The interaction of a single photon with an individual two-level system is the textbook example of quantum electrodynamics. Achieving strong coupling in this system has so far required confinement of the light field inside resonators or waveguides. Experiments with Rydberg superatoms [1,2] have demonstrated the ability to realize strong coupling to a propagating light pulse containing only a few photons in free space. In this talk, we will present the exact input-output formalism to describe the phenomenon of collective Rabi oscillations in a single Rydberg two-level superatom coupled to a photon field. The photonic mode then defines an effective one-dimensional system, while the large size of the atomic cloud provides a chiral coupling. Extending the theory to describe several atoms coupled to a one-dimensional waveguide also provides an approach to investigate coherent internal dynamics of the atomic cloud by the exchange of virtual photons [3]. We will also show how this approach can be used to describe electromagnetically induced transparency and slow light.

[1] A. Paris-Mandoki et al., Phys. Rev. X 7, 41010 (2017)

[2] N. Stiesdal et al., Phys. Rev. Lett. 121, 103601 (2018)

[3] J. Kumlin et al., Phys. Rev. Lett. 121, 013601 (2018)

(pizza and drinks served at 12pm; talk starts at 12:10pm)