We are interested in interplay between nonlinearity and non-adiabaticity in many-body quantum systems: Bose-Einstein condensates (BEC). Adiabatic (geometric) phases and dynamics of adiabatic invariants are conceptual issues in classical mechanics and have been used recently in studying dynamics of BEC and ingenious manipulations with it. We present here two our recent results related to this theme. The first concerns a vortex creation during splitting of a spinor BEC in a magnetic chip-based trap. Our study is motivated by a recent MIT experiment 1 , where a fork-like structure appeared in the interference fringes indicating the presence of a singly quantized (SQ) vortex in one of the interfering condensates. It is well-known that in a spin-1 BEC in a quadrupole trap a doubly quantized (DQ) vortex is topologically produced by a “slow” reversal of bias magnetic field Bz. This fact can be explained using an adiabatic (Berry) phase argument. Since in the experiment a DQ vortex had never been seen, Y. Shin et. al ruled out the topological mechanism and concentrated on the mechanical mechanism for explanation of the vortex creation. We find however that both mechanisms are possible: SQ vortices can be formed in a spin-1 BEC topologically during the magnetic field switching-off process 2 . We therefore provide a possible alternative explanation for the interference patterns observed in the experiment. The second example concerns dynamics of atom-molecule BEC systems being considered recently, in particular in relation to Feschbach resonance passage. We use rigorous separatrix crossing theory 3 to study dynamics of adiabatic invariants in corresponding models. Our results 4 contradict those of 5
We are interested in interplay between nonlinearity and non-adiabaticity in many-body quantum systems: Bose-Einstein condensates (BEC). Adiabatic (geometric) phases and dynamics of adiabatic invariants are conceptual issues in classical mechanics and have been used recently in studying dynamics of BEC and ingenious manipulations with it. We present here two our recent results related to this theme. The first concerns a vortex creation during splitting of a spinor BEC in a magnetic chip-based trap. Our study is motivated by a recent MIT experiment 1 , where a fork-like structure appeared in the interference fringes indicating the presence of a singly quantized (SQ) vortex in one of the interfering condensates. It is well-known that in a spin-1 BEC in a quadrupole trap a doubly quantized (DQ) vortex is topologically produced by a “slow” reversal of bias magnetic field Bz. This fact can be explained using an adiabatic (Berry) phase argument. Since in the experiment a DQ vortex had never been seen, Y. Shin et. al ruled out the topological mechanism and concentrated on the mechanical mechanism for explanation of the vortex creation. We find however that both mechanisms are possible: SQ vortices can be formed in a spin-1 BEC topologically during the magnetic field switching-off process 2 . We therefore provide a possible alternative explanation for the interference patterns observed in the experiment. The second example concerns dynamics of atom-molecule BEC systems being considered recently, in particular in relation to Feschbach resonance passage. We use rigorous separatrix crossing theory 3 to study dynamics of adiabatic invariants in corresponding models. Our results 4 contradict those of 5
We investigate several mechanisms of vortex creation during splitting of a spinor Bose-Einstein condensate (BEC) in a magnetic double-well trap controlled by a pair of current carrying wires and bias magnetic fields. Our study is motivated by a recent MIT experiment on splitting BECs with a similar trap [Y. Shin et al., Phys. Rev. A 72, 021604 (2005)], where an unexpected fork-like structure appeared in the interference fringes indicating the presence of a singly quantized vortex in one of the interfering condensates. It is well known that in a spin-1 BEC in a quadrupole trap, a doubly quantized vortex is topologically produced by a ``slow'' reversal of bias magnetic field Bz. Since in the experiment a doubly quantized vortex had never been seen, Shin et al. ruled out the topological mechanism and concentrated on the nonadiabatic mechanical mechanism for explanation of the vortex creation. We find, however, that in the magnetic trap considered both mechanisms are possible: singly quantized vortices can be formed in a spin-1 BEC topologically (for example, during the magnetic field switching-off process). We therefore provide a possible alternative explanation for the interference patterns observed in the experiment. We also present a numerical example of creation of singly quantized vortices due to ``fast'' splitting; i.e., by a dynamical (nonadiabatic) mechanism.
