The development of modern technologies relies on the exquisite knowledge of transport properties. Electronic devices and computers are indeed based on the possibility to generate and control currents of electrons, elementary particles which abound in materials. By exploiting their electric charge and their response to electromagnetic fields, these particles are minutely guided along circuits composed of fine conducting materials. Thus, the information transport from which we benefit daily is associated to an intrinsic property of the electron: its charge.
Besides a mass and a charge, the electron also possesses a property whose existence was only revealed with the birth of quantum mechanics. This property, called spin, can be imagined as coming from a fictitious rotation of the particle and plays a fundamental role in magnetism. Contrary to its charge, the electron's spin can take two distinct values and could therefore carry supplementary information. By manipulating this additional degree of freedom, researchers envisage a revolution of modern technologies, generally refereed to as spintronics.
Today, the realization and the control of spin currents constitutes a very hot field of research. In 2006, a novel class of materials named topological insulators has been discovered in laboratories. In contrast with usual insulators, which do not carry electrical currents, these fascinating materials are characterized by spin currents counter-propagating along their edge. Remarkably enough, this spin transport occurs without any energy loss and is shown to be extremely robust in the presence of external perturbations.
The robustness of this phenomenon, called "quantum spin Hall effect," is elegantly described at the theoretical level: when this effect occurs, the material enters a novel phase characterized by a topological invariant -- a mathematical number which remains constant in response to small deformations. This specific state of matter is exotic, in the sense that it cannot be classified according to the criteria developed by the phase transitions theory. However, the materials presenting the quantum spin Hall effect generally suffer from various imperfections, such as impurities and additional couplings between the particles. These undesired features prohibit the observation of interesting effects and would complicate the application of topological insulators at the industrial scale.
Consequently, a better understanding of topological insulators would require neater and handier setups. Cold atoms -- atoms trapped and cooled by lasers -- offer an ideal playground for the exploration of exotic states of matter. These artificial systems offer the unprecedented possibility to control the interactions between the particles and do not contain impurities, in contrast with usual materials. In that sense, cold atoms are quantum simulators, which exploit today's finest experimental technologies. In particular, these systems allow to manipulate the atomic spins with great precision.
The realization of topological insulators with cold atoms is extremely attractive, as it would offer an ideal quantum spin Hall effect within a laboratory. In an article recently published in the journal Physical Review Letters, an international team lay out such an experiment. This work stems from a collaboration set up between theoreticians [N. Goldman (Brussels), A. Bermudez, M. A. Martin-Delgado (Madrid), M. Lewenstein (Barcelona), I. Satija, P. Nikolic (Maryland, USA)] and the experimentalist I. Spielman from the National Institute of Standards and Technology (NIST). They have elaborated an ingenious setup which produces topological insulators using cold atoms on an atom chip.
The latter is constituted by a set of current carrying wires, creating well designed magnetic fields that confine and guide the atoms on the chip. By controlling these magnetic fields, using methods developed by I.
Spielman at NIST, these authors demonstrate that the atomic system can enter a topological phase leading to the quantum spin Hall effect. The authors also propose an efficient method to detect the presence of spin currents within this new framework. The realization of topological liquids with cold atoms will deepen our understanding of these exotic states of matter and will certainly drive the development of quantum computing.
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