title: Dicke quantum phase transition with a superfluid gas in an optical cavity.
abstract: A phase transition describes the sudden change of state of a physical system, such as melting or freezing. Quantum gases provide the opportunity to establish a direct link between experiments and generic models that capture the underlying physics. The Dicke model describes a collective matter-light interaction and has been predicted to show an intriguing quantum phase transition. Here we realize the Dicke quantum phase transition in an open system formed by a Bose-Einstein condensate coupled to an optical cavity, and observe the emergence of a self-organized supersolid phase. The phase transition is driven by infinitely long-range interactions between the condensed atoms, induced by two-photon processes involving the cavity mode and a pump field. We show that the phase transition is described by the Dicke Hamiltonian, including counter-rotating coupling terms, and that the supersolid phase is associated with a spontaneously broken spatial symmetry. The boundary of the phase transition is mapped out in quantitative agreement with the Dicke model. Our results should facilitate studies of quantum gases with long-range interactions and provide access to novel quantum phases.
authors.affiliation: Institute for Quantum Electronics, ETH Zürich, 8093 Zürich, Switzerland.
authors.forename: Kristian
authors.initials: K
authors.lastname: Baumann
authors.forename: Christine
authors.initials: C
authors.lastname: Guerlin
authors.forename: Ferdinand
authors.initials: F
authors.lastname: Brennecke
authors.forename: Tilman
authors.initials: T
authors.lastname: Esslinger