A., Minguzzi

A., Recati

Citro, Roberta

E., Orignac

T., Roscilde

Archivio della Ricerca - Università di Salerno

Eur. Phys. J. Special Topics 217, 1–2 (2013)c© EDP Sciences, Springer-Verlag 2013DOI: 10.1140/epjst/e2013-01748-8THE EUROPEANPHYSICAL JOURNALSPECIAL TOPICSEditorialNovel quantum phases and mesoscopic physicsin quantum gasesR. Citro, A. Minguzzi, E. Orignac, A. Recati, and T. RoscildeThis volume of The European Physical Journal Special Topics is a collection oforiginal papers presented either as posters or as oral contributions at the SixthInternational Workshop “Theory of Quantum Gases and Quantum Coherence” andCNRS E´cole The´matique “Physique Me´soscopique avec les gaz quantiques” held inLyon (France) from the 5th to the 8th of June 2012. This event has involved mainlystudents and young researchers both as speakers and participants, each session beingopened by an overview lecture given by senior researchers expert in the ﬁelds ofquantum gases and condensed matter.Since the observation of Bose-Einstein condensation (BEC) and of superﬂuidityin atomic Fermi gases, ultracold quantum gases have become a very versatile testingground for quantum many-body theories. The amazing experimental possibilitieswhich allow changing the dimensionality of the system, introduce disorder in acontrolled way, and create a whole variety of strongly correlated states, have madethe connection between the physics of ultracold quantum gases and condensed-matter physics stronger and stronger. This connection has been especially takenin consideration in the selection of contributions for the conference. In addition,attention has also been given to the recent activities devoted to the realizationof BEC in condensed matter systems (e.g. polaritons and magnons), which createfurther links to the condensed matter community. The papers collected in thistopical issue represent a cross section of the oral and poster contributions in theWorkshop/E´cole and contain original results on six relevant research directions inthe ﬁeld of condensed matter with ultracold gases.A ﬁrst important topic in condensed matter theory is the eﬀect of dimensionalityon correlated particles. In one dimension, a central concept is the Tomonaga-Luttingerliquid, in which the proper low energy excitations are not quasiparticles having thesame quantum numbers as the non-interacting particles but renormalized masses andweak residual interactions, but collective charge and spin excitations propagatingwith diﬀerent velocities (the so-called spin-charge separation phenomenon). Evenat zero temperature, a Tomonaga-Luttinger liquid only exhibits quasi-long rangeorder, with superﬂuid and density-wave correlations decaying as power laws withdistance with non-universal exponents. Trapping in two-dimensional optical lattices,or on-chip trapping allows the realization of quasi-one dimensional atomic gases inwhich the Tomonaga-Luttinger liquid can be observed. Moreover, the application ofa longitudinal periodic potential permits the observation of the Tomonaga-Luttingerliquid to Mott insulator transition. Atomic gases also oﬀer the unique possibilityto study the non-equilibrium dynamics of Tomonaga-Luttinger liquids. The physicsof quasi-one dimensional atomic gases has thus been one of the main topics of theworkshop, as shown by the contributions of Dalmonte et al., Jolicoeur, Nevado andPorras, Naddeo et al. and Langen et al.2 The European Physical Journal Special TopicsA second important topic of condensed matter theory is Anderson localization. Inthe absence of interaction, it is well known that one and two dimensional systems haveonly localized states in the presence of disorder, while in three dimensions for weakdisorder high energy extended states and low energy localized states are separatedby a mobility edge, and for strong disorder all states are localized. In the absence ofinteraction, the resulting metal insulator transition is well understood thanks to an-alytical and numerical work. In condensed matter physics however, electron-electronand electron-phonon interaction make the picture more complicated. In experimentson ultracold gases, electron-phonon interactions are absent, and the particle-particleinteraction can be tuned using Feshbach resonances. The observation of Andersonlocalization and of the Aubry-Andre´ transition in 2008 have led to a considerableactivity in the ﬁeld of disorder and cold atoms. Another route for studying the An-derson localization with cold atoms is by exploiting the relationship between a kickedrotor and the Prange model. The contributions of Alamir et al., Gaul and Mu¨ller,Jachymski and Idziaszek, Plisson et al., Piraud and Sanchez-Palencia, Seiringeret al., and Vermersch and Garreau cover that ﬁeld.A third important topic in condensed matter physics is the so-called “Diracmatter” in which the low-energy fermionic excitations are described by a two-dimensional analogue of the Dirac equation. In solid state physics, “Dirac matter”has been observed in graphene and in the “topological insulator” materials such asBi2Te3, Bi2Se3. Atomic trapping allows the realization of lattices analogous to theone of graphene or topological insulators paving the way for the study of “Diracmatter” in ultracold gases. That topic is covered in the contributions of Tarruellet al. and Goldman et al.A fourth important topic is the physics of strong interaction, in particular theBEC-BCS crossover, the unitary limit (where the scattering length diverges) and thepolaron eﬀect (where an impurity is moving in a bath of interacting particles). Thistheme is covered in the contributions of Enss, Bertaina and Casteels et al.A ﬁfth topic, which was the original motivation to study trapped atomic gases, isthe hydrodynamics of interacting Bose gases. Questions such as the critical velocity,and the formation of vortices in a superﬂuid can be realistically considered withinthe Bogoliubov or the Gross-Pitaevskii formalism, in contrast with superﬂuid 4He.Moreover, cold atoms allow the realization of spinorial Bose gases, in which a richerhydrodynamics can be expected. Also, BEC of magnons and polaritons oﬀer newpossibilites of probing the hydrodynamics of Bose gases. This topic is touched bythe contributions of Chestnov et al., Rougerie and Szirmai.A last topic is the interplay of coherent quantum transport and many bodyphysics considered in mesoscopic physics. With cold atoms, such interplay can beobserved in Bose-Josephson junctions, or in Bose-Hubbard lattices, allowing tocharacterize the entanglement and the decoherence of the eigenstates in thesesystems. Such a theme is covered in the contribution of Tarallo et al., Khripkov et al.and Mazzarella and Dell’Anna.All the papers have been peer reviewed following the usual standards of TheEuropean Physical Journal Special Topics. We would like to thank the Chief Editorsfor giving us the possibility of publishing this topical issue, and all the staﬀ andespecially Agne`s Henri. We would also like to thank the other organizer of theConference, Fre´de´rique Che´vy, who has contributed to the success of the event.This Workshop was made possible thanks to the ﬁnancial support of theCentre Europe´en de Calcul Atomique et Mole´culaire (CECAM, Lausanne), theCentre National de la Researche Scientiﬁque (CNRS), the re´gion Rhoˆne-Alpes,the E´cole Normale Supe´rieure de Lyon, the University Claude Bernard (Lyon-I), theFe´de´ration de Physique Andre´-Marie Ampe`re, the E´cole Doctorale PhAst (Lyon),the E´cole Doctorale des Physique (Grenoble), The Centre de The´orie en Physiquede Grenoble, the BEC:INO-CNR BEC Center in Trento, the Department of Physics“E.R. Caianiello”(Salerno).

Novel quantum phases and mesoscopic physics in quantum gases

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