Waveguide QED with Rydberg superatoms

The field of Waveguide QED investigates how light in a single mode propagates through a system of localized quantum emitters. If the coupling between individual photons and individual emitters is sufficiently strong, this at first glance simple situation can turn into an interesting quantum many-body system where the photons mediate an effective interaction between the emitters, or vice versa the cascaded interaction with saturated emitters can be interpreted as a photon-photon interaction.

We realize effective two-level emitters by exploiting the Rydberg blockade effect of atomic ensembles. By confining N~10.000 atoms to a single blockaded volume, the ensemble only supports a single excitation creating a so-called Rydberg superatom. Due to the collective nature of the excitation, the superatom effectively represents a single emitter coupling strongly to single photons. The directional emission of the superatom into the inital probe mode realizes a waveguide-like system in free-space without any actual light-guiding elements.

This talk will discuss how we scale this system from one to few strongly coupled superatoms to study how the propagation of quantized light fields through a small emitter chain results in photon-photon correlations and entanglement between the emitters. We also show how controlled dephasing of the collective excitation into collective dark states can be used to subtract exact photon numbers from an incoming light pulse.