to control the interaction between the atoms forming the pairs. When interactions are cancelled, the pairs are not stable and the atoms quickly fall apart, diffusing within a few milliseconds across the lattice and hopping from one site to the next. When repulsive interactions are restored, however, the separation process is halted. The pairs of atoms then live much longer together — several hundred milliseconds — clearly demonstrating that their stability is induced by the mutual repulsion of the constituents. The authors obtain additional information about the nature of the pairs by measuring the velocity distribution of the atoms and the energy of the bond. This new type of bound object remains stable because, in the structured environment of the lattice, the large repulsive interaction between the atoms cannot be converted into kinetic energy. This result is general, as magnetic fields could be used to tune the strength of the interaction between other atomic species. An even broader impact could be achieved by extending the concept to mixtures of different atoms, and also by playing with quantum gases of different nature, both bosonic and fermionic. Once more, it seems, optical lattices are demonstrating how useful they are for investigating many-body phenomena unobservable in other systems in which the interaction with the environment is too strong. As cold atoms are themselves a type of experimental simulator of quantum-physical effects, the result is a toolbox that kits us out to explore fascinating new avenues in quantum information and quantum states of matter. ■ Leonardo Fallani and Massimo Inguscio are at the European Laboratory for Non-Linear Spectroscopy (LENS), Università degli Studi di Firenze, Polo Scientifico, Via Nello Carrara 1, I-50019 Sesto Fiorentino, Italy. e-mail: inguscio@lens.unifi.it

Microbiology: Perspectives on plague

Polarity arrives earlySeveral decades of work in frogs and fish led to the hypothesis that ‘localized determinants’ deposited in the egg by the mother establish the dorsal axis of the developing embryo, but the nature of the determinants has remained a mystery. New work in the zebrafish embryo identifies Squint, a Nodal-related transforming growth factor-β signal, as a possible localized dorsal determinant. Squint is present in the two- and four-cell embryo, suggesting that embryonic polarity is established very early on in development. The presence of Squint element in mammals suggests that this mechanism of polarity determination is conserved in the higher vertebrates.