Quantum effects in light-matter interaction have always intrigued scientists. Such effects may result in quantum particle-particle correlations, for instance. The first measurement of an intensity-intensity correlation function was performed by Hanbury-Brown and Twiss , while the theoretical basis for the full quantum characterization of light was introduced by Glauber . Therefore, researchers now have efficient tools in their hands to probe light fields for quantum signatures. Remarkably, these correlations have now been measured for matter waves too, as described in Physical Review Letters by Karen Kheruntsyan at the University of Queensland, Australia, and colleagues . In this paper, they report violation of an important relation for atom number correlations, called the Cauchy-Schwarz inequality, which indicates clear quantum effects in the interaction of matter waves. The correlated atoms have large spatial separations and therefore this work opens new opportunities for extending fundamental quantum-nonlocality tests to ensembles of massive particles.