Phase transitions in photonic cavities: Exact vs. mean-field

A system of photonic cavities, each of which contains a single two level system, is a novel system in which to explore the boundary between quantum optics and condensed matter [1, 2, 3]. We examine this system by comparing results found using the mean-field approximation with exact diagonalisation techniques for small systems (up to six cavities), and find a good match between the quantum phase transition diagram for systems with the same number of nearest neighbours [4]. We thereby provide a pathway between proof-of-concept experiments, and complete photonic phase transition devices.