Supersolidity is a paradoxical phase of matter where both superfluid and crystalline orders coexist. Predicted 50 years ago, the existence of such a phase has been long debated in theory and in experiments. Here, we report on evidence for hallmarks of this exotic state in ultracold dilute atomic gases.

While most work has focused on achieving supersolidity in helium, researchers have recently turned to atomic gases, in particular, those with strong dipolar interactions. Recent experiments have revealed that such gases exhibit fundamental similarities with superfluid helium. These features lay the groundwork for reaching a state with both spontaneous density modulation and global phase coherence, which are indicators of supersolidity.

We experimentally create states showing these properties in both erbium and dysprosium quantum gases, and we connect our observations to theoretical phase diagrams. While in erbium the supersolid behavior is only transient, the dysprosium realization shows an unprecedented stability. Here, the supersolid behavior is not only long-lived but also can be directly achieved via evaporative cooling, starting from a thermal sample.

Our results with dysprosium offer exciting prospects for near-future experiments and theories, as the supersolid state is little affected by dissipative dynamics or excitations, thus paving the way for probing its excitation spectrum and its superfluid behavior.