This chapter introduces the fundamental concepts of Bose–Einstein condensates (BECs), providing a comprehensive theoretical foundation for subsequent investigation on their localization and numerical simulation. It begins with a historical overview of BECs, tracing their theoretical origins and experimental realization. The chapter then focuses on the theoretical perspective of scalar BECs and how it can be modelled by a single-component Gross–Pitaevskii (GP) equation in the mean-field description, a variable-coefficient nonlinear Schrödinger (NLS) equation encompassing a trapping potential and interatomic interaction. The mean-field description is then extended to govern the dynamics of vectorial condensates described by a coupled GP equation with intra and inter-species (atomic) interactions. Subsequently, the chapter discusses the origin of fermionic condensates, highlighting their distinct pairing mechanisms. Beyond mean-field approaches are then introduced to address the limitations of the GP framework, incorporating quantum fluctuations and correlations. The chapter concludes with an outline of the textbook, setting the stage for analytical and numerical investigations of ultracold atoms in ensuing chapters.
This chapter introduces the fundamental concepts of Bose–Einstein condensates (BECs), providing a comprehensive theoretical foundation for subsequent investigation on their localization and numerical simulation. It begins with a historical overview of BECs, tracing their theoretical origins and experimental realization. The chapter then focuses on the theoretical perspective of scalar BECs and how it can be modelled by a single-component Gross–Pitaevskii (GP) equation in the mean-field description, a variable-coefficient nonlinear Schrödinger (NLS) equation encompassing a trapping potential and interatomic interaction. The mean-field description is then extended to govern the dynamics of vectorial condensates described by a coupled GP equation with intra and inter-species (atomic) interactions. Subsequently, the chapter discusses the origin of fermionic condensates, highlighting their distinct pairing mechanisms. Beyond mean-field approaches are then introduced to address the limitations of the GP framework, incorporating quantum fluctuations and correlations. The chapter concludes with an outline of the textbook, setting the stage for analytical and numerical investigations of ultracold atoms in ensuing chapters.
This book offers a detailed introduction to Bose-Einstein condensates (BECs) and nonlinear quantum systems, tailored for advanced undergraduates and researchers. It bridges foundational concepts with modern developments in quantum condensed matter. Starting with the history and experimental milestones of BECs, it explores theoretical models including mean-field and beyond-mean-field approaches for both scalar and vectorial condensates. Analytical techniques such as inverse scattering and soliton theory are covered alongside numerical methods for solving the Gross-Pitaevskii equation. Applications span optical solitons, quantum vortices, and exciton-polariton physics. Each topic is supported by examples, problem sets, and computational tools, equipping readers with both theoretical insight and practical skills to engage with cutting-edge research areas like quantum turbulence and supersolids. Part of IOP Series in Advances in Optics, Photonics and Optoelectronics. Key features • Introduces the subject at the very basic level. • Includes the analytical and theoretical methods. • Contains numerical methods. • Provides worked examples, end-of-chapter summaries, and homework problems.
Abstract In the next few chapters we cover some further techniques based on NLS-type equations by focusing on a particularly attractive application in atomic physics: Bose-Einstein condensates (BECs). The mean-field dynamics of BECs can be modeled by the Gross-Pitaevskii (GP) equation which is a generalization of the NLS equation that includes an external potential. In this chapter we introduce BECs and some basic properties of the GP equation. In particular, we show how the external potential may render the, originally 3D, BEC into an effective 1D or 2D system. We also describe the large mass (Thomas-Fermi) limit and the low mass (linear) limit.
The achievement of Bose-Einstein condensation (BEC) in ultracold vapors of alkali atoms has given enormous impulse to the theoretical and experimental study of dilute atomic gases in condensed quantum states inside magnetic traps and optical lattices. This article offers a short survey on mathematical models and theories as well as numerical methods for BEC based on the mean field theory. We start with the Gross-Pitaevskii equation (GPE) in three dimensions (3D) for modeling one-component BEC of the weakly interacting bosons, scale it to obtain a three-parameter model and show how to reduce it to two dimensions (2D) and one dimension (1D) GPEs in certain limiting regimes. Mathematical theories and numerical methods for ground states and dynamics of BEC are provided. Extensions to GPE with an angular momentum rotation term for a rotating BEC, to GPE with long-range anisotropic dipole-dipole interaction for a dipolar BEC and to coupled GPEs for spin-orbit coupled BECs are discussed. Finally, some conclusions are drawn and future research perspectives are discussed.
