Flat-band Localization and Interaction-Induced Delocalization of Photons

Lattices with dispersionless, or flat, energy bands have attracted significant interest in part due to the strong dependence of particle dynamics on interactions. Using superconducting transmon qubits, we design a plaquette of a lattice whose band structure consists entirely of flatbands under the addition of a synthetic magnetic field of pi. We first observe compact localization in the dynamics of a single particle, the hallmark of all-bands-flat physics. Next, we initialize two photons bound by interactions on the same site and observe an interaction-enabled delocalized walk across the plaquette. These results mark the first experimental observation of a quantum walk that becomes delocalized due to interactions and establish superconducting circuits as a platform for studies of flat-band-lattice dynamics with strong interactions.