Solitary waves in trapped ultracold gases

by Prof. Lev Petrovič Pitaevskij (University of Trento and Kapitza Institute for Physical Problems RAS)

Monday 27 Nov 2017, 17:00 → 18:00 Europe/Rome

MLH (GSSI)

Solitary waves, that is localized perturbations of a media, are the fascinating topological excitations of nonlinear systems. Despite the fact that they do not correspond to the ground states of the systems, these solitary waves can be stable and live for a long time under certain physical conditions, which may have important applications for information processing. Because of the tunability of the interaction coupling constants and the absence of disorder, ultracold atomic gases provide an ideal playground for the observation of these excitations. Since the first realization of Bose-Einstein condensate with alkali atoms, various solitary waves and other quantum defects have been experimentally observed and/or theoretically investigated, such as scalar solitons, vector solitons, domain walls, and solitonic vortexes. Among various physical systems, ultracold atomic gases provide a prominent platform for the investigation of solitons which can be engineered by phase imprinting, density imprinting, quantum quenches. From the other hand the solitary waves exhibits a quite peculiar dynamic properties and their description arises non-trivial theoretical questions. For example, even a definition of the canonical momentum is a quite non-trivial problem. A variety of solitary waves, such as solitons, vortex rings, solitonic vortices, and more complex entities, have recently been predicted to exist. The simplest one is a dark soliton, which can be described by an analytical solution of the Gross-Pitaevskii equation. Other demands numerical solitions. Examples will be presented. The solitary waves can oscillate in superfluid ultracold gases along elongated traps.The theoretical description of this motion requires knowledge of the inertial soliton mass and the effective number of particles in it as functions of the soliton velocity. While these functions can be calculated by a microscopic theory, it is also possible to express them directly in terms of observable quantities, such as the order parameter phase jump and the particle number depletion in the soliton. An interesting new possibilities arise in mixtures of different spin states of ultra-cold gases, where can exist so-called magnetic solitons. The problem permits a simple analytical description. New unusual phenomena can be observed in the presence of so-called Rabi coupling between the spin states.