  Macroscopic realism is a classical worldview that a macroscopic system is
always determinately in one of the two or more macroscopically distinguishable
states available to it, and so is never in a superposition of these states. The
question of whether there is a fundamental limitation on the possibility to
observe quantum phenomena at the macroscopic scale remains unclear. Here we
implement a strict and simple protocol to test macroscopic realism in a
light-matter interfaced system. We create a micro-macro entanglement with two
macroscopically distinguishable solid-state components and rule out those
theories which would deny coherent superpositions of up to 76 atomic
excitations shared by 10^10 ions in two separated solids. These results provide
a general method to enhance the size of superposition states of atoms by
utilizing quantum memory techniques and to push the envelope of macroscopicity
at higher levels.
