Two-dimensional supersolidity in a dipolar quantum gas.

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. Although supersolidity has not been observed in solid helium (despite much effort), ultracold atomic gases provide an alternative approach, recently enabling the observation and study of supersolids with dipolar atoms. 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, quantum wires and theoretically in chains of individual dipolar particles. This opens the possibility of studying rich excitation properties, including vortex formation, and ground-state phases with varied geometrical structure in a highly flexible and controllable system.