Sound emission and annihilations in a programmable quantum vortex collider

Abstract

In quantum fluids, the quantisation of circulation forbids the diffusion of a vortex swirling flow seen in classical viscous fluids. Yet, a quantum vortex accelerating in a superfluid may lose its energy into acoustic radiation 1,2 , in a similar way an electric charge decelerates upon emitting photons. The dissipation of vortex energy underlies central problems in quantum hydrodynamics 3 , such as the decay of quantum turbulence, highly relevant to systems as varied as neutron stars, superfluid helium and atomic condensates 4,5 . A deep understanding of the elementary mechanisms behind irreversible vortex dynamics has been a goal for decades 3,6 , but it is complicated by the shortage of conclusive experimental signatures 7,8 . Here, we address this challenge by realising a programmable quantum vortex collider in a planar, homogeneous atomic Fermi superfluid with tunable inter-particle interactions. We create on-demand vortex configurations and monitor their evolution, taking advantage of the accessible time and length scales of our ultracold atomic gas. Engineering collisions within and between vortex-antivortex pairs allows us to decouple relaxation of the vortex energy due to sound emission and interactions with normal fluid, i.e. mutual friction. We directly visualise how the annihilation of vortex dipoles radiates a sound pulse in the superfluid. Further, our few-vortex experiments extending across different superfluid regimes suggest that fermionic quasiparticles localised inside the vortex core contribute significantly to dissipation, opening the route to exploring new pathways for quantum turbulence decay, vortex by vortex.

Vortex and soliton dynamics in particle-hole symmetric superfluids

“…In contrast, the KG vortex dipole will move along the dipole axis and annihilate each other, due to the absence of a velocity field. Related observations of vortex dynamics were reported in [44]. We note that the particle-hole symmetry is the origin of this qualitatively distinct behaviour from GP fluid dynamics.…”

“…Our work reveals that turbulence in a BCS superconductor and its scaling laws might deviate from Kolmogorow scaling laws [46], which apply to classical systems as well to GP fluids. We note that our predictions could be experimentally confirmed using refined experimental technique, such as in situ observations of two dimensional Fermi liquids when probing the BEC-BCS crossover in neutral atoms [47,48] or the well controlled imprinting of vortex dipole pairs [44].…”

“…We have shown analytically and numerically that for particle-hole symmetric superfluids with vanishing Noether charge the Magnus force is absent. This allows for a dipole pair of vortices to approach each other without transverse motion and to annihilate, reminiscent of recent observation in [44]. Another consequence of the vanishing Magnus force is that a soliton does not bend as it decays into vortices.…”

Superfluidity of a laser-stirred Bose-Einstein condensate

“…[22][23][24][25][26]. Laser stirring is also employed to generate controlled vortex distributions [27][28][29]. A number of theoretical studies was reported in Refs.…”

Generation of high winding-number superfluid circulation in Bose-Einstein condensates

“…This is particularly relevant for studies of quantum turbulence where one might want to reproducibly generate a many-vortex state to look for signatures of quantum turbulence in subsequent vortex dynamics. Progress towards a flexible experimental vortex architecture has been made in highly-oblate singlecomponent BECs regarding deterministic placement of individual vortex cores [25], with subsequent refinement made possible by the development of arbitrary configurable optical potentials [26,27]. However, a fully flexible architecture would benefit from the further development of additional experimental techniques that allow for controlled placement of a cluster of a fixed number of vortices, all with the same sign of circulation.…”