  The control of the electronic properties of materials via the vacuum fields
of cavity electromagnetic resonators is one of the emerging frontiers of
condensed matter physics. We show here that the enhancement of vacuum field
fluctuations in subwavelength split-ring resonators dramatically affects
arguably one of the most paradigmatic quantum protectorates, namely the quantum
Hall electron transport in high-mobility 2D electron gases. The observed
breakdown of the topological protection of the integer quantum Hall effect is
interpreted in terms of a long-range cavity-mediated electron hopping where the
anti-resonant terms of the light-matter coupling finally result into a finite
resistivity induced by the vacuum fluctuations. The present experimental
platform can be used for any 2D material and provides new ways to manipulate
electron phases in matter thanks to vacuum-field engineering
