  The physics of intense laser-matter interactions is described by treating the
light pulses classically, anticipating no need to access optical measurements
beyond the classical limit. However, the quantum nature of the electromagnetic
fields is always present. Here, we demonstrate that intense laser-atom
interactions may lead to the generation of highly non-classical light states.
This was achieved by using the process of high-harmonic generation in atoms, in
which the photons of a driving laser pulse of infrared frequency are
up-converted into photons of higher frequencies in the extreme ultraviolet
spectral range. The quantum state of the fundamental mode after the
interaction, when conditioned on the high-harmonic generation, is a so-called
Schr\"odinger cat state, which corresponds to a superposition of two distinct
coherent states: the initial state of the laser and the coherent state reduced
in amplitude that results from the interaction with atoms. The results open the
path for investigations towards the control of the non-classical states,
exploiting conditioning approaches on physical processes relevant to
high-harmonic generation.
