Dissipative Phase Transition in the Interaction of a Rydberg Polariton With a Single Rydberg Atom*

Interactions between Rydberg atoms underpin many recent developments in quantum science and technology, such as NISQ-era quantum computation, complex many-body simulation, entanglement-enhanced metrology and strong optical nonlinearity at the single-photon level. In the latter case, photons propagate within atomic clouds as collective excitations termed Rydberg polaritons, strongly interacting with each other or with nearby stationary Rydberg atoms. In this work, we observe the full range of interaction regimes between a single Rydberg atom and a Rydberg polariton, divided into blockade, coherent exchange and incoherent hopping. We demonstrate that the transition from blockade to exchange is, in fact, a dissipative phase transition, arising from the closing of the energy gap between two polariton eigenstates, and further investigate the utility of the atom-polariton interaction for fast, non-destructive detection of Rydberg atoms.