Based on a parameterized electronic many-body theory we calculate the spectrum of optical second-harmonic generation (SHG) on an antiferromagnetic (AF) surface: NiO (001). The occurrence of AF spectral lines is further exploited for the calculation of ultra-fast spin dynamics. This spin dynamics is observable in SHG. It is characterized by both a sharp drop of the AF signal within a few femtoseconds and a long persistence (up to nanoseconds) of the spin coherence. These two features constitute an ideal system for possible applications in both magnetic recording and quantum computing.