Emergence of nonclassical radiation in strongly laser-driven quantum systems

Nonclassical light sources are central to emerging quantum technologies, yet current platforms offer limited tunability and typically operate at low photon numbers. In parallel, strong-field physics provides widely tunable, bright coherent radiation through high-order harmonic generation (HHG), but its quantum optical character has remained largely unexplained. While recent experiments have revealed signatures of entanglement, squeezing, and quantum-state modification in both the driving and generated fields, a unified theoretical framework capable of identifying the origin and controllability of these effects has been missing. Here we introduce a fully quantum, analytically tractable theory of intense light-matter interaction that rigorously captures the emergence of nonclassicality in HHG. Our approach employs a parametric factorization of the coupled electron-field system into a driven electronic state and a dynamically perturbed quantum optical field, derived directly from the time-dependent Schrödinger equation without requiring conditioning, homodyne detection, or mode-selection techniques. We show how quantum correlations, squeezing, and Wigner-function negativity arise intrinsically from the interaction dynamics, and we identify the precise conditions under which specific nonclassical features are amplified or suppressed. The theory enables predictive design of bright, high-photon-number quantum states at tunable frequencies, and we demonstrate its utility by outlining realistic conditions for generating bright nonclassical ultraviolet light. Our results establish a comprehensive foundation for strong-field quantum optics and open new avenues toward tabletop quantum light sources for sensing, communication, and photonic quantum information processing.