# Free-Space Quantum Electrodynamics with a single Rydberg superatom

## TL;DR

This paper demonstrates strong coherent coupling between a single Rydberg superatom and a few-photon propagating light pulse, enabling observation of interaction effects at the single-photon level, advancing quantum network technologies.

## Contribution

It shows for the first time strong coupling between a Rydberg superatom and a propagating light pulse, matching theoretical predictions and enabling quantum network applications.

## Key findings

- Strong coherent coupling observed with a single Rydberg superatom.
- Interaction effects on the driving field detected at the single photon level.
- Results align quantitatively with two-level system theory.

## Abstract

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 so far required confinement of the light field inside resonators or waveguides. Here, we demonstrate strong coherent coupling between a single Rydberg superatom, consisting of thousands of atoms behaving as a single two-level system due to the Rydberg blockade, and a propagating light pulse containing only a few photons. The strong light-matter coupling in combination with the direct access to the outgoing field allows us to observe for the first time the effect of the interactions on the driving field at the single photon level. We find that all our results are in quantitative agreement with the predictions of the theory of a single two-level system strongly coupled to a single quantized propagating light mode. The demonstrated coupling strength opens the way towards interfacing photonic and atomic qubits and preparation of propagating non-classical states of light, two crucial building blocks in future quantum networks.