Observing Higgs and Goldstone modes in a supersolid quantum gas

We report on the realization of a supersolid with continuous translational symmetry breaking. This U(1) invariance is engineered via symmetry enhancement by coupling a Bose-Einstein condensate to the modes of two optical cavities with individual $\mathbb{Z}_2$ symmetries. Spectroscopic measurements reveal the presence of a Goldstone and a Higgs mode, and our data show their energy across the supersolid phase transition. The finite cavity leakage offers a glance into real-time dynamics of the system, while the choice of cavity detunings facilitates control of symmetry-breaking fields that tune the mass of the Goldstone mode.