  The development of sources delivering non-classical states of light is one of
the main needs for applications of optical quantum information science. Here,
we demonstrate the generation of non-classical states of light using
strong-laser fields driving a solid-state system, by using the process of
high-order harmonic generation, where an electron tunnels out of the parent
site and, later on, recombines on it emitting high-order harmonic radiation, at
the expense of affecting the driving laser field. Since in solid-state systems
the recombination of the electron can be delocalized along the material, the
final state of the electron determines how the electromagnetic field gets
affected because of the laser-matter interaction, leading to the generation of
entanglement between the electron and the field. These features can be enhanced
by applying conditioning operations, i.e., quantum operations based on the
measurement of high-harmonic radiation. We study non-classical features present
in the final quantum optical state, and characterize the amount of entanglement
between the light and the electrons in the solid. The work sets the foundation
for the development of compact solid-state-based non-classical light sources
using strong-field physics.
