  Quantum mechanics predicts microscopic phenomena with undeniable success.
Nevertheless, current theoretical and experimental efforts still do not yield
conclusive evidence that there is, or not, a fundamental limitation on the
possibility to observe quantum phenomena at the macroscopic scale. This
question prompted several experimental efforts producing quantum superpositions
of large quantum states in light or matter. Here we report on the observation
of entanglement between a single photon and an atomic ensemble. The certified
entanglement stems from a light-matter micro-macro entangled state that
involves the superposition of two macroscopically distinguishable solid-state
components composed of several tens of atomic excitations. Our approach
leverages from quantum memory techniques and could be used in other systems to
expand the size of quantum superpositions in matter.
