Observation of supersolid-like sound modes in a driven quantum gas

Driven systems are of fundamental scientific interest, as they can exhibit properties that are radically different from the same system at equilibrium. In certain cases, long-lived states of driven matter can emerge, which exhibit new material properties. In this work, we probe the excitation spectrum of an emergent patterned state in a driven superfluid, finding that its response is identical to that of a one-dimensional supersolid. In order to extract physical quantities that parametrize the observed sound modes, we apply an effective hydrodynamic theory of superfluid smectics, which is agnostic to microscopic processes. We therefore use the conceptual framework of supersolids to characterize an otherwise dynamic and far-from-equilibrium state.