  Oxide growth with semiconductor-like accuracy has led to atomically precise
thin films and interfaces that exhibit a plethora of phases and functionalities
not found in the oxide bulk material. This yielded spectacular discoveries such
as the conducting, magnetic or even superconducting LaAlO$_3$/SrTiO$_3$
interfaces separating two prototypical insulating perovskite materials. All
these investigations, however, consider the static state at the interface,
although studies on fast oxide interface dynamics would introduce a powerful
degree of freedom to understanding the nature of the LaAlO$_3$/SrTiO$_3$
interface state. Here we show that the polarization state at the
LaAlO$_3$/SrTiO$_3$ interface can be optically enhanced or attenuated within
picoseconds. Our observations are explained by a model based on charge
propagation effects in the interfacial vicinity and transient polarization
buildup at the interface.
