  We observe non-perturbative high harmonic generation in solids driven by a
macroscopic quantum state of light, bright squeezed vacuum (BSV), which we
generate in a single spatiotemporal mode. The BSV-driven process is
considerably more efficient in the generation of high harmonics than classical
light of the same mean intensity. Due to its broad photon-number distribution,
covering states from $0$ to $2 \times 10^{13}$ photons per pulse, and sub-cycle
electric field fluctuations over $\pm1\hbox{V}/\hbox{\r{A}}$, BSV provides
access to free carrier dynamics within a much broader range of peak intensities
than accessible with classical light. Our findings contribute to recent
developments of quantum optics with extreme intensities, moving beyond its
traditional focus on low photon numbers, and providing a new method for
exploring extreme nonlinearities in solids.
