Quantum Droplets

Dear Colleagues,

One of the major advances in the field of ultracold atoms is the recent prediction and observation of ultradilute quantum liquids. In normal liquids, the distance between atoms is of the order of the van der Waals radius, set by the competition between the short-range repulsion and long-range attraction. Instead, ultradilute quantum liquids are formed due to a near cancellation of mean-field and beyond mean-field energy terms, which is possible due to different scaling of those terms with the density. This balance has made it possible to experimentally realize liquids at densities which are eight orders of magnitude more dilute compared to that of usual liquids, such as helium, which is a paradigmatic quantum liquid. Another important feature of the novel class of liquids is that they are almost fully coherent, while the condensate fraction in a liquid helium is smaller than ten percent. Ultradilute quantum liquids have already been created in two types of settings: (i) dipolar atoms with tunable short-range interactions and (ii) two-component mixtures with short-range interactions between atoms. Ultracold atoms offer an exquisite degree of controllability of the interactions (Feshbach resonances, confinement-induced resonances, etc.), a variety of external fields (traps, optical lattices, etc.) and various geometries (three-, two- and one-dimensional), resulting in the rapid expansion of the field. There is great interest in understanding and quantifying the properties of already available ultradilute liquids, as well as the prediction for novel species. A significant number of proposals have been elaborated for this type of quantum matter for different geometries, statistics (Bose, Fermi and mixtures) and nonlinearities, including the settings with the spin-orbit and Rabi couplings, multi-component compositions, etc. The aim of this Topic is to collect important contributions in the field of ultradilute quantum liquids, including theoretical and experimental works.

Prof. Dr. Boris Malomed
Dr. Chiara D’Errico
Prof. Dr. Enguo Chen
Dr. Grigori E. Astrakharchik
Topic Editors