Emergence of nonclassical radiation in strongly laser-driven quantum systems

TL;DR

This paper develops an analytical framework linking strong laser-driven quantum dynamics to the emergence of nonclassical light, revealing how nonlinear electronic responses generate various quantum optical phenomena in high-order harmonic generation.

Contribution

It introduces a parametric factorization approach that simplifies the analysis of nonclassical light emergence in strongly driven quantum systems, connecting dynamics to quantum optical properties.

Findings

Nonclassicality arises from nonlinear electronic dipole responses to the light mode.

Constant dipole yields coherent radiation; linear dependence results in squeezing.

Higher-order nonlinearities lead to Wigner-function negativity.

Abstract

We present an analytical framework for the emergence of nonclassical radiation in strongly laser-driven quantum systems, with a focus on high-order harmonic generation (HHG). Starting from a Pauli-Fierz description, we employ a parametric factorization of the coupled light-matter wavefunction that reduces the dynamics to coupled equations for a field-driven electronic state and a quantized light mode. Within this framework, we identify a simple and predictive mechanism for nonclassicality: it originates from the nonlinear dependence of the electronic dipole response on the light-mode coordinate. An approximately constant dipole yields coherent radiation, a linear dependence produces squeezing, and higher-order nonlinearities give rise to Wigner-function negativity. We illustrate this mechanism for atomic and molecular model systems and analyze its scaling in multi-emitter configurations, indicating routes toward high-photon-number nonclassical radiation in HHG. Our results provide a transparent connection between strong-field dynamics and quantum-optical properties of emitted light, offering a basis for engineering nonclassical states in intense laser-matter interactions.