Two-dimensional supersolidity in a dipolar quantum gas

Supersolids are quantum states combining the phase coherence of a superfluid with the spatial periodicity of a solid. Predicted 60 years ago and long searched in solid helium, supersolidity was recently observed in dipolar gases [1-3]. In these systems, in a narrow interaction-parameter range, it is possible to reach spontaneous density modulation and global phase coherence. So far, supersolidity in dipolar systems has only been demonstrated along a single direction, as a linear chain of phase-coherent “droplets”. Differently from other experiments, we perform direct evaporative cooling into the supersolid state from a thermal cloud, which allows us to reach high atom numbers and states with large number of droplets. By loosening the transversal trap confinement, we observe a phase transition from a linear to a zig-zag structure, establishing supersolid properties in two dimensions [4]. This step brings our system closer to the supersolid states predicted in solid helium and opens up the possibility of studying rich excitation spectra, as well as vortex formation.