Supersolidity in ultracold dipolar gases

Supersolidity is an intriguing state of matter that combines superfluid and crystal features. Theoretically predicted in the 1960s, it has only recently been observed in atomic gases that exhibit typical supersolid properties such as spontaneous density modulations combined with coherence effects and the occurrence of new Goldstone modes. Can a gas behave like a crystal? Supersolidity is an intriguing and challenging state of matter that combines key features of superfluids and crystals. Predicted a long time ago, its experimental realization has been recently achieved in Bose–Einstein condensed atomic gases inside optical resonators, spin–orbit-coupled Bose–Einstein condensates and atomic gases interacting with long-range dipolar forces. The activity on dipolar gases has been particularly vibrant in the past few years. This Perspective article summarizes the main experimental and theoretical achievements concerning supersolidity in the field of dipolar gases, like the observation of the density modulations caused by the spontaneous breaking of translational invariance, the effects of coherence and the occurrence of novel Goldstone modes. A series of important issues for the future experimental and theoretical research are outlined including, among others, the possible realization of quantized vortices inside these novel crystal structures, the role of dimensionality, the characterization of the crystal properties and the nature of the phase transitions. At the end, a brief overview on some other (mainly cold atomic) platforms, in which supersolidity has been observed or supersolidity is expected to emerge, is provided.