The synthesis of strong-field physics and quantum optics is one of the nowadays challenges in these areas (https://doi.org/10.48550/arXiv.2509.26602).
We will focus on entirely quantum effects within the high-harmonic generation process from single-atom perspective, it is explained by the famous three-step model. First, the field ionizes the atom, then the electron is driven by the field and accelerated towards the parent ion emitting XUV field (https://doi.org/10.1103/PhysRevLett.71.1994). The resulting field contains multiples of the frequency of the primary field, which gives the name to the process. This description relies on using a classical field of the form E(t) = A*cos(ω*t). Our goal will be to replace E(t) by quantum light (e.g. squeezed-vacuum or Schrödinger-cat states) and study the generated radiation.