|Durham Atomic and Molecular Physics|
Solitons and Vortices in Atomic BECs
Atomic Bose-Einstein condensates support macroscopic excitations in the form of bright solitons, dark solitons, and quantized vortices. Our research is based upon the Gross-Pitaevskii equation (GPE) which describes atomic BECs at ultracold temperature,
Collisions of bright solitons
Bright solitons are localised nonlinear wavepackets which have found important applications in optical communication. Recently bright atomic solitons have been generated experimentally in atomic BECs. While an isolated soliton tends to behave as a classical particle, multiple solitons interact in a non-trivial manner. Here we study the dynamics of multiple solitons in a harmonic trap, and in particular the effect of collisions between the solitons.
Formation of vortex lattices
Recent experiments have generated vortex lattices in atomic BECs by rotating the condensate in an elliptical trap.
We successfully describe the evolution using the GPE in 2D, and find 4 stages: (i) Fragmentation: an unstable
quadrupole leads to the fragmentation of the condensate. (ii) Symmetry-breaking: the two-fold symmetry of the system becomes
macroscopically broken. (ii) Turbulence: A disordered state of vortices and sound waves is formed, with the characteristics of Kolmogorov
turbulence. (iv) Crystallisation: Through vortex-sound interactions (see below), the vortices orders themselves into a lattice.
Accelerating vortices in atomic BECs decay via the emission of sound waves. In a harmonic trap, the
inhomogeneous density causes a vortex to precess, and this induces sound emission (left, upper plots). However, in these confined systems, reabsorption
of the emitted
sound can occur. A dimple trap embedded in a weak outer trap can be used to control these vortex-sound interactions. By simulating these dynamics, we
have shown that the power radiated is proportional to the acceleration squared. Similarly, vortices can
due to the interaction with other vortices, and this also induces sound emission (left, lower plot).
Decay, stabilisation and parametric driving of dark solitons
Dark solitons (localised density dips) are technically 1D objects. When embedded
in 3D systems they are
unstable to transverse modes, and decay into vortex rings and sound waves
(right, top). However, in quasi-1D geometries featuring strong transverse
confinement, dark solitons are metastable to this effect.
In analogy to vortices, dark solitons radiate sound waves under
acceleration, with the power emitted being proportional to the acceleration
confined systems, sound reabsorption can occur, which can partially or fully
stabilise this decay. By employing a dimple trap of variable depth, within a
weaker trap, these soliton-sound interactions can be controlled.
We can further manipulate the soliton-sound interactions. By 'shaking' the
condensate, we can parametrically drive the dark soliton and increase its
energy. This technique could be employed to stabilise the soliton against
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