Strictly nonclassical behavior of a mesoscopic system

We experimentally demonstrate the strictly nonclassical behavior in a many-atom system using a recently derived criterion [E. Kot et al., Phys. Rev. Lett. 108, 233601 (2012)] that explicitly does not make use of quantum mechanics. We thereby show that the magnetic-moment distribution measured by McConnell et al. [Nature (London) 519, 439 (2015)] in a system with a total mass of 2.6 x 10(5) atomic mass units is inconsistent with classical physics. Notably, the strictly nonclassical behavior affects an area in phase space 10(3) times larger than the Planck quantum (h) over bar.