Topical issue on Novel Quantum Phases and Mesoscopic Physics in Quantum Gases

This topical issue of The European Physical Journal B is a collection of articles originating from the Fourth International Workshop ``Theory of Quantum Gases and Quantum Coherence'' and of the CNRS Ecole Thematique ``Physique M\'esoscopique avec les gaz quantiques'' held in Grenoble (France) from June the 3rd to June the 7th 2008. This event has involved mainly students and young researchers both as speakers and participants, each session being opened by an overview lecture given by senior researchers expert in the fields of quantum gases and condensed matter. Since the achievement of Bose-Einstein condensation (BEC) and superfluidity with atomic Fermi gases, ultracold quantum gases have become a very versatile testing ground for quantum many-body theories. The amazing experimental possibilities which allow changing the dimensionality of the system, introduce disorder in a controlled way, and create a whole variety of strongly correlated states, have made the connection between the physics of ultracold quantum gases and condensed-matter physics stronger and stronger. This connection has been especially taken in consideration in the selection of contributions for the conference. In addition, attention has been also devoted to the recent activities devoted to the realization of BEC in other condensed matter systems (eg polaritons, magnons in Helium-3), which create further links to the condensed matter community. A topic of major relevance at the moment concerns the presence of disorder, both for light and matterwaves. The very recent realization of Anderson localisation of non interacting matter-waves with Bose-Einstein condensates in quasi-one-dimensional geometries, opens up the very promising possibility of understanding the problem of disorder in the presence of interactions and/or in higher dimensionality. Using optical lattices it is also possible to create several geometries suited to the study of low dimensional systems, quantum phase transitions, and the creation of supersolid and paired phases of matter. Especially fascinating results arise from the possibility of tuning the atomic interactions. Use of Fano-Feshbach resonances to change the magnitude and sign of the s-wave scattering length, has allowed, e.g., the observation of collapsing Bose condensates and of the crossover from a BEC to a Bardeen-Cooper-Schrieffer-type transition. More recent experiments have demonstrated that the range of the interactions can also be manipulated. Dipole interactions with long-range anisotropic character have been observed in $^{52}$Cr atoms and this has opened up a rich and active research field. The reduced dimensionality has also become a very useful tool for making possible the realization of the Berezinskii-Kosterlitz-Thouless superfluid phase transition occuring in two-dimensional systems. The target list of the present topical issue includes various contributions on the subjects above but also contributions on the quantum magnetic phases of atoms in optical lattices (ordered or spin liquids), the problem of spin squeezing in a bimodal condensate, and the static properties of Bose-Fermion mixtures. Many of these systems pose longstanding theoretical and experimental condensed matter questions, which might find in ultracold atomic systems a first non controversial answer.