Structured Squeezed Light Allows for High-Harmonic Generation in Classical Forbidden Geometries

High-harmonic generation (HHG) is a nonlinear process in which a strong driving field interacts with a material, resulting in the frequency up-conversion of the driver into its high-order harmonics. This process is highly sensitive to the field's polarization: circular polarization, for instance, inhibits HHG. In this Letter, we demonstrate that the use of nonclassical structured light enables HHG in this otherwise prohibitive configuration for classical drivers. We consider circularly polarized light with nonclassical fluctuations, introduced via squeezing along one polarization direction, and show that these nonclassical features prompt the HHG process. We find that the spectral properties of the emitted harmonics depend on the type of squeezing applied and, by analyzing the inner electron dynamics, we relate the observed differences to modifications of the HHG three-step mechanism induced by the specific squeezing type. This approach opens new pathways for integrating quantum optics in HHG, providing novel means of controlling the light-matter interaction dynamics.