Ultrastrong coupling and critical phenomena

Coupling between light and matter has long been limited to the weak coupling regime, because of the small value of the fine structure constant.

In the last two decades, however, various experiments have reached the ultrastrong coupling regime, where the interaction strength becomes comparable with the bare frequencies of the subsystems. In this regime, light and matter can no longer be considered separate entities, and form hybrid compounds called polaritons. In this regime, various exotic phenomena can take place, such as entangled ground-states or phase transitions. Importantly, these phenomena are not only limited to light and matter, but can also be observed in optomechanical systems, and more generally in most systems involving bosonic fields, strong interactions, and non-linearities.
In this talk, I will give an introduction to ultrastrong coupling, and how it can lead to critical phenomena. I will briefly discuss our recent work on the use of these critical behaviors for sensing applications; finally, I will present some ideas and open questions about how these phenomena could be implemented with levitated nanoparticles.