Session T5: Bose-Einstein Condensates

Ground State Properties of a Homogeneous Bose-Einstein Condensate

We will present measurements of the coherence, energy and free expansion of a quasi-homogeneous atomic Bose-Einstein condensate (BEC) in an optical box potential. We have measured the ground state wave function of a trapped quasi-pure BEC in momentum space using Bragg spectroscopy and compare this with the real-space wave function. We find excellent quantitative agreement with the Heisenberg uncertainty principle and also confirm the expected scaling of the momentum uncertainty with the box length. In addition, by varying the condensate atom number, we have studied the effect of interactions on the momentum distribution and mean-field energy of the condensate. Finally, we will present measurements of the evolution in time of both the mean-field energy and the momentum distribution of a freely expanding condensate.   

Thermodynamics and Dynamics of Bose condensation in a quasi-homogeneous gas

We present an experimental study of the thermodynamics and dynamics of Bose-Einstein condensation (BEC) in an optical-box trap. We first characterize the critical point for BEC, and observe saturation of the thermal component in a partially condensed cloud, in agreement with Einstein's textbook picture of a purely statistical phase transition. We also observed the quantum Joule-Thomson effect, namely isoenthalpic cooling of a non-interacting gas [1]. We then investigate the dynamics of Bose condensation in the box potential following a rapid temperature quench through the phase transition, and focus on the time-evolution of the condensed fraction, the coherence length and the mean-field shift, that we probe via Bragg spectroscopy. \\[4pt] [1] T.F. Schmidutz, I. Gotlibovych, A.L. Gaunt, R.P. Smith, N. Navon, Z. Hadzibabic, Phys. Rev. Lett. 112, 040403 (2014)   

Classical fields and quantum measurement for Bose-Einstein condensate

We analyze a process of splitting of the Bose-Einstein condensate and the mutual coherence of two separated atomic clouds. Of interest is a statistics of the population difference between the two wells. Within the classical fields approximation we show that coherence between clouds is degraded if atoms interact and if we account for the sufficiently long observation time. We also show, that upon recombination, the coherence across the sample is restored. The coherence is not fully degraded if the splitting potential remains sufficiently penetrable. We calculate the variance of atom number difference for this time-averaging measurement and show that for low temperatures it can be well below Poissonian limit like it was observed in the experiments.   

Bose-Einstein condensation of $^{86}$Sr

We demonstrate a Bose-Einstein condensate of the alkaline-earth atom $^{86}$Sr. We use a magneto-optical trap to load atoms into a pancake-shaped optical dipole trap. The large volume of the dipole trap allows forced evaporation to proceed despite the large scattering length ($\sim$ 800 a$_{0}$) of $^{86}$Sr. Optical Feshbach resonances will allow us to tune the scattering length during future experiments.   

Half-Quantum Vortex Bound States in a Rotating Two-Component Dipolar Bose Gas

We consider a rotating two-component Bose-Einstein condensate in quasi-two dimensional geometry, wherein one component exhibits dipole-dipole interactions. We model numerically the interaction potential between a half-quantized vortex (HQV) in the dipolar species and a HQV in the other species. We find that for sufficiently strong dipolar interactions a bound state between HQVs occurs as a result of rotonic features induced by the dipolar vortex. We then simulate a rapidly rotating system confined in an oblate harmonic trap and observe novel ground state vortex configurations including HQV molecules and chains of bound vortices. Finally we present a phase diagram which elucidates the effects of dipolar interactions on the planar vortex geometry.   

Local Number Fluctuations Within a Dipolar Bose Einstein Condensate

In dipolar Bose-Einstein condensates a range of new physics is expected to arise, including a momentum dependent interaction and a roton-like dispersion relation for certain trap geometries (which is yet to be observed experimentally). We present theoretical results demonstrating that the analysis of number fluctuations within cells of various shapes and sizes is a highly sensitive tool for probing roton modes experimentally.   

Explicit contribution of molecule radius in dynamical properties of dipolar BECs

We make generalization of the mean-field theory of dipolar BECs to account finite size of molecules. We do this generalization in two steps. First we consider full potentials of dipole-dipole interactions, which are different for point-like electric and magnetic dipoles. We use it to show analytically different behavior of electric and magnetic dipolar BECs revealing change of the dipolar part of the Bogoliubov spectrum on 30-100 percent depending on direction of wave propagation. For instance we find that electric dipolar BECs reveal no roton instability, when magnetic dipolar BECs can reveal it for all directions of wave propagation. At second step we present method of including of the finite size of molecules in hydrodynamic equations for electric dipolar BECs. We calculate spectrum of collective excitations. We find that finite size of particles decreases dipolar part of the spectrum on 10 percent (for molecule radius 4 $10^{-8}$ cm) in compare with point-like particles. Hence we present an updated mean-field theory of dipolar BECs, which explicitly contains size of molecules and gives large contribution in non-equilibrium collective properties of dipolar BECs.   

Spinor Bose-Einstein Condensates of Positronium

Bose-Einstein condensates (BECs) of positronium (Ps) have been of experimental and theoretical interest due to their potential application as the gain medium of a $\gamma$-ray laser. Ps BECs are intrinsically spinor due to the presence of ortho-positronium (o-Ps) and para-positronium (p-Ps), whose annihilation lifetimes differ by three orders of magnitude. In this paper, we study the spinor dynamics and annihilation processes in the p-Ps/o-Ps system using both solutions of the time-dependent Gross-Pitaevskii equations and a semiclassical rate-equation approach. The spinor interactions have an $O(4)$ symmetry which is broken to $SO(3)$ by an internal energy difference between o-Ps and p-Ps. For an initially unpolarized condensate, there is a threshold density of $\approx 10^{19}$ cm$^{-3}$ at which spin mixing between o-Ps and p-Ps occurs. Beyond this threshold, there are unstable spatial modes accompanied by spin mixing. To ensure a high production yield above the critical density, a careful choice of external field must be made to avoid the spin mixing instability.