A direct measurement of the quantum spin tunneling splitting energy can be achieved using single spin scanning tunneling inelastic spectroscopy, that permits to measure the spin excitations of individual atoms on surfaces. Using this technique, the spin excitation spectrum of an individual integer spin was obtained by Hirjibehedin et al. for a S=2 single Fe atom on a surface of Cu 2 N/Cu(100), that made it possible to measure a quantum spin tunneling splitting of 0.2 meV. Using the same technique other groups measured the spin excitations of S=1 Fe phthalocyanine molecule on a copper surface and a S=1 Fe atom on InSb, both of which had a quantum spin tunneling splitting of the S z = ± ± 1 doublet larger than 1 meV.