The quantum hydrodynamic description of quantum gases with different interactions

TL;DR

This paper reviews the development of quantum hydrodynamics for ultracold Bose gases, deriving equations that describe their behavior with various interactions, including short-range, three-particle, nonlocal, and dipole-dipole interactions, and connecting them to generalized nonlinear Schrödinger equations.

Contribution

It introduces a comprehensive derivation of quantum hydrodynamics equations for Bose gases with different interactions, extending the Gross-Pitaevskii framework to include higher-order and nonlocal effects.

Findings

Derivation of quantum hydrodynamics equations for Bose gases.

Extension of the Gross-Pitaevskii equation to include three-particle interactions.

Generalization of the GP equation for dipolar Bose gases.

Abstract

We describe recent development of quantum hydrodynamics for ultracold Bose particle studying and consider different kinds of interactions. The method of derivation of equations describing the evolution of the neutral Bose particle system at low temperatures is described. Despite the fact that we consider the neutral particles we account the short-range interaction between particles. We consider the particles in the Bose-Einstein condensate (BEC) state. This method is called the method of quantum hydrodynamics, because natural for of the quantum mechanics rewritten in terms of material fields of observable quantities in three dimensional space is the set of equations, which look like the hydrodynamics equations. It can be shown that from the quantum hydrodynamics equations can be derived macroscopic non-linear Schrodinger equation. Most famous non-linear Schrodinger equation is the Gross-Pitaevskii (GP) equation, which contains nonlinearity of the third degree. There are generalizations of the GP equation. New term appears in the GP equation at account of the three-particle interaction. This term contains nonlinearity of the fifth degree. At more detailed account of the two particle interaction we come to the nonlocal non-linear Schrodinger equation. This equation contains spatial derivatives of the order parameter in the non-linear terms caused by the interaction. In this terminology the GP equation corresponds to the first order by the interaction radius. For the BEC of the neutral particles with anisotropic long-range dipole-dipole interaction the generalization of the GP equation was also suggested. Detailed analyses of the applicability conditions shows that this equation valid for the system of dipoles parallel to each other, which do not change their direction, and where the dipole-dipole interaction interferences translational motion of particles.