![]() ![]() A very striking consequence of the dipoles tilted towards the plane is the formation of vortex stripes 30, 39, 40. These interaction properties are predicted to give rise to a vortex lattice structure that can follow a triangular pattern 30, 34, as is typical for non-dipolar BECs 11, or a square lattice for attractive or zero contact interactions 36, 37, 38 when the DDI is isotropic (dipoles aligned with the rotation axis). For vortex pairs, the anisotropic DDI is expected to alter the lifetime and dynamics 33, 35 and can even suppress vortex–antivortex annihilation 33. For instance, theoretical works predict single vortices to exhibit an elliptic-shaped core for a quasi-2D setting with in-plane dipole orientation 30, 31, 32, 33 or the presence of density oscillations around the vortex core induced by the roton minimum in the dispersion relation 30, 31, 32, 33, 34. ![]() The dipolar interaction is predicted to also intimately change the properties of vortices in quantum gases 29. This intriguing platform provided the key to observe, for example, extended Bose–Hubbard dynamics 20, roton excitations 21, 22, 23, the quantum version of the Rosensweig instability 24 and supersolid states of matter 25, 26, 27, 28, and is foreseen to host novel phenomena for quantum simulation and metrology 18, 19. Such a system, providing a quantum analogue of classical ferrofluids, enables access to the physics of dipolar BECs, in which atoms feature a strong long-range anisotropic dipole–dipole interaction (DDI) 18, 19 on top of the traditional contact-type isotropic one. Recently, a new class of ultracold quantum gases are being created in various laboratories around the world, using strongly magnetic lanthanide atoms 16, 17. ![]() Moreover, vortices play a fundamental role in the description of the Berezinskii–Kosterlitz–Thouless transition in two-dimensional (2D) systems 12, as well as in the evolution of quantum turbulence 13, 14, and have been observed in interacting Fermi gases along the Bose-Einstein condensate to Bardeen-Cooper-Schrieffer crossover 8, 15. In contact-interacting BECs, vortical singularities have been observed experimentally in the form of single vortices 5, 6, vortex–antivortex pairs 7, solitonic vortices 8, 9, vortex rings 10 and vortex lattices 6, 11 using a number of different techniques. It can be understood as a type of topologically protected singularity with a 2π phase winding that preserves the single-valuedness of the superfluid wave function and the irrotational nature of its velocity field. In the quantum realm, a quantized vortex may emerge as a unique response of a superfluid to rotation. Their classical counterparts have as well fascinated scientists from different epochs and fields as vortices are found in many scales of physical systems, from tornadoes in the atmosphere to ferrohydrodynamics. Their very existence sets a unifying concept encompassing a variety of quantum fluids from liquid helium 1 to the core of neutron stars 2 and from superconductors 3 to quantum fluids of light 4. Since the first experiments on gaseous Bose–Einstein condensates (BECs), the observation of quantized vortices has been considered the most fundamental and defining signature of the superfluid nature of such systems.
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