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My research involves theoretical studies of emergent phenomena in ultra cold atomic gases due to the collective many-body physics of strongly interacting quantum systems. One particular topic I am currently looking at is developing theories of light propagation through cold dipolar gases, mainly involving Rydberg atoms. Rydberg atoms have highly excited electronic states, where the outermost electron is easily polarisable, resulting in large dipole moments and strong interactions between atoms. If the atoms are exposed to light, they can become excited. However, due to the strong dipole interactions, only a single atom in a medium can become excited, which leads to strong photon interactions in the medium. The photons now behave as non-equilibrium hard-core bosons, meaning that the usual models of light moving through a media no longer hold and new theories need to be developed.
In Plain English
Collective phenomena is any occurence where particles behave together and is a key area of study in condensed matter physics, leading to interesting effects such as Bose Einstein condensation, magnetism and superconductivity. When we consider how light interacts with a material, we usually treat the atoms as independent oscillators, where the electron and atom behave as if joined by a spring and the light causes the spring to move back and forth. This model is relatively successful at describing most light-matter interactions we see in everyday life. However, if we make the temperature low enough, we have to take into account quantum effects and interactions between the atoms. This can give rise to collective behaviour which changes how light behaves when it enters a material and means we have to formulate new models to describe these processes.