Trapped Bose-Einstein condensates in the presence of a current nonlinearity

We investigate the effect of a current nonlinearity on the evolution of a trapped atomic Bose-Einstein condensate. We have implemented techniques from the field of nonlinear optics to provide new insights into the irregular dynamics associated with chiral superfluids. We have found that the current nonlinearity can be treated as a Kerr-like nonlinearity modulated by a spatiotemporal function that can lead to a number of processes such as broadening and compression of the wave function. In the long time scale limit, the wave function is drastically deformed and delocalised compared to its initial state. However, localised modes which oscillate with the period of the inverse trap frequency can still be observed.Comment: A short note on the links between nonlinear gauge potentials and nonlinear optics. Comments are welcom

On quantum time crystals and interacting gauge theories in atomic Bose-Einstein condensates

We study the dynamics of a Bose-Einstein condensate trapped circumferentially on a ring, and which is governed by an interacting gauge theory. We show that the associated density-dependent gauge potential and concomitant current nonlinearity permits a ground state in the form of a rotating chiral bright soliton. This chiral soliton is constrained to move in one direction by virtue of the current nonlinearity, and represents a time crystal in the same vein as Wilczek's original proposal.Comment: New version with general improvements and correction

Not all Crises are Detrimental for the Government: The Global Economic Crisis and the Swedish Case

Not all crises have the same impact. In this article the argument is made that we should make a distinction between a “homemade” and an “externally” caused crisis when analyzing the implications of an economic downturn. The case is Sweden and the analyses include both citizens’ and MPs’ attitudes towards democracy before and after the economic crisis of 2008. The article also makes an effort to compare the economic crisis of 2008 to a previous economic crisis that occurred at the beginning of the 1990s. Both crises were related to international turmoil, and they had a clearly negative impact on the Swedish economy. Yet, the crises were also very different. In the crisis of the 1990s, Sweden was seen as one of the countries that suffered the most, while in the aftermath of the economic crisis 2008, Sweden was seen as one of the most successful countries in handling the crisis. By comparing the two crises, we can get a better understanding of the extent to which attribution of responsibility depends on the context of the crisi

Coupled eigenmodes in a two-component Bose-Einstein condensate

We have studied the elementary excitations in a two-component Bose-Einstein condensate. We concentrate on the breathing modes and find the elementary excitations to possess avoided crossings and regions of coalescing oscillations where both components of the condensates oscillate with same frequency. For large repulsive interactions between the condensates, their oscillational modes tend to decouple due to decreased overlap. A thorough investigation of the eigenmodes near the avoided crossings is presented.Comment: Replacement, 17 pages, 9 figure

Elementary excitations of chiral Bose-Einstein condensates

We study the collective modes of a Bose-Einstein condensate subject to an optically induced density-dependent gauge potential. The corresponding interacting gauge theory lacks Galilean invariance, yielding an exotic superfluid state. The nonlinear dynamics in the presence of a current nonlinearity and an external harmonic trap are found to give rise to dynamics which violate Kohn's theorem; where the frequency of the dipole mode strongly depends on the strength of the mass current in the gas. The linearised spectrum reveals how the centre of mass and shape oscillations are coupled, whereas in the strongly nonlinear regime the dynamics is irregular.Comment: General improvements, corrections and references adde

Quantized vortices in interacting gauge theories

We consider a two-dimensional weakly interacting ultracold Bose gas whose constituents are two-level atoms. We study the effects of a synthetic density-dependent gauge field that arises from laser-matter coupling in the adiabatic limit with a laser configuration such that the single-particle zero-order vector potential corresponds to a constant synthetic magnetic field. We find a new exotic type of current non-linearity in the Gross-Pitaevskii equation which affects the dynamics of the order parameter of the condensate. We investigate the rotational properties of this system, focusing in particular on the physical conditions that make the nucleation of a quantized vortex in the system energetically favourable with respect to the non rotating solution. We point out that two different physical interpretations can be given to this new non linearity: firstly it can be seen as a local modification of the mean field coupling constant, whose value depends on the angular momentum of the condensate. Secondly, it can be interpreted as a density modulated angular velocity given to the cloud. Looking at the problem from both of these viewpoints, we analyze the physical conditions that make a single vortex state energetically favourable. In the Thomas-Fermi limit, we show that the effect of the new nonlinearity is to induce a rotation to the condensate, where the transition from non-rotating to rotating states depends on the density of the cloud.Comment: 6 pages, one figure. General improvement

Black-hole lasing in coherently coupled two-component atomic condensates

We theoretically study the black-hole lasing phenomenon in a flowing one-dimensional, coherently coupled two component atomic Bose-Einstein condensate whose constituent atoms interact via a spin-dependent s-wave contact interaction. We show by a numerical analysis the onset of the dynamical instability in the spin branch of the excitations, once a finite supersonic region is created in this branch. We study both a spatially homogeneous geometry and a harmonically trapped condensate. Experimental advantages of the two-component configuration are pointed out, with an eye towards studies of back-reaction phenomena.Comment: General improvements, corrections and references adde

Vortex dynamics in superfluids governed by an interacting gauge theory

We study the dynamics of a vortex in a quasi two-dimensional Bose gas consisting of light matter coupled atoms forming two-component pseudo spins. The gas is subject to a density dependent gauge potential, hence governed by an interacting gauge theory, which stems from a collisionally induced detuning between the incident laser frequency and the atomic energy levels. This provides a back-action between the synthetic gauge potential and the matter field. A Lagrangian approach is used to derive an expression for the force acting on a vortex in such a gas. We discuss the similarities between this force and the one predicted by Iordanskii, Lifshitz and Pitaevskii when scattering between a superfluid vortex and the thermal component is taken into account.Comment: 9 pages. Comments are welcom