  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, in a
similar way an electric charge decelerates upon emitting photons. The
dissipation of vortex energy underlies central problems in quantum
hydrodynamics, such as the decay of quantum turbulence, highly relevant to
systems as varied as neutron stars, superfluid helium and atomic condensates. A
deep understanding of the elementary mechanisms behind irreversible vortex
dynamics has been a goal for decades, but it is complicated by the shortage of
conclusive experimental signatures. 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 Fermi 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.
