Rydberg quantum optics achieves optical non-linearities at the single-photon level by mapping the strong dipolar interactions between Rydberg atoms in cold atomic gases onto light fields using electromagnetically-induced transparency and photon storage. The non-linearities are a direct consequence of the long-range character of the interaction which allows a single photon to modify the optical response in a volume containing many atoms. In this thesis, the long-range character of the resulting effective photon-photon interaction is directly observed as photons propagating in non-overlapping optical modes are stored as collective Rydberg excitations in adjacent and non-overlapping microscopic clouds of 87Rb atoms. While stored, van-der-Waals interactions imprint spatially non-uniform phase shifts in the collective excitations. These distort the photons' retrieval modes resulting in anti-correlated retrieval between the original modes.In this first demonstration of contactless effective interactions between photons, these effects are observed between photons separated by more than 15 times their wavelength, well above the optical diffraction limit.This represents a promising step towards the implementation of scalable, multichannel quantum optical devices such as quantum gates. The experiments are enabled by a new, specialised experimental setup centred around a pair of in-vacuo aspheric lenses. These provide optical resolution of order 1 µm to optically trap and address the ensembles separated by distances well below the range of Rydberg interactions. The ensembles are prepared in approximately 100 ms thanks to efficient loading of a magneto-optical trap (MOT) from an atomic beam produced by a 2D MOT. Combined with the ability to recycle the ensembles > 20000 times, effective cycle times exceeding 100 kHz enable the acquisition of large datasets for the analysis of photon statistics within a matter of minutes.