Bose–Einstein Condensation (BEC) of atomic gases has attracted a renewed theoretical and experimental interest in quantum many-body systems at extremely low temperatures (Pethick & Smith; 2002). This excitement stems from two favorable features: (1) by applying magnetic fields and lasers, most of the system parameters, such as the shape, dimensionality, internal states of the condensates, and even the strength of the interatomic interactions, are controllable; (2) due to the diluteness, the mean-field theory explains experiments quite well. In particular, the Gross–Pitaevskii (GP) equation demonstrates its validity as a basic equation for the condensate dynamics. The GP equation is the counterpart of the nonlinear Schrodinger (NLS) equation in nonlinear optics. Thus, a study based on nonlinear analysis is possible and important. In nonlinear physics, a soliton is remarkable object not only for the fact that exact solutions can be obtained but also for its usefulness as a communications tool due to its robustness. In general, solitons are formed under the balance between nonlinearity and dispersion. For atomic condensates, the former is attributed to the interatomic interactions, while the latter comes from the kinetic energy. Either dark or bright solitons are allowable depending on the positive or negative sign of the interatomic coupling constants g, respectively, and indeed have been observed in a quasione-dimensional (q1D) optically constructed waveguide (Strecker et al.; 2002) (Khaykovich et al.; 2002). Such matter-wave solitons are expected in atom optics for applications in atom laser, atom interferometry, and coherent atom transport (Meystre; 2002). In this chapter, we extend the analysis of the matter-wave solitons to a multicomponent case by considering the so-called spinor condensate (Stenger et al.; 1998) whose spin degrees of freedom are liberated under optical traps. Based on theoretical and experimental results, we introduce a new integrable model which describes the dynamical properties of the matter-wave soliton of spinor condensates (Ieda et al.; 2004a). We employ the inverse scattering method to solve this model exactly. As a result, we predict the occurrence of undiscovered physical phenomena such as macroscopic spin precession and spin switching. The chapter is organized as follows. In Sec. 2, the mean field theory of condensate is briefly reviewed. Section 3 introduces an effective interatomic coupling in a q1D condensate. Using these results, we consider a spinor condensate in q1D regime in Sec. 4. Then, in Sec. 5, we
Trapped Bose-condensed atomic gases are remarkable because, in spite of the fact that these are very dilute systems, they exhibit robust coherent dynamic behaviour when perturbed. These quantum “wisps of matter” are a new phase of highly coherent matter. While binary collisions are very infrequent, the large coherent mean field associated with the Bose condensate ensures that interactions play a crucial role in determining the collective response of these trapped superfluid gases. In our discussion of the theory of collective oscillations of atomic condensates, the macroscopic Bose wavefunction Φ( r , t ) plays a central role. This wavefunction is the BEC order parameter. As discussed in Chapter 1, the initial attempts at defining this order parameter began with the pioneering work of London (1938a), were further developed by Bogoliubov (1947) and finally extended to deal with any Bose superfluid using the general quantum field theoretic formalism developed by Beliaev (1958a). Almost all this early theoretical work was limited to T = 0 where, in a dilute weakly interacting Bose gas, all the atoms are in the condensate. The first extension of these ideas to nonuniform Bose condensates was by Pitaevskii (1961) and, independently, by Gross (1961), which led to the now famous Gross–Pitaevskii (GP) equation of motion for Φ( r , t ). Before the discovery of BEC in trapped gases, the time-dependent GP equation was mainly used to study vortices in Bose superfluids, which involve a spatially nonuniform ground state. Apart from this application, the GP equation was largely unknown. The situation changed overnight in 1995 with the creation of trapped nonuniform Bose condensates in atomic gases.
This chapter focuses on exploring localized excitations in the variable coefficient nonlinear Schrodinger (NLS) equation and its generalizations. The motivation comes from the fact that the NLS equation and its variants appear in several branches/topics of physics, including nonlinear optics and Bose-Einstein condensates (BECs). The chapter reviews critically the variety of matter wave solitons and their interactions in single- and multi-component Bose-Einstein condensates. It provides a brief overview of the mean Gross- Pitaevskii (GP) equation, derivation of the one-dimensional GP equation, and mapping of the 1D GP equation to the standard NLS equation. The chapter discusses matter wave single and multi bright and dark solitons of Bose– Einstein condensates with varying trap potentials and atomic interactions. It shows various solitons solutions of multi-component BECs described by the two coupled GP equations. In this case, the coupled GP equation has been mapped onto a two coupled NLS equation.