Impurity in a bosonic Josephson junction: Swallowtail loops, chaos, self-trapping, and Dicke model

We study a model describing N identical bosonic atoms trapped in a double-well potential together with a single-impurity atom, comparing and contrasting it throughout with the Dicke model. As the boson-impurity coupling strength is varied, there is a symmetry-breaking pitchfork bifurcation which is analogous to the quantum phase transition occurring in the Dicke model. Through stability analysis around the bifurcation point, we show that the critical value of the coupling strength has the same dependence on the parameters as the critical coupling value in the Dicke model. We also show that, like the Dicke model, the mean-field dynamics goes from being regular to chaotic above the bifurcation and macroscopic excitations of the bosons are observed. Although the boson-impurity system behaves like a poor man's version of the Dicke model, we demonstrate a self-trapping phenomenon which thus far has not been discussed in the realm of the Dicke model.