A supersolid is a phase of matter possessing the seemingly incompatible properties of crystalline order and superfluidity. The crystal structure breaks the continuous translational symmetry, giving solids mechanical stability. Superfluidity breaks the global gauge symmetry, leading to a coherent, macroscopic behaviour of quantum particles. Consequently, persistent and dissipationless flow can exist. These two spontaneously broken symmetries are the smoking gun of the supersolid phase, and now Luca Tanzi and co-workers, Mingyang Guo and co-workers, and Gabriele Natale and co-workers have reported the observation of their signatures.
Breaking the two continuous symmetries causes the emergence of two gapless excitations. In harmonically trapped systems, however, only the collective modes with finite excitation energies can be observed. To characterize the supersolid phase realized in a trapped dipolar quantum gas of magnetic lanthanide atoms, the teams explored several specific collective modes and their relation to the two broken symmetries. Based on that they could identify the emergence of the supersolid phase and its transition to other phases.