The condensation of weakly interacting alkali atom gases observed in 1995 is a manifestation of the macroscopic quantum phenomenon called Bose-Einstein condensation. In contrast to superconductivity and Helium superfluids, the dilute alkali atom condensates are directly observable using optical imaging techniques and they constitute highly controllable quantum systems. Hence, they offer an ideal test bench to investigate the applicability of quantum field theories and to predict the properties of the condensates starting from the first principles.\n\nSince the first experimental realizations of dilute Bose-Einstein condensates, a great interest has been focused on this field, resulting in numerous experimental studies concerning, for example, nonlinear dynamics and the elementary excitations of these many-particle quantum systems. The existence of topological phase singularities, quantized vortices, first predicted theoretically and finally observed experimentally in 1999, led to another series of pioneering theoretical and experimental studies.\n\nSeveral methods to create single-quantum vortices were realized already in 2001 but, however, the puzzle of creating multiply quantized vortices still remained. In this thesis, the so-called topological method to create two and four times quantized vortices is studied in detail. Since the instrumentation required for the experimental realization of the topological method had already been installed in many laboratories, the theoretical predictions of this thesis were experimentally verified shortly after their publication. Multiply quantized vortices are energetically unstable and tend to split into singly quantized vortices. However, the dissipation of energy in the condensates is minimal at low temperatures and, consequently, can lead to long lifetimes for the multiquantum vortices. Nevertheless, one prediction of this thesis is that the lifetimes of doubly quantized vortices are relatively short, owing to the known dynamical instability of the state. The splitting of the doubly quantized vortex has recently been observed experimentally.\n\nThe stability of vortices in dilute Bose-Einstein condensates is intimately related to the excitation spectrum of the stationary vortex state. However, developing an accurate, computationally feasible finite-temperature quantum field theory remains an open problem. In this thesis, we study the elementary excitations of an irrotational condensate within a recently developed systematic second order perturbation theory. The collapse and revival of certain elementary excitations is discovered.\n\nThe most recent studies of this thesis are devoted to stationary, but rotationally asymmetric vortex states. In weakly interacting condensates, a novel stationary state, a vortex tripole holding finite angular momentum is presented. Also the lowest elementary excitations of the so-called vortex dipole, tripole, and quadrupole states are studied.
Abstract Chapter 4 addresses the progress of studies on superfluid dynamics, mainly related to the dynamics of quantized vortices, in cold-atom BECs. First, we give a minimal description of the physics of cold atoms, e.g. cooling, trapping, control of interatomic interactions by external fields. We then turn to the argument of superfluid hydrodynamics in a scalar BEC characterized by a single-component order parameter. The properties of the vortices can be well described on the basis of the GP model, which allows a quantitative comparison between theory and experiment. We will discuss topics such as single vortex dynamics, interacting dynamics of a few vortices, vortex lattice formation in a rotating BEC, and collective properties of a vortex lattice in a rapidly rotating BEC, which have been unraveled experimentally by direct visualization of vortices in condensed gases. Next, we will discuss the properties of vortices in multicomponent BECs characterized by the multicomponent order parameters. We will first explain the vortex physics in two-component BECs as a simple case, finding that their structures and dynamics are highly nontrivial due to the presence of the intercomponent interaction; we will cover the topics of vortex structure composed of two-component order parameters, dynamics of composite vortices, exotic vortex lattices, and hydrodynamic instabilities associated with the interface dynamics between immiscible condensates. Finally, we explain the vortices in a spinor BEC. The vortex structures are extremely rich due to the complicated order parameter manifold, which is clarified by the discussion on vortices in spin-1 and spin-2 spinor BECs.
Since its realization in dilute bosonic atomic gases [], [], Bose-Einstein condensation of alkali atoms and hydrogen has been produced and studied extensively in the laboratory [], and has permitted an intriguing glimpse into the macroscopic quantum world. In view of potential applications [], [], [], the study of quantized vortices, which are well-known signatures of superfluidity, is one of the key issues. In fact, bulk superfluids are distinguished from normal fluids by their ability to support dissipationless flow. Such persistent currents are intimately related to the existence of quantized vortices, which are localized phase singularities with integer topological charge []. The superfluid vortex is an example of a topological defect that is well known in superconductors [] and in liquid helium []. The occurrence of quantized vortices in superfluids has been the focus of fundamental theoretical and experimental work []. Different research groups have obtained quantized vortices in Bose-Einstein condensates (BECs) experimentally, e.g., the JILA group [], [], the ENS group [] and the MIT group [], []. Currently, there are at least two typical ways to generate quantized vortices from a BEC ground state: (i) impose a laser beam rotating with an angular velocity on the magnetic trap holding the atoms to create a harmonic anisotropic potential [], [], []; or (ii) add to the stationary magnetic trap a narrow, moving Gaussian potential, representing a far-blue detuned laser [], [], [], [], [], [].
We studied the dynamics of quadruply quantized vortices in 87 Rb Bose–Einstein condensates. Vortices were created in magnetically trapped condensates with hyperfine spin F = 2 by employing topological phase imprinting technique, wherein the direction of atomic spin is adiabatically reversed by applying a bias magnetic field. Vortices were observed for a holding time of up to 10 ms. Disappearance of the vortices was attributed to considerable expansion and excitation of the condensates, which were caused by the distortion of the magnetic potential. In order to observe the long-term behavior of the vortices, we transferred the condensates to a crossed-type optical dipole force trap after creating the vortices. In this case, the vortices were observed for a holding time of up to 22 ms. We also observed density profiles, which indicated the presence of split vortices.
This report addresses our recent study on pattern formation in two-component Bose-Einstein condensates(BECs), focusing on the dynamics of multiple-domain formation and the structure of vortex states in rotating condensates. Cross-phase modulation instability gives rise to formation of multiple domains that alternate two components, which are consistent with the experimental observation by the MIT group. Rotating two-component BECs exhibit a rich variety of vortex states such as square lattices and vortex sheets, which depend on the strength of intercomponent interaction and rotation frequency.