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Register a new userDepletion of a Bose-Einstein condensate by laser-iduced dipole-dipole interactions
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We study a gaseous Bose-Einstein condensate with laser-induced dipole-dipole interactions using the Hartree-Fock-Bogoliubov theory within the Popov approximation. The dipolar interactions introduce long-range atom-atom correlations, which manifest themselves as increased depletion at momenta similar to that of the laser wavelength, as well as a roton dip in the excitation spectrum. Surprisingly, the roton dip and the corresponding peak in the depletion are enhanced by raising the temperature above absolute zero.
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We consider a trapped cigar-shaped atomic Bose-Einstein condensate irradiated by a single far-off resonance laser polarized along the cigar axis. The resulting laser induced dipole-dipole interactions between the atoms significantly change size of the condensate, and can even cause its self-trapping.
We derive an exact solution to the Thomas-Fermi equation for a Bose-Einstein condensate which has dipole-dipole interactions as well as the usual s-wave contact interaction, in a harmonic trap. Remarkably, despite the non-local anisotropic nature of the dipolar interaction the solution is an inverted parabola, as in the pure s-wave case, but with a different aspect ratio. Various properties such as electrostriction and stability are discussed.
A quantum vortex dipole, comprised of a closely bound pair of vortices of equal strength with opposite circulation, is a spatially localized travelling excitation of a planar superfluid that carries linear momentum, suggesting a possible analogy with ray optics. We investigate numerically and analytically the motion of a quantum vortex dipole incident upon a step-change in the background superfluid density of an otherwise uniform two-dimensional Bose-Einstein condensate. Due to the conservation of fluid momentum and energy, the incident and refracted angles of the dipole satisfy a relation analogous to Snells law, when crossing the interface between regions of different density. The predictions of the analogue Snells law relation are confirmed for a wide range of incident angles by systematic numerical simulations of the Gross-Piteavskii equation. Near the critical angle for total internal reflection, we identify a regime of anomalous Snells law behaviour where the finite size of the dipole causes transient capture by the interface. Remarkably, despite the extra complexity of the surface interaction, the incoming and outgoing dipole paths obey Snells law.
We have measured the quantum depletion of an interacting homogeneous Bose-Einstein condensate, and confirmed the 70-year old theory of N.N. Bogoliubov. The observed condensate depletion is reversibly tuneable by changing the strength of the interparticle interactions. Our atomic homogeneous condensate is produced in an optical-box trap, the interactions are tuned via a magnetic Feshbach resonance, and the condensed fraction probed by coherent two-photon Bragg scattering.
Three distinct types of behaviour have recently been identified in the two-dimensional trapped bosonic gas, namely; a phase coherent Bose-Einstein condensate (BEC), a Berezinskii-Kosterlitz-Thouless-type (BKT) superfluid and normal gas phases in order of increasing temperature. In the BKT phase the system favours the formation of vortex-antivortex pairs, since the free energy is lowered by this topological defect. We provide a simple estimate of the free energy of a dilute Bose gas with and without such vortex dipole excitations and show how this varies with particle number and temperature. In this way we can estimate the temperature for cross-over from the coherent BEC to the (only) locally ordered BKT-like phase by identifying when vortex dipole excitations proliferate. Our results are in qualitative agreement with recent, numerically intensive, classical field simulations.
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