A Single Rydberg Atom Controlled Optical Mirror formed by Subwavelength Arrays of Atoms

Efficient and versatile interfaces for the interaction of light with matter are an essential cornerstone for quantum science. A fundamentally new avenue of controlling light-matter interactions has been recently proposed based on the rich interplay of photon-mediated dipole-dipole interactions in structured subwavelength arrays of quantum emitters. Here we report on the direct observation of the cooperative subradiant response of a two-dimensional (2d) square array of atoms in an optical lattice. We observe a spectral narrowing of the collective atomic response well below the quantum-limited decay of individual atoms into free space. Through spatially resolved spectroscopic measurements, we show that the array acts as an efficient mirror formed by only a single monolayer of a few hundred atoms.   We also show how the optical properties of the entire array can be switched via a single Rydberg impurity that is deterministically prepared in the center of the array. This opens the path towards novel structured quantum light matter interfaces with unique properties as well as the path towards entanglement generation between light and matter.