  Enhancing light-matter coupling at the level of single quanta is essential
for numerous applications in quantum science. The cooperative optical response
of subwavelength atomic arrays has been found to open new pathways for such
strong light-matter couplings, while simultaneously offering access to multiple
spatial modes of the light field. Efficient single-mode free-space coupling to
such arrays has been reported, but the spatial control over the modes of
outgoing light fields has remained elusive. Here, we demonstrate such spatial
control over the optical response of an atomically thin mirror formed by a
subwavelength array of atoms in free space using a single controlled ancilla
atom excited to a Rydberg state. The switching behavior is controlled by the
admixture of a small Rydberg fraction to the atomic mirror, and consequently
strong dipolar Rydberg interactions with the ancilla. Driving Rabi oscillations
on the ancilla atom, we demonstrate coherent control of the transmission and
reflection of the array. These results represent a step towards the realization
of quantum coherent metasurfaces, the demonstration of controlled atom-photon
entanglement and deterministic engineering of quantum states of light.
