Structure and properties of quantum systems with elevated stability of the stationary state

Boris I Stepanov

Robert Gilmore, Lorenzo M. Narducci

The connection between the critical properties of the Dicke model, subject to equilibrium and nonequilibrium boundary conditions, is explicitly exhibited. The Langevin equations generated by the Dicke Hamiltonian lead to a single nonlinear order-parameter equation that characterizes the stationary states of the system. We prove the existence of two essentially identical manifolds of stationary states, one for equilibrium, the other for nonequilibrium boundary conditions. These manifolds can each be derived from a potential obtained from the nonlinear order-parameter equations. In the equilibrium case the potential is the free energy of the system; in the nonequilibrium case it can be identified with the so-called "laser potential." The reduced-density operator for the field and atomic subsystems factors into a geometric and a physical part. The geometric part is determined by the system signal; the physical part, by the system noise. This is exhibited explicitly with an example taken from the theory of photoelectron counting. The identification of the stationary-state manifolds for the Dicke model subject to equilibrium and nonequilibrium conditions with the same geometric (cusp) manifold allows a real analytic continuation of model properties from the equilibrium configuration to the nonequilibrium dissipative regime. Three types of stability are considered: static, dynamical, and structural. Structural-stability considerations lead to strong conclusions about the effects of additional perturbations on the Dicke model.

Vasily E Tarasov

Sami M. M. Virtanen

Since the first realizations of Bose-Einstein condensation in dilute alkali-atom vapours in 1995, these novel quantum gases have attracted wide interest. The main focus of the research has been on finding out the coherence and superfluidity properties of the condensates. From the theoretical point of view, these systems provide a rare possibility to model an interacting many-particle system exhaustively and quantitatively accurately from first-principles quantum field theories. However, the challenge of developing a correct and computationally feasible finite-temperature formalism still remains.\n\nThe coherence and superfluidity properties are intimately related to the existence and stability of quantized vortices. In this thesis, the structure and stability of vortex states in weakly interacting Bose-Einstein condensates is investigated within microscopic finite-temperature mean-field theories. An effective scheme for computationally solving the Bogoliubov-de Gennes quasiparticle eigenequations is developed, and applied to study the microscopic structure of vortex states in partially condensed vapours. In contrast to experimental observations and the predictions of the zero-temperature Bogoliubov approximation, stationary vortex states are found to be metastable within the self-consistent Popov approximation and its extensions even in the zero-temperature limit.\n\nThe reasons underlying this discrepancy are analysed in detail. A criterion for the validity of stationary mean-field approximations in modelling slowly time-dependent systems is derived, and applied to precessing off-axis vortex states. Under typical experimental conditions moving vortices are shown to be deformed due to the inability of the thermal cloud to follow the vortex core rigidly. Consequently, the validity of the commonly used stationary mean-field theories in describing such states is shown to be questionable. Finally, an analysis of uniformly precessing off-axis vortex states within a fully time-dependent Popov approximation is presented. Even dynamically deformed vortices are shown to become energetically metastable as the vortex core is partially filled with noncondensed gas in the course of thermalization. This self-stabilization mechanism of vortex states, shown to occur under very general circumstances, is an inherent property of all self-consistent formalisms having a structure similar to the Popov approximation. However, it is argued to be affected by the improper treatment of the noncondensate dynamics within these formalisms, and the need for a theory taking properly into account the coupled dynamics of the condensate and the noncondensate in modelling vortex states is emphasized. The computational scheme developed is also applied to investigate possibilities to energetically stabilize multiply quantized vortex states by piercing an externally rotated condensate with a blue-detuned laser beam.

Andrea López-Incera

Hoi-Yin Hui, Ryan Barnett, J. V. Porto +1 more

In this work, we consider excited many-body mean-field states of bosons in a double-well optical lattice by investigating stationary Bloch solutions to the nonlinear equations of motion. We show that, for any positive interaction strength, a loop structure emerges at the edge of the band structure whose existence is entirely due to interactions. This can be contrasted to the case of a conventional optical (Bravais) lattice where a loop appears only above a critical repulsive interaction strength. Motivated by the possibility of realizing such nonlinear Bloch states experimentally, we analyze the collective excitations about these nonlinear stationary states and thereby establish conditions for the system's energetic and dynamical stability. We find that there are regimes that are dynamically stable and thus apt to be realized experimentally.