  We directly trace the near- and mid-infrared transmission change of a VO$_2$
thin film during an ultrafast insulator-to-metal transition triggered by
high-field multi-terahertz transients. Non-thermal switching into a metastable
metallic state is governed solely by the amplitude of the applied terahertz
field. In contrast to resonant excitation below the threshold fluence, no
signatures of excitonic self-trapping are observed. Our findings are consistent
with the generation of spatially separated charge pairs and a cooperative
transition into a delocalized metallic state by THz field-induced tunneling.
The tunneling process is a condensed-matter analogue of the Schwinger effect in
nonlinear quantum electrodynamics. We find good agreement with the pair
production formula by replacing the Compton wavelength with an electronic
correlation length of 2.1 $\AA$.
