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Supersolids are states of matter that spontaneously break two continuous symmetries: translational invariance owing to the appearance of a crystal structure and phase invariance owing to phase locking of single-particle wavefunctions, responsible for superfluid phenomena. Although originally predicted to be present in solid helium1–5, ultracold quantum gases provided a first platform to observe supersolids6–10, with particular success coming from dipolar atoms8–12. Phase locking in dipolar supersolids has been investigated through, for example, measurements of the phase coherence8–10 and gapless Goldstone modes13, but quantized vortices, a hydrodynamic fingerprint of superfluidity, have not yet been observed. Here, with the prerequisite pieces at our disposal, namely a method to generate vortices in dipolar gases14,15 and supersolids with two-dimensional crystalline order11,16,17, we report on the theoretical investigation and experimental observation of vortices in the supersolid phase (SSP). Our work reveals a fundamental difference in vortex seeding dynamics between unmodulated and modulated quantum fluids. This opens the door to study the hydrodynamic properties of exotic quantum systems with numerous spontaneously broken symmetries, in disparate domains such as quantum crystals and neutron stars18. Magnetostirring is used to rotate the magnetic field and set a dipolar supersolid composed of ultracold atoms spinning, revealing fundamental differences in vortex seeding dynamics between modulated and unmodulated quantum fluids. [ABSTRACT FROM AUTHOR]

Supersolid states simultaneously feature properties typically associated with a solid and with a superfluid. Like a solid, they possess crystalline order, manifesting as a periodic modulation of the particle density; but unlike a typical solid, they also have superfluid properties, resulting from coherent particle delocalization across the system. Such states were initially envisioned in the context of bulk solid helium, as a possible answer to the question of whether a solid could have superfluid properties.sup.1-5. Although supersolidity has not been observed in solid helium (despite much effort).sup.6, ultracold atomic gases provide an alternative approach, recently enabling the observation and study of supersolids with dipolar atoms.sup.7-16. However, unlike the proposed phenomena in helium, these gaseous systems have so far only shown supersolidity along a single direction. Here we demonstrate the extension of supersolid properties into two dimensions by preparing a supersolid quantum gas of dysprosium atoms on both sides of a structural phase transition similar to those occurring in ionic chains.sup.17-20, quantum wires.sup.21,22 and theoretically in chains of individual dipolar particles.sup.23,24. This opens the possibility of studying rich excitation properties.sup.25-28, including vortex formation.sup.29-31, and ground-state phases with varied geometrical structure.sup.7,32 in a highly flexible and controllable system. Two-dimensional supersolidity is demonstrated using highly magnetic, ultracold dysprosium atoms., Author(s): Matthew A. Norcia [sup.1] , Claudia Politi [sup.1] [sup.2] , Lauritz Klaus [sup.1] [sup.2] , Elena Poli [sup.2] , Maximilian Sohmen [sup.1] [sup.2] , Manfred J. Mark [sup.1] [sup.2] [...]