QUROPE Quantum Information Processing and Communication in Europe

Interference is central to quantum physics and occurs when indistinguishable paths exist, as in a double-slit experiment. Replacing the two slits with single atoms introduces optical nonlinearities for which non-trivial interference phenomena are predicted. Their observation, however, has been hampered by difficulties in preparing the required atomic distribution, controlling the optical phases and detecting the faint light. Here we overcome all of these experimental challenges by combining an optical lattice for atom localization, an imaging system with single-site resolution and an optical resonator for light steering. We observe resonator-induced saturation of resonance fluorescence for constructive interference and non-zero emission with huge photon bunching for destructive interference. The latter is explained by atomic saturation and photon-pair generation, similar to predictions for free-space atoms. Our experimental setting allows realization of the Tavis-Cummings model for any number of atoms and photons, exploration of fundamental aspects of light–matter interaction and implementation of new quantum information processing protocols.