Myung-Joong, Ricardo and Martin have published work showing that a two-level atom coupled to a single-mode cavity field undergoes a second order quantum phase transition when the atomic transition frequency is much larger than the cavity frequency. This result has been chosen as an Editor Suggestion in Physical Review Letters.
You can read the full details in
Quantum Phase Transition and Universal Dynamics in the Rabi Model
Myung-Joong Hwang, Ricardo Puebla, and Martin B. Plenio
Phys. Rev. Lett. 115, 180404 (2015) – Published 29 October 2015
Their work shows that superradiant quantum phase transitions, which are generally studied for systems of thermodynamically many atoms, can actually be investigated within systems containing just a single atom and a single radiation mode. The limit of larger particle number, normally considered as the thermodynamics limit, is replaced by a limit in the coupling strength such that an increasingly large Hilbert space becomes accessible. To confirm that the system behaves like a critical system they studied the Kibble-Zurek mechanism and saw all the right scaling relations.