Observation of long-lived metastable structures in a quantum gas with long-range interactions

We study relaxation of a quantum gas after quenches across a phase transition and in the presence of competing long-range interactions. The interactions are mediated by two cavity modes, which induce competing spatial ordering. The quenches are implemented by changing the detuning between an external laser frequency and the cavity resonances. Using the real-time access to the order parameters provided by the leaking cavity fields, we observe metastability for a large range of parameters. The atoms remain frozen in the initial pattern with lifetimes that exceed any natural time scale of the system before relaxing to the stable configuration. From an ab-initio treatment we derive a Vlasov equation. We show that its fixed points are the metastable configurations, which can be understood as quasi-stationary states due to the long-range interactions. By this mean we theoretically reproduce the characteristic time scale of relaxation and their dependence on the physical parameters. We attribute the observed metastability to the competing global range interactions.